B. 
i 




B 



i 












of % 
^airifr ^jBunlap Iiserliatory 



presentee bg 

Dr.. ..C,.. .A*. ..Chan: 



Jm 1 :!?..."!.?... .19.4-1 



san 







n 1&8& Safe 



TC Hi 







H 



^v. 



91 

HB1 



2h Bfl 






BEf5 






9Z0 
H 



THE WORK OF HERTZ 



AND 



SOME OF HIS SUCCESSORS. 



BEING THE SUBSTANCE OP 



Jl fttctaxt bdibmb at t\je tlaual Institution 

ON 

FRIDAY EVENING, JUNE I, 1894, 

BY 

PROF. OLIVER LODGE, F.R.S. 



WITH ADDITIONS AND APPENDICES. 



Reprinted from "THE ELECTRICIAN;' and Revised by Prof. Lodge. 



NEW YORK : 
THE D. VAN NOSTRAND COMPANY, 

23, Murray Street, and 27 Warren Street. 

LONDON : 
"THE ELECTRICIAN" PRINTING AND PUBLISHING COMPANY, 

LIMITED, 

Salisbury Court, Fleet Street. 



All Rights Reserved. 




Works by Dr. (). J. LODGE. 



Lightning Conductors and Lightning Cuards. 15s. 
Pioneers of Science. 7s. 6d. 
Modern Views of Electricity. 6s. 6d. 
Elementary Mechanics. 3s. 
Protection of Buildings from Lightning. Is. 
Secondary Batteries and the Electrical Storage 
of Energy. 1s. 



"C 



/ 



aL 



#t-c 









THE WORK OF HERTZ 



AND 



SOME OF HIS SUCCESSOES. 



Introductory. 

The untimely end of a young and brilliant career cannot fail to 
strike a note of sadness and awaken a chord of sympathy in the 
hearts of his friends and fellow-workers. Of men thus cut down in 
the early prime of their powers there will occur to us here the 
names of Fresnel, of Carnot, of Clifford, and now of Hertz. His 
was a strenuous and favoured youth ; he was surrounded from his 
birth with all the influences that go to make an accomplished man 
of science accomplished both on the experimental and on the 
mathematical side. The front rank of scientific workers is weaker 
by his death, which occurred on January 1, 1894, the thirty-seventh 
year of his life. Yet did he not go till he had effected an achieve- 
ment which will hand his name down to posterity as the founder 
of an epoch in experimental physics. 

In mathematical and speculative physics others had sown the 
seed. It was sown by Faraday, it was sown by Thomson and by 
Stokes, by Weber also doubtless, and by Helmholtz ; but in this 
particular department it was sowed by none more fruitfully and 
plentifully than by Clerk Maxwell. Of the seed thus sown Hertz 
reaped the fruits. Through his experimental discovery, Germany 
awoke to the truth of Clerk Maxwell's theory of light, of light and 
electricity combined, and the able army of workers in that country 
(not forgetting some in Switzerland, France, and Ireland) have 
done most of the gleaning after Hertz. 

B 



2 THE WORK OF HERTZ 

This is the work of Hertz which is best known, the work which 
brought him immediate fame. It is not always that public notice 
is so well justified. The popular instinct is generous and trustful, 
and it is apt to be misled. The scientific eminence accorded to a 
few energetic persons by popular estimate is more or less amusing 
to those working on the same lines. In the case of Hertz no such 
mistake has been made. His name is not over well-known, and 
his work is immensely greater in every way than that of several 
who have made more noise. 

His best known discovery is by no means his only one. I 
have here a list of eighteen Papers contributed to German 
periodicals by him, in addition to the Papers incorporated in his 
now well-known book on electric waves. I would like to suggest 
that it would be an act of tribute, useful to students in this country, 
if the Physical Society of London saw their way to translate and 
publish a collection of, at any rate, some of these Papers : 

1878-79. Wied. Ann., 1880, vol. 10, p. 414. Experiments to establish an 
Upper Limit for the Kinetic Energy of Electric Flow. 

1880. Inaugural Dissertation (Doctor Thesis) on Induction in Rotating 
Spheres. 

1881. Vol.13, Wied. Ann., p. 266. On the Distribution of Electricity on 
the Surface of Moving Conductors. 

1881. Crelle, vol. 92, p. 156. On the Contact of Solid Elastic 
Bodies. 

1881. Vol. 14, Wied. Ann., p. 581. Upper Limits for the Kinetic Energy 
of Moving Electricity. 

1882. Verhandlungen des Vereins des Gewcrbfleisses (Sonder&bdruck). On 
the Contact of Solid Elastic Bodies and on Hardness. 

1882. Wied. Ann., vol. 17, p. 177. On the Evaporation of Liquids, 
especially of Quicksilver, in Air-Free Space, and on the Pressure of 
Mercury Vapour. 

1882. Vcrliandln. d. phys. Geselhchaft in Berlin, p. 18. On a New 
Hygrometer. 

1883. March. Schldmilch Zeitscltrift, p. 125. On the Distribution of 
Pressures in an Elastic Circular Cylinder. 

1883. Wied. Ann., vol. 19, p. 78. On an appearance accompanying 
Electric Discharge. 

1883. lb. , vol. 19, p. 782. Experiments on Glow Discharge. 

1883. Wied. Ann., vol. 20, p. 279. On the Property of Benzine as an 
Insulator and as showing Elastic Reaction (Ruckstandsbildner). 

1883. Zcitschrift fur Instrumentcnhindc. Dynamometric Contrivance 
of Small Resistance and Infinitesimal Self-induction. 



AND SOME OF HIS SUCCESSORS. 3 

1884. Met. Zeitschrift, November-December. Graphic Methods for the 
Determination of the Adiabatic Changes of Condition of Moist Air. 

1884. Wicd. Ann., vol. 22, p. 449. On the Equilibrium of Floating 
Elastic Plates. 

1884. Uk, vol. 23. On the Connection between Maxwell's Electro- 
dynamic Fundamental Equations and those nf Opposition Electro- 
dynamics. 

1885. lb., vol. 24, p. 114. On the Dimension of a Magnetic Pole in 
different Systems of Units. 

1887-1889. Papers incorporated in his book, "Ausbreitung der Elek- 
trischen Kraft," translated under the title of " Electric Waves." 

1892. Wied. Ann., vol. 45, p. 28. On the Passage of Cathode Kays 
through Thin Metal Sheets. 



Portrait Slide* 

The portrait exhibited at the lecture, though excellent as a 
photograph, failed to represent Hertz at his best ; perhaps because 
it was not taken till after the pharyngeal trouble had set in which 
ultimately carried him off. The frontispiece to this pamphlet, a 
steel-plate contributed by the Proprietors of The Electrician, is 
here used to replace it, with advantage. 

In closing these introductory and personal remarks, I should 
like to say that the enthusiastic admiration for Hertz's spirit and 
chai'acter, felt and expressed by students and works who came into 
contact with him, is not easily to be exaggerated. Never was a 
man more painfully anxious to avoid wounding the susceptibilities 
of others ; and he was accustomed to deprecate the prominence 
given to him by speakers and writers in this country, lest it might 
seem to exlialt him unduly above other and older workers among 
his own sensitive countrymen. 

Speaking of the other great workers in physics in Germany, it is 
not out of place to record the sorrow with which we have heard of 
the recent death of Dr. August Kundt, Professor in the University 
of Berlin, successor to Von Helmholtz in that capacity. 

When I consented to discourse on the work of Hertz, my inten- 
tion was to repeat some of his actual experiments, and especially 
to demonstrate his less-known discoveries and observations. But 
the fascination exerted upon me by electric oscillation experiments, 
when I, too, was independently working at them in the spring of 

b2 



4 THE WORK OF HERTZ 

1888,* resumed its hold, and my lecture will accordingly consist of 
experimental demonstrations of the outcome of Hertz's work rather 
than any precise repetition of portions of that work itself. 

In case a minority of my audience are in the predicament of not 
knowing anything about the subject, a five minutes' explanatory 
prelude may be permitted, though time at present is very far from 
being " infinitely long." 

> The simplest way will be for me hastily to summarise our know- 
ledge of the subject before the era of Hertz. 

Just as a pebble thrown into a pond excites surface ripples, which 
can heave up and down floating straws under which they pass, so a 
struck bell or tuning-fork emits energy into the air in the form of 
what are called sound waves, and this radiant energy is able to set 
up vibrations in other suitable elastic bodies. 




Fig. 1. Oscillations of Dumb-bell Hertz Vibrator (after Bjerknes). 



If the body receiving them has its natural or free vibrations 
violently damped, so that when left to itself it speedily returns 
to rest (Fig. 1), then it can respond fully to notes of almost any 
pitch. This is the case with your ears and the tones of my voice. 
Tones must be exceedingly shrill before they cease to excite the 
ear at all. 

If, on the other hand, the receiving body has a persistent period 
of vibration, continuing in motion long after it is left to itself 
(Fig. 2) like another tuning-fork or bell, for instance, then far 
more facility of response exists, but great accuracy of tuning is 

* Phil. Mag., XXVI. pp. 229, 230 August, 1888 ; or " Lightning Con- 
ductors and Lightning Guards" (Whittaker), pp. 104, 105 ; also P roc. Roy. 
Soc, Vol. 50, p. 27. 



AND SOME OF HIS SUCCESSORS. 5 

necessary if it is to be fully called out ; for if the receiver is not 
thus accurately syntonised with the source, it fails more or less 
completely to resound. 

Conversely, if the source is a persistent vibrator, correct tuning 
is essential, or it will destroy at one moment (Fig. 3) motion Avhich 
it originated the previous moment. Whereas, if it is a dead-beat or 




Fig. 2. 



-Oscillation of Ring-shaped Hertz Resonator excited by syntonic 
Vibrator (after Bjerknes). 



strongly-damped exciter, almost anything will respond equally well 
or equally ill to it. 

What I have said of sounding bodies is true of all vibrators in a 
medium competent to transmit waves. Now a sending telephone 
or a microphone, when spoken to, emits waves into the ether, and 




Fig. 3. Oscillation of Ring Resonator not quite syntonic with Radiator. 
(For method of obtaining these curves see Fig. 14.) 



this radiant energy is likewise able to set up vibration in suitable 
bodies. But we have no delicate means of directly detecting 
these electrical or etheriai waves ; and if they are to produce a 
perceptible effect at a distance, they must be confined, as by a 
speaking-tube, prevented from spreading, and concentrated on the 
distant receiver. 



6 THE WORK OF HERTZ 

This is the function of the telegraph wire ; it is to the other 
what a speaking-tube is to air. A metal wire in air (in function, 
not in details of analogy) is like a long hollow cavity surrounded 
by nearly rigid but slightly elastic walls. 

Sphere charged from EUctrophoras. 

Furthermore, any conductor electrically charged or discharged 
with sufficient suddenness must emit electrical waves into the 
ether, because the charge given to it will not settle down instantly, 
but will surge to and fro several times first ; and these surgings or 
electric oscillations must, according to Maxwell, start waves in the 
ether, because at the end of each half-swing they cause electrostatic, 
and at the middle of each half swing they cause electromagnetic 
effects, and the rapid alternation from one of these modes of 
energy to the other constitutes etherial waves.* If a wire is handy 
they will run along it, and may be felt a long way off. If no wire 
exists they will spread out like sound from a bell, or light from a 
spark, and their intensity will decrease according to the inverse 
square of the distance. 

Maxwell and his followers well knew that there would be such 
waves ; they knew the rate at which they would go, they knew that 
they would go slower in glass and water than in air, they knew 
that they would curl round sharp edges, that they would be partly 
absorbed but mainly reflected by conductors, that if turned back 
upon themselves they would produce the phenomena of stationary 
waves, or interference, or nodes and loops ; it was known how to 
calculate the length of such waves, and even how to produce them 
of any required or predetermined wave-length from 1,000 miles to 
a foot. Other things were known about them which would take 
too long to enumerate ; any homogeneous insulator would transmit 
them, would refract or concentrate them if it were of suitable 
shape, would reflect none of a particular mode of vibration at a 
certain angle, and so on, and so on. 

* Strictly speaking, in the waves themselves there is no lag or difference 
of phase between the electric and the magnetic vibrations ; the difference 
exists in emitter or absorber, but not in the transmitting medium. True' 
radiation of energy does not begin till about a quarter wave-length from 
the source, and within that distance the initial quarter period difference of 
phase is obliterated. 



AND SOME OF HIS SUCCESSORS. 



All this was knotcn, I say, known with varying degrees of con- 
fidence ; but by some known with as great confidence as, perhaps 
even more confidence than, is legitimate before the actuality of 
experimental verification. 

Hertz supplied the verification. He inserted suitable conductors 
in the path of such waves, conductors adapted for the occurrence 
in them of induced electric oscillations, and to the surprise of 
everyone, himself doubtless included, he found that the secondary 
electric surgings thus excited were strong enough to display them- 
selves by minute electric sparks. 





Slider for Tun.ng 



Fig. 4. Experiment with syntonic Ley den Jars. 



Syntonic Leyden Jars. 
I shall show this in a form which requires great precision of 
tuning or syntony, both emitter and receiver being persistently 
vibrating things giving some 30 or 40 swings before damping has a 
serious effect. I take two Leyden jars with circuits about a yard 
in diameter, and situated about two yards apart (Fig. 4). I charge 
and discharge one jar, and observe that the surgings set up in the 
other can cause it to overflow if it is syntonised with the first.* 



* See Nature, Vol. XLL, p. 
Researches," p. 395. 



368 ; or J. J. Thomson, " Recent 



8 THE WORK OF HERTZ 

A closed circuit such as this is a feeble radiator and a feeble 
absorber, so it is not adapted for action at a distance. In fact, 
I doubt whether it will visibly act at a range beyond the \\ at 
which true radiation of broken-off energy occurs. If the coatings 
of the jar are separated to a greater distance, so that the dielectric 
is more exposed, it radiates better ; because in true radiation the 
electrostatic and the magnetic energies are equal, whereas in a ring 
circuit the magnetic energy greatly predominates. By separating 
the coats of the jar as far as possible we get a typical Hertz 
vibrator (Fig. 5), whose dielectric extends out into the room, and 
this radiates very powerfully. 




Fig. 5. Standard Hertz Radiator. 

Ordinary Size Hertz Vibrator. 

In consequence of its radiation of energy, its vibrations are 
rapidly damped, and it only gives some three or four good strong 
swings (Fig. 1). Hence it follows that it has a wide range of 
excitation ; i.e., it can excite sparks in conductors barely at all 
in tune with it. 

The two conditions, conspicuous energy of radiation and persis- 
tent vibration electrically produced, are at present incompatible. 
Whenever these two conditions coexist, considerable power or 
activity will, of course, be necessary in the source of energy. At 
present they only coexist in the sun and other stars, in the electric 
arc, and in furnaces. 

Two Circular Vibrators sparling in sympathy. 
The receiver Hertz used was chiefly a circular resonator (Fig. 6), 
not a good absorber but a persistent vibrator, well adapted for 
picking up disturbances of precise and measurable wave-length. 
Its mode of vibration when excited by emitter in tune with it 



AXB SOME OF HIS SUCCESSORS. 



depicted in Fig. 2. 1 find that the circular resonators can act as 
senders too ; here is one exciting quite long sparks in a second one. 
Electric Syntony. That was his discovery, but he did not stop 
there. He at once proceeded to apply his discovery to the verifica- 
tion of what had already been predicted about the waves, and by 
laborious and difficult interference experiments he ascertained that 




Fig. 6. Circular Resonator. (The knobs ought to nearly touch each other.) 

the previously calculated length of the waves was thoroughly borne 
out by fact. These interference experiments in free space are his 
greatest achievement. 

He worked out every detail of the theory splendidly, separately 
analysing the electric and the magnetic oscillation, using language 




Fig. 6a. Any circular resonator can be used as a sender by bringing its 
knobs near the sparking knobs of a coil ; but a simple arrangement is to 
take two semi-circles, as in above figure, and make them the coil terminals. 
The capacity of the cut ends can be varied, and the period thereby 
lengthened, by expanding them into plates. 

not always such as we should use now, but himself growing in 
theoretic insight through the medium of what would have been to 
most physicists a confusing maze of troublesome facts, and disen- 
tangling all their main relations most harmoniously. 



10 



THE WORK OF HERTZ 



Holtc Machine, A and B Sparks ; Glass and Quartz Panes in 

Screen. 

While Hertz was observing sparks such as these, the primary 
or exciting spark and the secondary or excited one, he observed 
as a bye issue that the secondary spark occurred more easily if the 
light from the primary fell upon its knobs. He examined this new 
influence of light in many ways, and showed that although spark 
light and electric brush light were peculiarly effective, any source 
of light that gave very ultra-violet rays produced the same result. 




Fig. 7. Experiment arranged to show effect on one spark of light from 
another. The B spark occurs more easily when it can see the A spark 
through the window, unless the window is glazed with glass. A quartz 
pane transmits the effect. 

The above figure represents my way of showing the experiment. 
It will be observed that with this arrangement the B knobs are at 
the same potential up to the instant of the flash, and- at the same 
potential up to the instant of the flash, and in that case the ultra- 
violet portion of the light of the A spark assists the occurrence of 
the B spark. But it is interesting to note what Elster and Geitel 
have found (see Appendix I., Fig. A), that if the B knobs were 
subjected to steady strain instead of to impulsive rush e.g., if 



AND SOME OF HIS SUCCESSORS. 



11 



they were connected to the inner coats of the jars instead of the 
outer coatings that then the effect of ultra-violet light on either 
spark-gap would exert a deterrent influence, so that the spark 
would probably occur at the other, or non-illuminated, gap. With 
the altered connections it is, of course, not feasible to illuminate 
one spark by the light of the other ; the sparks are then alternative, 
not successive. 

Wiedemann and Ebert, and a number of experimenters, have 
repeated and extended this discovery, proving that it is the cathode 
knob on which illumination takes effect ; and Hallwachs and Righi 
made the important observation, which Elster and Geitel, Stoletow, 




Fig. 8. Zinc Rod in Arc Light, protected by Glass Screen. The lenses 
are of quartz, but there is no need for any lenses in this experiment ; 
leakage begins directly the glass plate is withdrawn. 

Branly, and others have extended, that a freshly-polished zinc 
or other oxidisable surface, if charged negatively, is gradually 
discharged by ultra-violet light. 

It is easy to fail in reproducing this experimental result if the 
right conditions are not satisfied ; but if they are it is absurdly 
easy, and the thing might have been observed nearly a century ago. 

Zinc discharging Negative Electricity in Light ; Gold Leaf 

Electroscope; Glass and Quartz Panes ; Quart: Prism. 
Take a piece of zinc, clean it with emery paper, connect it to a 
gold leaf electroscope, and expose it to an arc lamp. (Fig. 8). If 



12 THE WORK OF HERTZ 

charged positively nothing appears to happen, the action is very 
slow ; but a negative charge leaks away in a few seconds if the light 
is bright. Any source of light rich in ultra-violet rays will do ; the 
light from a spark is perhaps most powerful of all. A pane of glass 
cuts off all the action ; so does atmospheric air in sufficient thickness 
(at any rate, town air), hence sunlight is not powerful. A pane of 
quartz transmits the action almost undiminished, but fluor-spar 
may be more transparent still. Condensing the arc rays with a 
quartz lens and analysing them with a quartz prism or reflection 
grating, we find that the most effective part of the light is high up 
in the ultra-violet, surprisingly far beyond the limits of the visible 
spectrum* (Fig. 9). 



* While preparing for the lecture it occurred to me to try, if possible 
during the lecture itself, some new experiments on the effect of light on 
negatively charged bits of rock and ice, because if the effect is not limited 
to metals it must be important in connection with atmospheric electricity. 
When Mr. Branly coated an aluminium plate with an insulating varnish, 
he found that its charge was able to soak in and out of the varnish during 
illumination (Comptes Rendus, Vol. CX., p. 898, 1890). Now, the mountain 
tops of a negatively charged earth are exposed to very ultra-violet rays, 
and the air is a dielectric in which quiet up-carrying and sudden downpour 
of electricity could go on in a manner not very unlike the well-known 
behaviour of water vapour ; and this perhaps may be the reason, or one of 
the reasons, why it is not unusual to experience a thunderstorm after a 
few fine days. I have now tried these experiments on such geological 
fragments as were handy, and find that many of them discharge negative 
electricity under the action of a naked arc, especially from the side of the 
specimens which was somewhat dusty, but that when wet they discharge 
much less rapidly, and when positively charged hardly at all. Ice and 
garden soil discharge negative electrification, too, under ultra-violet illu- 
mination, but not so quickly as limestone, mica schist, ferruginous quartz ( 
clay, and some other specimens. Granite barely acts ; it seems to insulate 
too well. The ice and soil were tried in their usual moist condition, but, 
when thoroughly dry, soil discharges quite rapidly. No rock tested 
was found to discharge as quickly as does a surface of perfectly bright 
metal, such as iron, but many discharged much more quickly than ordinary 
dull iron, and rather more quickly than when the bright iron surface was 
thinly oiled or wetted with water. To-day (June 5) I find that the leaves 
of Geranium discharge positive electrification five times as quickly as 
negative, under the action of an arc-light, and that glass cuts the effect off 
while quartz transmits it. [For ELster and Geitel's experiments, and those 
of Righi, see Appendix]. 



AND SOME OF HIS SUCCESSORS. 



13 



This is rather a digression, but I have taken some pains to show- 
it properly because of the interest betrayed by Lord Kelvin on this 
matter, and the caution which he felt about accepting the results 
of the Continental experimenters too hastily. 

It is probably a chemical phenomenon, and I am disposed to 
express it as a modification of the Volta contact effect* with 
illumination. 












Fig. 9. Zinc Rod discharging Negative Electricity in the very Ultra- 
violet Light of a Spectrum formed by a Quartz Train. 

Return now to the Hertz vibrator, or Leyden jar with its coatings 
well separated, so that we can get into its electric as well as its 
magnetic field. Here is a great one giving waves 30 metres long, 
radiating while it lasts with an activity of 100 h.p., and making 
ten million complete electric vibrations per second. 



* See Brit. Assoc. Report, 1884, 
pp. 267-352. 



pp. 502-519, or Phil. Mag., Vol. XIX. ; 



14 THE WORK OF HERTZ 

Large Hertz Vibrator in action; Abel's Fuse; Vacuum Tube 
Striking of an Arc. 

Its great radiating power damps it down very rapidly, so that it 
does not make above two or three swings ; but neverthless, each 
time it is excited, sparks can be drawn from most of the reasonably 
elongated conductors in this theatre. 

A suitably situated gas-leak can be ignited by these induced sparks. 
An Abel's fuse connecting the water pipes with the gas pipes will 
blow off; vacuum tubes connected to nothing will glow (this fact has 
been familiar to all who have worked with Hertz waves since 1889), 
electric leads, if anywhere near each other, as they are in some 
incandescent lamp holders, may spark across to each other, thus 
striking an arc and blowing their fuses. This blowing of fuses by 
electric radiation frequently happened at Liverpool till the suspen- 
sions of the theatre lamps were altered. 



ho OH 



Fig. 10. Hertz Oscillator on reduced scale. T Vth inch to a foot. 

The striking of an arc by the little reverberating sparks between 
two lamp-carbons connected with the 100-volt mains I incidentally 
now demonstrate. An arc is started directly the large Hertz 
vibrator is excited at a distance. 

There are some who think that lightning flashes can do none of 
these secondary things. They are mistaken. 

Specimens of emitters and receivers. 
On the table are specimens of various emitters and receivers 
such as have been used by different people; the orthodox 
Hertz radiator of dumb-bell type (Fig. 5), and the orthodox Hertz 
receivers : a circular ring (Fig. 6) for interference experiments, 
because it is but little damped, and a straight wire for receiving 
at a distance, because it is a much better absorber. Beside 
these are the spheres and ellipsoids (or elliptical plates), which 
I have mainly used, (Fig. 19) because they are powerful radiators 
and absorbers, and because their theory has been worked out by 
Horace Lamb and J. J. Thomson. Also dumb-bells (Fig. ll)without 



AND SOME OF HIS SUCCESSORS. 



15 



air-gap, and many other shapes, the most recent of mine being the 
inside of a hollow cylinder with sparks at ends of a diameter (Fig. 12); 
this being a feeble radiator, but a very persistent vibrator, * and, 
therefore, well adapted for interference and diffraction experi- 
ments. But, indeed, spheres can be made to vibrate longer than 




l? Inches ju 3'A Inches 




Fig. 11. A Dumb-bell Form of Radiator. 

usual by putting them into copper hats or enclosures, in which an 
aperture of varying size can be made to let the waves out (Figs. 
20 and 21). 

Many of these senders will do for receivers too, giving off sparks 
to other insulated bodies or to earth ; but, besides the Hertz type 

m 




Fig. 12. Dr. Lodge's Hollow Cylindrical Radiator, arranged horizontally 
against the Outside of a Metal-lined Box. Half natural size. Emitting 
3in. waves. 

of receiver, many other detectors of radiation have been employed. 
Vacuum tubes can be used, either directly or on the trigger prin- 
ciple, as by Zehnder (Fig. 13), t the resonator spark precipitating a 
discharge from some auxiliary battery or source of energy, and so 
making a feeble disturbance very visible. Explosives may be used 

* J. J. Thomson, "Recent Researches," 344. 
f Wied. Ann., XLVIL, p. 77. 



16 



THE WORK OF HERTZ 



for the same purpose, either in the form of mixed water-gases or 
in the form of an Abel's fuse. Fitzgerald found that a tremendously 
sensitive galvanometer could indicate that a feeble spark had 
passed, by reason of the consequent disturbance of electrical 
equilibrium which settled down again through the galvanometer.* 
This was the method he used in this theatre four years ago. Blyth 
used a one-sided electrometer, and V. Bjerknes has greatly de- 
veloped this method (Fig. 14), abolishing the need for a spark, and 
making the electrometer metrical, integrating, and satisfactory.! 
With this detector many measurements have been made at Bonn 
by Bjerknes, Yule, Barton, and others on waves concentrated 
and kept from space dissipation by guiding wires. 






Fig. 13. Zehnder's Trigger Tube. Half Natural Size. The two right- 
hand terminals, close together, are attached to the Hertz receiver ; another 
pair of terminals are connected to some source just not able to make the 
tube glow until the scintilla occurs and makes the gas more conducting 
as observed by Schuster and others. 

Mr. Boys has experimented on the mechanical force exerted by 
electrical surgings, and Hertz also made observations of the same 
kind. 

Various Detectors. 

Going back to older methods of detecting electrical radiation, 

we have, most important of all, a discovery made long before man 

existed, by a creature that developed a sensitive cavity on its skin ; 

a creature which never so much as had a name to be remembered 

by (though perhaps we now call it trilobite). Then, in recent 

times we recall the photographic plate and the thermopile, with its 

modification, the radiomicrometer ; also the so-called bolometer, 

or otherwise-known Siemens' pyrometer, applied to astronomy by 

* Fitzgerald, Nature, Vol. XLL, p. 295, and Vol. XLIL, p. 172. _ 
t Wied. Ann., 44, p. 74. 



AND SOME OF HIS SUCCESSORS. 



17 



Langley, and applied to the detection of electric waves in wires by 
Rubens and Hitter and Paalzow and Arons. The thermal junction 
was applied to the same purpose by Kolacek, D. E. Jones, and 
others. 

And, before all these, the late Mr. Gregory, of Cooper's Hill, 
made his singularly sensitive expansion meter, whereby waves in 
free space could be detected by the minute rise of temperature they 
caused in a platinum wire, a kind of early and sensitive form of 
Cardew voltmeter. 



IA 





Fig. 14. Bjerknes' Apparatus, showing (1) a Hertz vibrator connected 
to an induction coil ; (2) a nearly-closed-circuit receiver properly tuned 
with the vibrator : and (3) a one-sided electrometer for inserting in the 
air-gap of 2. The receiver is not provided with knobs, as shown, but its 
open circuit is terminated by the cmadrants of the electrometer, which is 
shown on an enlarged scale alongside. The needle is at zero potential and 
is attracted by both quadrants. By calculation from the indications of this 
electrometer Bjerknes plotted the curves 1, 2, and 3 on pages 4 and 5. 
Fig. 1 represents the oscillations of the primary vibrator, rapidly damped by 
radiation of energy. Fig. 2 represents the vibrations thereby set up in the 
resonating circuit when the two are accurately in tune ; and which persist 
for many swings. Fig. 3 shows the vibrations excited in the same circuit 
when slightly out of tune with the exciter. A receiver of this kind makes 
many swings before it is seriously danced. 

Going back to the physiological method of detecting surgings, 
Hertz tried the frog's-leg nerve and muscle preparation, which to 
the steadier types of electrical stimulus is so surpassingly sensitive, 
and to which we owe the discovery of current electricity. But he 
failed to get any result. Hitter has succeeded ; but, in my expe- 
rience, failure is the normal and proper result. Working with my 
colleague, Prof. Gotch, at Liverpool, I too have tried the nerve and 

c 



13 THE WORK OF HERTZ 

muscle preparation of the frog (Fig. 15), and we find that an ex- 
cessively violent stimulus of a rapidly alternating character, if pure 
and unaccompanied by secondary actions, produces no effect no 
stimulating effect, that is, even though the voltage is so high that 
sparks are ready to jump between the needlea in direct contact 
with the nerve. 

All that such oscillations do, if continued, is to produce a tem- 
porary paralysis or fatigue of the nerve, so that it is unable to 
transmit the nerve impulses evoked by other stimuli, from which 
paralysis it recovers readily enough in course of time. 

This has been expected from experiments on human beings, such 
experiments as Tesla's and those of d'Arsonval. But an entire 
animal is not at all a satisfactory instrument wherewith to attack 
the question ; its nerves are so embedded in conducting tissues 
that it may easily be doubted whether the alternating type of 

B D 




A C 

Fig. 15. Experiment of Gotch and Lodge on the physiological effect of 
rapid pure electric alternations. Nerve-muscle preparation, with four 
needles, or else non-polarisable electrodes applied to the nerve. C and D 
are the terminals of a rapidly alternating electric current from a conductor 
at zero potential, while A and B are the terminals of an ordinary very 
weak galvanic or induction coil stimulus only just sufficient to make the 
muscle twitch. 

stimulus ever reaches them at all. By dissecting out a nerve and 
muscle from a deceased frog after the historic manner of physi- 
ologists, and applying the stimulus direct to the nerve, at the same 
time as some other well known yJoth of a volt stimulus is applied 
to another part of the same nerve further from the muscle, it can 
be shown that rapid electric alternations, if entirely unaccompanied 
by static charge or by resultant algebraic electric transmission, 
evoke no excitatory response until they are so violent as to give 
rise to secondary effects such as heat or mechanical shock. Yet, 
notwithstanding this inaction, they gradually and slowly exert a 
paralysing or obstructive action on the portion of the nerve to which 
they are applied, so that the nerve impulse excited by the feeble 
just perceptible T^jth-volt stimulus above is gradually throttled 



AND SOME OF JUS SUCCESSORS. 



19 



on its way down to the muscle, and remains so throttled for a time 
varying from a few minutes to an hour after the cessation of the 
violence. 

I did not show this experiment at the lecture, but we hope to 
show it to the Physiological Section at Oxford. 

Air Gap ami Electroscope charged by Glass Bod and discharged by 

moderately distant Sphere excited by Coil. 

Among trigger methods of detecting electric radiation, I have 

spoken of the Zehnder vacuum tubes ; another method is one used 

by Boltzmann.* A pile of several hundred volts is on the verge 




TK3 



ry-r 



Fig. 16. Air-gap for Electroscope. Natural Size. The bottom plate is 
connected to, and represents, the cap of an electroscope ; the " knob " 
above it, mentioned in text, is the polished end of the screw, whose 
terminal is connected with the case of the instrument or " earth." 

of charging an electroscope through an air gap just too Avide to 
break down. Very slight electric surgings precipitate the discharge 
across the gap, and the leaves diverge. I show this in a modified 
and simple form. On the cap of an electroscope is placed a 
highly-polished knob or rounded end connected to the sole, and 
just not touching the cap, or rather just not touching a plate 
connected with the cap (Fig. 16), the distance between knob and 
plate being almost infinitesimal, such a distance as is appreciated 
in spherometry. Such an electroscope overflows suddenly and 



* Wied. Ann., 40, p. 399. 



c2 



20 THE WORK OF HERTZ 

completely with any gentle rise of potential. Bring excited 
glass near it, the leaves diverge gradually and then suddenly col- 
lapse, because the air space snaps ; remove the glass, and they 
rediverge with negative electricity ; the knob above the cap being 
then charged positively, and to the verge of sparking. In this 
condition any electrical waves, collected if weak by a foot or so of 
wire projecting from the cap, will discharge the electroscope by 
exciting surgings in the wire, and so breaking down the air-gap. 
The chief interest about this experiment seems to me the extremely 
definite dielectric strength of so infinitesimal an air space. More- 
over, it is a detector tor Hertz waves that might have been used 
last century ; it might have been used by Benjamin Franklin. 

For to excite them no coil or anything complicated is necessary ; 
it is sufficient to flick a metal sphere or cylinder with a silk hand- 
kerchief and then discharge it with a well-polished knob. If it is 
not well polished the discharge is comparatively gradual, and the 
vibrations are weak ; the more polished are the sides of an air-gap, 
the more sudden is the collapse and the more vigorous the con- 
sequent radiation, especially the radiation of high frequency, the- 
hjgher harmonics of the disturbance. 

For delicate experiments it is sometimes well to repolish the- 
knobs every hour or so. For metrical experiments it is often, 
better to let the knobs get into a less efficient but more permanent 
state. This is true of all senders or radiators. For the generation 
of the, so to speak, "infra-red" Hertz waves any knobs will do,, 
but to generate the " ultra-violet " high polish is essential. 

Microphonic Detectors. 

Receivers or detectors which for the present I temporarily call 
microphonic are liable to respond best to the more rapid vibra- 
tions. Their sensitiveness is to me surprising, though of course it 
does not approach the sensitiveness of the eye ; at the same time, I 
am by no means sure that the eye differs from them in kind. It is 
these detectors that I wish specially to bring to your notice. 

Prof. Minchin, whose long and patient Avork in connection with 
photo-electricity is now becoming known, and who has devised an 
instrument more sensitive to radiation than even Boys' radiomicro- 
meter, in that it responds to the radiation of a star while the 



AXD SOME OF HIS SUCCESSORS. 21 

racliomicrometer does not, found some years ago that some of his 
light-excitable cells lost their sensitiveness capriciously on tap- 
ping, and later he found that they frequently regained it again 
while Mr. Gregory's Hertz-wave experiments were going on in the 
same room. 

These "impulsion-cells," as he terms them, are troublesome 
things for ordinary persons to make and work with at least I 
have never presumed to try but in Mr. Minchin's hands they are 
surprisingly sensitive to electric waves.* 

The sensitiveness of selenium to light is known to everyone, and 
Mr. Shelford Bid well has made experiments on the variations of 
conductivity exhibited by a mixture of sulphur and carbon. 

Nearly four years ago M. Edouard Branly found that a burnished 
coat of porphyrised copper spread on glass diminished its resist- 
ance enormously, from some millions to some hundreds of ohms 
when it was exposed to the neighbourhood, even the distant neigh- 
bourhood, of Leyden jar or coil sparks. He likewise found that 
a tube of metallic filings behaved similarly, but that this recovered 
its original resistance on shaking. Mr. Croft exhibited this fact 
recently at the Physical Society. M. Branly also made pastes and 
solid rods of filings, in Canada balsam and in sulphur, and found 
them likewise sensitive. t 

With me the matter arose somewhat differently, as an outcome 
of the air-gap detector employed with an electroscope by Boltzmann. 
For I had observed in 1889 that two knobs sufficiently close together, 
far too close to stand any voltage such as an electroscope can show, 
could, when a spark passed between them, actually cohere ; con- 
ducting an ordinary bell-ringing current if a single voltaic cell was 
in circuit ; and, if there were no such cell, exhibiting an electro- 
motive force of their own sufficient to disturb a low resistance gal- 
vanometer vigorously, and sometimes requiring a faintly perceptible 
amount of force to detach them. The experiment was described to 
the Institution of Electrical Engineers, X and Prof. Hughes said he 
had observed the same thing. 

* Phil. Mag., Vol. XXXI., p. 223. 

+ E. Branly, Combes Rendus, Vol. CXI., p. 785 ; and Vol. CXII., p. 90. 

\ Journal Institution of Electrical Engineers, 1890, Vol. XIX., pp. 352-4 ; 

or " Lightning Conductors and Lightning Guards" (Whit taker), pp. 382-4. 



22 



THE WORK OF HERTZ 



Coherer in open, responding to Feeble Stimuli : Small Sphere, 
Gas-lighter, Distant Sphere, Electrophorus* 

Well, this arrangement, which I call a coherer, is the most asto- 
nishingly sensitive detector of Hertz waves. It differs from an 
actual air-gap in that the insulating film is not really insulating ; the 
film breaks down not only much more easily, but also in a less dis- 
continuous and more permanent manner, than an air-gap. A tube 
of filings, being a series of bad contacts, clearly works on the same 
plan ; and though a tube of filings is by no means so sensitive, yet 
it is in man; respects easier to work with, and, except for very 





Fig. 17. Coherer, consisting of a spiral of thin iron wire mounted on an 
adjustable spindle and an aluminium plate. When the lever is moved 
clockwise, the tip of the iron wire presses gently against the aluminium 
plate. 



feeble stimuli, is more metrical. If the filings used are coarse, say 
turnings or borings, the tube approximates to a single coherer ; if 
they are fine, it has a larger range of sensibility. In every case 
what these receivers feel are sudden jerks of current ; smooth 
sinuous vibrations are ineffective. They seem to me to respond best 
to waves a few inches long, but doubtless that is determined chiefly 
by the dimensions of some conductor with which they happen to be 
associated. (Figs. 17 and 18.) 



AND SOME OF HIS SUCCESSORS. 



23 



Filings in open, responding to Sphere, to Electrophoms, to Spark, 
from Gold-leaf Electroscope. 

I picture to myself the action as follows : Suppose two fairly 
clean pieces of metal in light contact say two pieces of iron con- 
nected to a single voltaic cell ; a film of what may be called oxide 
intervenes between the surfaces, so that only an insignificant cur- 
rent is allowed to pass, because a volt or two is insufficient to 
break down the insulating film, except perhaps at one or two atoms.* 
If the film is not permitted to conduct at all, it is not very sensi- 
tive ; the most sensitive condition is attained when an infinitesimal 
current passes, strong enough just to show on a moderate gal- 
vanometer. 

Now let the slightest surging occur, say by reason of a sphere 
being charged and discharged at a distance of forty yards ; the film 

n 



ChOLAM* " 



y* 






Fig. 18. Iron Borings Tube, one-third natural size. 



at once breaks down perhaps not completely, that is a question 
of intensity but permanently. As I imagine, more molecules get 
within each other's range, incipient cohesion sets in, and the 
momentary electric quiver acts somewhat like a flux. It is a 
singular variety of electric welding. A stronger stimulus enables 
more molecules to hold on, the process is surprisingly metrical ; 
and, as far as I roughly know at present, the change of resistance 
is proportional to the energy of the electric radiation, from a 
source of given frequency. 

It is to be specially noted that a battery current is not needed 
to effect the cohesion, only to demonstrate it. The battery can be 
applied after the spark has occurred, and the resistance will be 
found changed as much as if the battery had been on all the time. 

* See Phil. Mag., Jan., 1894, p. 94. 



24 THE WORK OF HERTZ 

The incipient cohesion electrically caused can be mechanically 
destroyed. Sound vibrations, or any other feeble mechanical dis- 
turbances, such as scratches or taps, are well adapted to restore the 
contact to its original high-resistance sensitive condition. The more 
feeble the electrical disturbance the slighter is the corresponding 
mechanical stimulus needed for restoration. When working with the 
radiating sphere (Fig. 19) at a distance of forty yards out of window, 
I could not for this reason shout to my assistant, to cause him 
to press the key of the coil and make a spark, but I showed him a 
duster instead, this being a silent signal which had no disturbing 
effect on the coherer or tube of filings. I mention 40 yards, because 
that was one of the first outdoor experiments ; but I should think 
that something more like half an mile was nearer the limit of sensi- 
tiveness. However, this is a rash statement not at present verified > 
At 40 or 60 yards the exciting spark could be distinctly heard, and 
it was interesting to watch the spot of light begin its long excursion 
and actually travel a distance of 2in. or 3in. before the sound arrived. 
This experiment proved definitely enough that the efficient cause 
travelled quicker than sound, and disposed completely of any 
sceptical doubts as to sound-waves being, perhaps, the real cause 
of the phenomenon. 

Invariably, when the receiver is in good condition, sound or 
other mechanical disturbance acts one way, viz., in the direction of 
increasing resistance, while electrical radiation or jerks act the 
other way, decreasing it. While getting the receiver into condition 
or when it is getting out of order, vibrations and sometimes electric 
discharges act irregularly ; and an occasional good shaking does the 
filings good. I have taken rough measurements of the resistance, 
by the simple process of restoring the original galvanometer 
deflection by adding or removing resistance coils. A half -inch tube 
8in. long, of selected iron turnings (Fig. 18) had a resistance of 
2,500 ohms in the sensitive state. A feeble stimulus, caused 
by a distant electrophorus spark, brought it down 400 ohms. 
A rather stronger one reduced it by 500 and 600, while a trace 
of spark given to a point of the circuit itself, ran it down 1,400 
ohms. 

This is only to give an idea of the quantities. I have not yet 
done any seriously metrical experiments. 



AND SOME OF II IS SUCCESSORS. 



25 











* \ 












d 








, 4 




to 


<72 <D O 




d 


e^* 'flj 




e 


0) J3 'S 




3 


^ ^>~ 


**s 


. o 


H^ o 




o 
ft 


a 2-= 

.2 .2" 


P (3 




stimul 
applied 
microph 




o 
u 
o 


elen 

puis 
(Mil 


s 


a 2 




s 


m a 

1 1 




to 


unless 
who 
the i 














* 






W 








s- 




So" 






* - 


?H 




O to p 




'cS 
o 








'a & 

.3 ft.-*= 




'3 

J3 


as 




r3 t; 

0) c 

r5 * 




'i <u S 
re 2: o 

^ -5 ft 




o 
a; 

8 


s 


ft 3 
go""" 




respond to 
'ut suggest 
es that its 






w 














a> 








/ ''-" s, 


ft ^ 


. 4= -|3 


OF RADIATION. 


"c8 
u 

o 

w 


p.p 

MS 


{Telephone ; Air-gap 
and Arc. 
(Lodge.) 


Vacuum Tube. 
(Dragoumis.) 

Galvanometer. 

(Fitzgerald.) 

Air-gap and Electrosco 

(Boltzniann.) 

Trigger Tube. 

(Warburg and Zehnder. 


es not appear really to 
hings are unimportant, 
gainst Selenium indica 












O tf tc 






/^s 


. 


*^3 t_. 


o 






V 


2 


licates that it 
gainst the othi 
rrogation mar 


DETECT 


a 

EH 


O 1 
i- 


-*3 
CD 

a = 

Is 


spanding Wi 

(Gregory.) 

ermal Juncti 

(Klemencik.) 








f 


W Eh 


's leg in< 
names a 
Hie inte 






0> 


id 

0) 


15 




j 


s 


02 
CO 


O 


60 a> 

O .a 




O 


^o 


a 








a 








-*j 


0) ,o 




ft 


> 


o 


.13 




0) 

o 


60 
O 


o 
'ft 


'oj 

o 

4-3 


ist t 
nner 
adia 
ful. 






-4J 


* 





.S re i -g 






O 


H 


ft 


,3.0^ 






ft 




he cross a 

secondary 

to electr 

nay be do 


13 
To 




60.2 






o 

*f0 




DO , , 

'60 = 

o 3 




H 2 o a 

x 1 a 




>s 




s 




g^ 3 




Ph 




x S 




.3T3 O 


1 












26 THE WORK OF HERTZ 

From the wall diagram which summarises the various detectors, 
and which was prepared a month or so ago, I see I have omitted 
selenium, a substance which in certain states is well known to 
behave to visible light as these other microphonic detectors behave 
to Hertz waves. It is now inserted, but with a query to indicate 
that its position in the table is not certainly known. 

Electrical Theory of Vision. 

And I want to suggest that quite possibly the sensitiveness of 
the eye is of the same kind. As I am not a physiologist I cannot 
be seriously blamed for making wild and hazardous speculations in 
that region. I therefore wish to guess that some part of the retina 
is an electrical organ, say like that of some fishes, maintaining an 
electromotive force which is prevented from stimulating the nerves 
solely by an intervening layer of badly conducting material, or of 
conducting material with gaps in it ; but that when light falls upon 
the retina these gaps become more or less conducting, and the 
nerves are stimulated. I do not feel clear which part is taken by 
the rods and cones, and which part by the pigment cells ; I must 
not try to make the hypothesis too definite at present. 

If I had to make a demonstration model of the eye on these lines, 
I should arrange a little battery to excite a frog's nerve-muscle 
preparation through a circuit completed all except a layer of filings 
or a single bad contact. Such an arrangement would respond to 
Hertz waves. Or, if I wanted actual light to act, instead of grosser 
waves, I would use a layer of selenium. 

But the bad contact and the Hertz waves are the most instruc- 
tive, because we do not at present really know what the selenium 
is doing, any more than what the retina is doing. 

And observe that (to my surprise, I confess) the rough outline 
of a theory of vision thus suggested is in accordance with some of 
the principal views of the physiologist . Hering. The sensation of 
light is due to the electrical stimulus ; the sensation of black is due 
to the mechanical or tapping back stimulus. Darkness is physio- 
logically not the mere cessation of light. Both are positive sen- 
sations, and both stimuli are necessary ; for until the filings are 
tapped back vision is persistent. In the eye model the period of 



AND SOME OF HIS SUCCESSORS. 27 

mechanical tremor should be, say, j^jth second, so as to give the 
right amount of persistence of impression. 

No doubt in the eye the tapping back is done automatically by 
the tissues, so that it is always ready for a new impression, until 
fatigued. And by mounting an electric bell or other vibrator on 
the same board as a tube of filings, it is possible to arrange so that 
a feeble electric stimulus shall produce a feeble steady effect, a 
stronger stimulus a stronger effect, and so on ; the tremor asserting 
its predominance, and bringing the spot back whenever the electric 
stimulus ceases. 

An electric bell thus close to the tube is, perhaps, not the best 
vibrator ; clockwork might do better, because the bell contains in 
itself a jerky current, which produces one effect, and a mechanical 
vibration, which produces an opposite effect ; hence the spot of 
light can hardly keep still. By lessening the vibration say, by 
detaching the bell from actual contact with the board, the electric 
jerks of the intermittent current drive the spot violently up the 
scale ; mechanical tremor brings it down again. 

You observe that the eye on this hypothesis is, in electrometer 
language, heterostatic. The energy of vision is supplied by the 
organism ; the light only pulls a trigger. Whereas the organ of 
hearing is idiostatic. I might diaw further analogies between this 
arrangement and the eye, e.g., about the effect of blows or disorder 
causing irregular conduction and stimulation, of the galvanometer 
in the one instrument, of the brain cells in the other. 

A handy portable exciter of electric waves is one of the ordinary 
hand electric gas-lighters, containing a small revolving doubler 
i.e., an inductive or replenishing machine. A coherer can feel a 
gas-lighter across a lecture theatre. Minchin often used them for 
stimulating his impulsion cells. I find that when held near they 
act a little even when no ordinary spark occurs, plainly because of 
the little incipient sparks at the brushes or tinfoil contacts inside. 
A Voss machine acts similarly, giving a small deflection while 
working up before it sparks. 

Holt:: Sparks not exciting Tube : except by help of a polished knob. 
And notice here that our model eye has a well-defined range of 
vision. It cannot see waves too long for it. The powerful dis- 



28 



THE WORK OF HERTZ 



turbance caused by the violent flashes of a Wimshurst or Voss 
machine it is blind to. If the knobs of the machine are well 
polished it will respond to some high harmonics, due to vibrations 
in the terminal rods ; and these are the vibrations to which it 
responds when excited simply by an induction-coil. The coil 
should have knobs instead of points. Sparks from points or dirty 
knobs hardly excite the coherer at all. But hold a well-polished 
sphere or third knob between even the dirty knobs of a Voss 
machine, and the coherer responds at once to the surgings got up 
in it. 




15 



A~ 



* 










Fig. 19. Radiator used in the library of the Royal Institution, exciting 
the Coherer (Fig. 17) on the lecture table in the theatre. 



Feeble short sparks again are often more powerful exciters 
than are strong long ones. I suppose because they are more 
sudden. 

This is instructively shown with an electrophorus lid. Spark 
it to a knuckle, and it does very little. Spark it to a knob and it 
works well. But now spark it to an insulated sphere, there is 
some effect. Discharge the sphere, and take a second spark, with - 
out recharging the lid ; do this several times ; and at last, when 



AND SOME OF HIS SUCCESSORS. 29* 

the spark is inaudible, invisible, and otherwise imperceptible, the 
coherer some yards away responds more violently than ever, and 
the spot of light rushes from the scale. 

If a coherer be attached by a side wire to the gas pipes, and an 
electrophorus spark be given to either the gas pipes or the water 
pipes or even to the hot-water system in any other room of the 
building, the coherer responds. 

In fact, when thus connected to gas-pipes one day when I tried 
it, the spot of light could hardly keep five seconds still. Whether 
there was a distant thunderstorm, or whether it was only picking 
up telegraphic jerks, I do not know. The jerk of turning on or 
off an extra Swan lamp can affect it when sensitive. I hope to 
try for long-wave radiation from the sun, filtering out the ordi- 
nary well-known waves by a black-board or other sufficiently 
opaque substance. 

We can easily see the detector respond to a distant source of 
radiation now, viz., to a 5in. sphere placed in the library between 
secondary coil knobs ; separated from the receiver, therefore, by 
several -walls and some heavily gilded paper, as "well as by 20 or 
30 yards of space. 

Also I exhibit a small complete detector made by my assistant, 
Mr. Davies, which is quite portable and easily set up. The essen- 
tials (battery, galvanometer, and coherer) are all in a copper 
cylinder three inches by two. A bit of wire a few inches long, 
pegged into it, helps it to collect waves. It is just conceivable 
that at some distant date, say by dint of inserting gold wires or 
powder in the retina, we may be enabled to see waves which at 
present we are blind to. 

Observe how simple the production and detection of Hertz waves 
are now. An electrophorus or a frictional machine serves to excite 
them ; a voltaic cell, a rough galvanometer, and a bad contact serves 
to detect them. Indeed, they might have been observed at the 
beginning of the century, before galvanometers were known. A 
frog's leg or an iodide of starch paper would do almost as well. 

A bad contact was at one time regarded as a simple nuisance, 
because of the singularly uncertain and capricious character of the 
current transmitted by it. Hughes observed its sensitiveness to 
sound-waves, and it became the microphone. Now it turns out 



30 THE WORK OF HERTZ 

to be sensitive to electric waves, if it be made of any oxidisable 
metal (not of carbon),* and we have an instrument which might be 
called a micro- something, but which, as it appears to act by cohe- 
sion, I call at present a coherer. Perhaps some of the capricious- 
ness of an anathematised bad contact was sometimes due to the 
fact that it was responding to stray electric radiation. 

The breaking down of cohesion by mechanical tremor is an 
ancient process, observed on a large scale by engineers in railway 
axles and girders ; indeed, the cutting of small girders by persis- 
tent blows of hammer and chisel reminded me the other day of the 
tapping back of our cohering surfaces after they have been exposed 
to the welding effect of an electric jerk. 

Receiver in Metal Enclosure. 

If a coherer is shut up in a complete metallic enclosure, waves 
cannot get at it, but if wires are led from it to an outside ordinary 
galvanometer, it remains nearly as sensitive as it was before (nearly, 
not quite), for the circuit picks up the waves and they run along 
the insulated wires into the closed box. To screen it effectively, it 
is necessary to enclose battery and galvanometer and every bit of 
wire connection ; the only thing that may be left outside is the 
needle of the galvanometer. Accordingly, here we have a compact 
arrangement of battery and coil and coherer, all shut up in a copper 
box (Fig. 21). The coil is fixed against the side of the box at such 
height that it can act conveniently on an outside suspended compass 
needle. The slow action of the coil has no difficulty in getting 
through copper, as everyone knows ; only a perfect conductor 
could screen off that, but the Hertz waves are effectively kept out 
by sheet copper. 

It must be said, however, that the box must be exceedingly well 
closed for the screening to be perfect. The very narrowest chink 
permits their entrance, and at one time I thought I should have to 
solder a lid on before they could be kept entirely out. Clamping a 

* Fitzgerald tells me that he has succeeded with carbon also. My 
experience is that the less oxidisable the metal, the more sensitive and also 
the more troublesome is the detector. Mr. Robinson has now made me a 
hydrogen vacuum tube of brass filings, which beats the coherer for 
sensitiveness. July, 1894. 



AND SOME OF HIS SUCCESSORS. 



31 



copper lid on to a flange in six places was not enough. But by 
the use of pads of tinfoil, chinks can be avoided, and the inside of 
the box becomes then electrically dark. 

If even an inch of the circuit protrudes, it at once becomes 
slightly sensitive again ; and if a mere single wire protrudes 
through the box, provided it is insulated where it passes through, 
the waves will utilise it as a speaking tube, and run blithely in. 
And this whether the wire be connected to anything inside or not, 
though it acts more strongly when connected. 

In careful experiments, where the galvanometer is protected in 
one copper box and the coherer in another, the wires connecting 






3 U 




1% 






// 



Fig. 20. Spherical Radiator for emitting a Horizontal Beam, arranged 
inside a Copper Hat, fixed against the outside of a metal-lined Box. One 
eighth natural size. The wires pass into the box through glass tubes not 
shown. 



the two must be encased in a metal tube (Fig. 21), and this tube 
must be well connected with the metal of both enclosures, if nothing 
is to get in but what is wanted. 

Similarly, when definite radiation is desired, it is well to put the 
radiator in a copper hat, open in only one direction. And in order 
to guard against reflected and collateral surgings running along the 
wires which pass outside to the exciting coil and battery, as they 
are liable to do, I am accustomed to put all these things in a 



32 THE WORK OF HERTZ 

packing case lined with tinfoil, to the outside of which the sending 
hat is fixed, and to pull the key of the primary exciting circuit by 
a string from outside. 

Even then, with the lid of the hat well clamped on, something 
gets out, but it is not enough to cause serious disturbance of quali- 
tative results. The sender must evidently be thought of as emitting 
a momentary blaze of light which escapes through every chink. 
Or, indeed, since the waves are some inches long, the difficulty of 
keeping them out of an enclosure may be likened to the difficulty 
of excluding sound ; though the difficulty is not quite so great as 
that, since a reasonable thickness of metal is really opaque. I 
fancied once or twice I detected a trace of transparency in such 
metal sheets as ordinary tinplate, but unnoticed chinks elsewhere 
may have deceived me. It is a thing easy to make sure of as soon 
as I have more time. 

One thing in this connection is noticeable, and that is how little 
radiation gets either in or out of a small round hole. A narrow 
long chink in the receiver box lets in a lot ; a round hole the size 
of a shilling lets in hardly any, unless indeed a bit of insulated 
wire protrudes through it like a collecting ear trumpet. 

It may be asked how the waves get out of the metal tube of an 
electric gas-lighter. But they do not ; they get out through the 
handle, which being of ebonite is transparent. Wrap up the 
handle tightly in tinfoil, and a gas-lighter is powerless. 

Optical Experiments. 

And now in conclusion I will show some of the ordinary optical' 
experiments with Hertz waves, using as source either one of two 
devices ; either a 5in. sphere with sparks to ends of a diameter 
(Fig. 19), an arrangement which emits 7in. waves but of so dead-beat 
a character that it is wise to enclose it in a copper hat to prolong 
them and send them out in the desired direction ; or else a 2in. 
hollow cylinder with spark knobs at ends of an internal diameter 
(Fig. 12). This last emits 3in. waves of a very fairly persistent 
character, but with nothing like the intensity of one of the outside 
radiators. 

As receiver there is no need to use anything sensitive, so I 
employ a glass tube full of coarse iron filings, put at the back of 



AND SOME OF HIS SUCCESSORS. 



33 



a copper hat with its mouth turned well askew to the source, 
which is put outside the door at a distance of some yards, so that 
only a little direct radiation can reach the tube. Sometimes 
the tube is put lengthways in the hat instead of cros sways, which 
makes it less sensitive, and has also the advantage of doing 
away with the polarising, or rather analysing, power of a crossway 
tube. 

The radiation from the sphere is still too strong, but it can be 
stopped down by a diaphragm plate with holes in it of varying 
size clamped on the sending hat (Fig. 21). 




!(,' "";, 




Fig. 21. General arrangement of experiments with the Copper " Hat," 
showing Metal Box on a Stool, inside which the Radiators were fixed ; 
the Copper Hat containing the Coherer, with the Metal Box containing 
Battery and Galvanometer Coil connected to it by a compo pipe conveying 
the wires ; a Paraffin Prism ; and a Polarising Grid. 

Reflection. 
Having thus reduced the excursion of the spot of light to a foot 
or so, a metal plate is held as reflector, and at once the spot travels 
a couple of yards. A wet cloth reflects something, but a thin glass 
plate, if dry, reflects next to nothing, being, as is well known, too 
thin to give anything but "the black spot." I have fancied that it- 
reflects something of the 3in. waves. 



D 



34 THE WORK OF HERTZ 

With reference to the reflecting power of different substances 
it may be interesting to give the following numbers, showing the 
motion of the spot of light when 8in. waves were reflected into the 
copper hat, the angle of incidence being about 45deg., by the 
following mirrors : 

Sheet of window glass or at most 1 division. 

Human body 7 divisions. 

Drawing board 12 

Towel soaked with tap-water 12 

Tea-paper (lead ?) 40 

Dutch metal paper 70 ,, 

Tinfoil 80 

Sheet copper 100 and up against stops. 



Refracting Prism and Lens. 

A block of paraffin about a cubic foot in volume is cast into the 
shape of a prism with angles 75deg., 60deg., and 45deg. Using 
the large angle, the rays are refracted into the receiving hat 
(Fig. 21), and produce an effect much larger than when the prism 
is removed. 

An ordinary 9in. glass lens is next placed near the source, and by 
means of the light of a taper it is focussed between source and re- 
ceiver. The lens is seen to increase the effect by concentrating 
the electric radiation. 

Arago Disc ; Grating ; and Zone-plate. 
The lens helps us to set correctly an 18in. circular copper disc 
in position for showing the bright diffraction spot. Removing the 
disc the effect is much the same as when it was present ; in accor- 
dance with the theory of Poisson. Add the lens and the effect is 
greater. With a diffraction grating of copper strips 2in. broad 
and 2in. apart, I have not yet succeeded in getting good results. 
It is difficult to get sharp nodes and interference effects with 
these sensitive detectors in a room. I expect to do better when 
I can try out of doors away from so many reflecting surfaces ; 
indoors it is like trying delicate optical experiments in a small 
whitewashed chamber well supplied with looking-glasses ; nor have 
I ever succeeded in getting clear concentration with this zone- 



AND SOME OF HIS SUCCESSORS, 



35 



plate having Newton's rings fixed to it in tinfoil. But really there 
is nothing of much interest now in diffraction effects except the 
demonstration of the waves and the measure of their length. There 
was immense interest in Hertz's time, because then the wave 
character of the radiation had to be proved ; but every possible kind 
of wave must give interference and diffraction effects, and their 
theory is, so to say, worked out. More interest attaches to polari- 
sation, double refraction, and dispersion experiments. 




Fig. 22. Zoneplate of Tinfoil on Glass. Every circular strip is of area 

equal to central space. 

Polarising and Analysing Grids. 
Polarisation experiments are easy enough. Radiation from a 
sphere is already strongly polarised, and the tube acts as a partial 
analyser, responding much more vigorously when its length is 
parallel to the line of sparks than when they are crossed ; but a 
convenient extra polariser is a grid of wires something like what 
was used by Hertz, only on a much smaller scale ; say an 18in. 
octagonal frame of copper strip with a harp of parallel copper wires 
{see Fig. 21, on floor). The spark-line of the radiator being set at 
45deg., a vertical grid placed over receiver reduces the deflection 
to about one-half, and a crossed grid over the source reduces it 
to nearly nothing. 

d2 



36 THE WORK OF HERTZ 

Rotating either grid a little, rapidly increases the effect, which 
becomes a maximum when they are parallel. The interposition of 
a third grid, with its wires at 45deg. between two crossed grids, 
restores some of the obliterated effect. 

Radiation reflected from a grid is strongly polarised, of course, 
in a plane normal to that of the radiation which gets through 
it. They are thus analogous in their effect to Nicols, or to a pile 
of plates. 

The electric vibrations which get through these grids are at right 
angles to the wires. Vibrations parallel to the wires are reflected 
or absorbed. 

Eeflecting Paraffin Surface; Direction of Vibrations in Polarised 

Light. 

To demonstrate that the so-called plane of polarisation of the 
radiation transmitted by a grid is at right angles to the electric 
vibration,* i.e., that when light is reflected from the boundary of a 
transparent substance at the polarising angle the electric vibrations 
of the reflected beam are perpendicular to the plane of reflection, 
I use the same paraffin prism as before ; but this time I use its 
largest face as a reflector, and set it at something near the 
polarising angle. When the line of wires of the grid over the 
mouth of the emitter is parallel to the plane of incidence, in 
which case the electric vibrations are perpendicular to the plane 
of incidence, plenty of radiation is reflected by the paraffin face. 
Turning the grid so that the electric vibrations are in the plane 
of incidence, we find that the paraffin surface set at the proper 
angle is able to reflect hardly anything. In other words, the 
vibrations contemplated by Fresnel are the electric vibrations ; 
those dealt with by McCullagh are the magnetic ones. 

Thus are some of the surmises of genius verified and made obvious 
to the wayfaring man. 



* Cf. Trouton, in Nature, Vol. 39, p. 393 ; and many optical experi- 
ments by Mr. Trouton, Vol. 40. p. 398. 



AND SOME OF HIS SUCCESSORS. 37 



NOTE. 



It may be well to explain that in my Royal Institution lecture I 
made no reference to the transmission of waves along wires. I 
regard the transmission of waves in free space as the special dis- 
covery of Hertz. Their transmission along wires is a much older 
thing, Von Bezold saw them in 1870, and I myself got quantitative 
evidence of nodes and loops in wires when working with Mr. 
Chattock in the session 1887-8 (see, for instance, reports of the 
Bath meeting of the British Association, 1888), and I exhibited 
them some time afterwards to the Physical Society. 



r 



Fig. 23. 

It may be worth mentioning that the arrangement frequently 
referred to in Germany by the name of Lecher (viz., that shown in 
the figure), and on which a great number of experiments have been 
made, is nothing but a pair of Leyden jars with long wires leading 
from their outer coats. The use of air dielectric instead of glass 
permits the capacity to be adjusted, and also readily enables the 
capacity to be small, and the frequency, therefore, high ; but 
otherwise the arrangement is the same in principle as had fre- 
quently been used by myself in the series of experiments called 
"the recoil kick." For these and other reasons no reference has 
been made in my lecture to the excellent work done on wires by 
Sarasin and De la Rive ; nor to work done by Lecher, Rubens, 
Arons, Paalzow, Ritter, Blondlot, Curie, D. E. Jones, Yule, 
Barton, and other experimenters. 



AND SOME OF HIS SUCCESSORS. d9 



Bppenbices, 



APPENDIX I. 



ON THE DISELECTPJFICATION OF METALS AND 
OTHER BODIES BY LIGHT. 

Referring to a footnote to my Royal Institution lecture on page 
12, Messrs. Elster and Geitel have been good enough to call my 
attention to a great deal of work done by them in the same direction. 
To make amends for my ignorance of this work at the time of my 
Royal Institution lecture, and to make it better known in this- 
country, I make abstract of their Papers as follows : 

Wiedemann's Annalen, 38, p. 40. "On the Dissipation of Negative 
Electricity by Sun and Daylight." 

With a view to Arrhenius' theory concerning atmospheric elec- 
tricity, we arranged experiments on the photo-electric power of 
sunlight and diffuse daylight at "Wolfenbiittel from the middle of 
May to the middle of June, 1889. Hocr alone had observed the 
effect of sunlight ; other experimenters had failed to find it, but 
we find a discharging effect even in diffuse daylight. 

We take an insulated zinc dish, 20 cm. diameter, connect it 
to a quadrant electrometer or an Exner's electroscope, and expose 
it in the open so that it can be darkened or illuminated at pleasure. 
Sunlight makes it lose a negative charge of 300 volts in about 60 
seconds. A positive charge of 300 volts is retained. The dissipa- 
tion of negative electricity ceases in the dark, and is much weakened 
by the interposition of glass. But light from the blue sky has a 
distinct effect. Fill the dish with water, or stretch a damp cloth 
over it, and the action stops. A freshly-scrubbed plate acquires a 
positive charge of 2h volts, which can be increased by blowing. 



40 THE WORK OF HERTZ 

With freshly-cleansed wires of zinc, aluminium, or magnesium 
attached to the knob of the electroscope, a permanent negative 
charge is impossible in open sunlight. Indeed, magnesium shows 
a dissipating action in diffuse evening light. Such wires act like 
glowing bodies. Exposing an electroscope so provided in an open 
space it acquires a positive charge from the atmosphere. No 
abnormal dissipation of positive electricity has been observed. 

Wied. Ann., 38, p. Ifi7. Continuation of Same Subject. 

Our success last time was largely due to the great clearness of 
the sky in June, and we wished to see if we could get the same 
effect at the beginning of the winter. 

The following is our summary of results : 

Bright fresh surfaces of the metals zinc, aluminium, magnesium 
were discharged by both sun and daylight when they were 
negatively charged ; and they spontaneously acquired a positive 
charge, whose amount could be increased by blowing.* A still 
more notable sensitiveness to light is shown by the amalgams of 
certain metals, viz., in the order of their sensitiveness, K, Na, Zn, 
Sn. Since pure mercury shows no effect, the hypothesis is per. 
misssible that the active agent is the metal dissolved in the 
mercury. If so, the following are the most active metals : 

K, Na, (Mg, Al), Zn, Sn. 

All other metals tried, such as Sn, Cd, Pb, Cu, Fe, Hg, Pt, and 
gas carbon, show no action. The same is true of nearly all non- 
metallic bodies ; but one of them namely, the powder of 
Balmain's luminous jxdnt acted remarkably well in sunlight. Of 
liquids, hot and cold water, and hot and cold salt solution were 
completely inactive ; consequently, wetting the surfaces of metals 
destroys their sensibility to light. 

The illumination experiments can be arranged in either of two 
ways. For experiments in free space we use zinc, aluminium, or 
magnesium wires, or small amalgamated spheres of zinc provided 
with an iron rod. With these it can be easily shown that the 
illuminated surface of certain metals acts in the same way as a 
flame-collector. 

* A fact noticed bv Bichat and Blondlot. 



AND SOME OF HIS SUCCESSORS. 



41 



For demonstration experiments the apparatus described* is 
better, and with this we show the following : 

Amalgamated zinc, negatively charged, discharges almost instantly 
in sunlight ; and if near a positively-electrified body charges itself 
positively. 

The same thing happens, though more slowly, in diffuse daylight. 
Red glass stops the action, but the following let some through : 
Selenite, mica, window glass, blue (cobalt) glass. 

X Source of Light 



c= 




Screen. 




Influence Machine. 
Fig. A. 
Explanation of Fig. A. B' is a brightly polished amalgamated zinc 
plate attached to the negative pole of a Holtz machine, with the positive 
knob from 6 to 10 centimetres distant. The source of light is a strip of 
burning magnesium ribbon 30 to 50 centimetres away. Whenever the 
spark is able just to choose the path BB', light shining on the zinc plate 
checks it and transfers the spark to A A'. 

Wied. Ann., 39, >. 382. On a Checking Action of Illumination on 
Electric Spark and Brush Discharge. 
If sparks are just able to occur between a brass knob and a clean 
amalgamated zinc cathode, illumination of the latter by ultra- 

* In this apparatus the mercury amalgams of K and Xa are run through 
a fine funnel, so that the freshly-formed surface of the drops may be illumi- 
nated. Under these circumstances, while pure murcury fell from 185 to 
175 volts in 30sec., amalgam of zinc fell from 195 to 116 in 15sec, 
amalgam of sodium fell from 195 to in lOsec, and an amalgam of 
potassium fell from 195 to in 5 seconds. 



42 THE WORK OF HERTZ 

violet light tends to check them. [This is a curious inversion of 
Hertz's fundamental experiment on the subject. It is an effect 
I have not yet observed ; but Elster and Geitel's arrangement 
differs from mine* in that the surfaces are at a steady high poten- 
tial before the spark, so that light can exert its discharging 
influence, whereas in mine the surfaces were at zero potential 
until the spark-rush occurred. Hertz's arrangement was more like 
mine, inasmuch as he illuminated the knobs of an induction coil 
on the verge of sparking. It appears, then, that whereas the 
action of light in discharging negative electricity from clean oxidis- 
able metallic surfaces is definite enough, its influence on a spark 
discharge differs according to the conditions of that discharge in 
cases of "steady strain" it tends to hinder the spark ; in cases of 
"sudden rush " it tends to assist it. O. J. L.] 

Wied. Ann., J+l, p. 161. On the Use of Sodium- Amalgam in 
Photo-electric Experiments. 

Elster and Geitel have repeated some of Righi's experiments on 
the discharge of negative electricity from metals in rarefied air, and 
find, in agreement with him, that a reduction of pressure to about 
one millimetre increases the discharge velocity about six or seven 
times. They proceed to try sodium-amalgam exposed to daylight 
in exhausted tubes, and describe apparatus for the purpose. Such 
an arrangement simply cannot hold a negative charge in bright day- 
light, even although it be unprovided with quartz windows. Even 
paraffin lamps and sodium flames exert some action. 

They observe that under the action of light the boundary surface 
of the metal and glass changes, and the metal begins to cling to the 
glass. They suppose that Warburg's vacuum tubes of pure sodium 
may behave similarly, and show photo-electric sensibility. 

The Same, p. 166. On a Checking Action of Magnetism on Photo- 
electric Discharge in Barefied Gases. 
The authors point out analogies between the above effects and 
those they had observed in the action of glowing bodies in air, 
and they mention Lenard and Wolf's experiment's (Wied. Ann. 

* Sec Fig. 7, page 10. 



AND SOME OF II IS SUCCESSORS. 



43 



XXXVII., p. 443), tending to show that the effect is due to a 
disintegrating or evaporative effect of light on surfaces. Elster 
and Geitel had observed that the discharging power of glowing 
bodies was diminished by application of a magnetic field, the effect 
being the same as if the temperature was lowered ; and they 
proceed to try if the discharge of negative electricity from 
illuminated surfaces in highly-rarefied gas could also be checked 
or hindered by a magnetic field. They find that it can. 




To Electroscope 



Fig. B. 
Explanation of Fig. B. The sodium and mercury are introduced 
through the tube S into the globe K. The tube S is then closed, a pump 
applied to X, and exhaustion carried on for some days. T is an open 
funnel sealed into the tube (as is done in some vacuum tubes made by 
Holtz to show a curious unilateral conductivity of rarefied gas. The object 
of this funnel is to permit metal from the interior, free from scum, to be 
introduced from K to D when the whole is tilted. Thus a bright surface 
is exposed to the earth ring R. It can be charged negatively, and its leak 
under illumination be measured, through the terminal D. Sometimes the 
tube is inverted, so that the active surface may be at D', further from the 
earth wire. 

Using the light from sparks admitted through a quartz window 
into the vacuum tube when a negatively-charged amalgamated zinc 
surface was exposed near an earth-connected platinum ring, and 
between the poles of a small electro-magnet, they found that when 
the tube was full of air at 10mm. pressure the magnet had but 



44 



THE WORK OF HERTZ 



little effect, but that at 0*15mm., whereas without the magnet the 
charge of 270 volts disappeared completely in five seconds, when 
the magnet was excited it only fell about half that amount in the 
same time. With hydrogen at 0'24mm. the result was much the 
same, and at either greater or less pressure in both cases the 
magnet had less effect. In oxygen the loss of charge was not quite 
so rapid ; and, again, at a pressure of O'lmm, the magnet more 
than halved the rate. But in C0 2 the rapidity of loss was 



; ,,Z<\Z,Y,Yl 



4 



.. v./:-:.- - 



/^ 




IZD 



Fig. C. 
Explanation of Fig. C. P is the plate of amalgamated zinc, and R is the 
earth ring, as before. Ultra-violet light is introduced through a quartz 
window Q from a spark gap r. The vessel has a joint at the middle, so 
that the sensitive plate can be got at and changed. Magnet poles are 
applied outside tills vessel in various positions. 

extreme.* Either at l'linm. or at '005mm. the charge of -270 
volts leaked away completely in two seconds when the magnet was 
not excited ; but in the latter case (low pressure) exciting the 
magnet reduced the speed by about one-half. At the pressure of 
rirara. the magnet did not seem to produce an effect. With 
daylight the results are similar. 

* Corresponding to the activity of this gas as found by Wiedmann and 
Ebert {Wicd. Ann., XXXIIL, p. 258), in their researches on the influence 
of light on ease of sparking. 



AND SOME OF II IS SUCCESSORS. 45 

The authors then discuss the meaning of this result, and its. 
bearing on the opposition hypotheses of Lenard and Wolf and 
of Righi. Lenard and Wolf's view is that the loss of negative 
electricity is due to dust disintegrated from the surface by the 
action of light, but whose existence they consider is established by 
an observed effect on steam jets. Righi, on the other hand, 
believes that gas molecules themselves act the part of electric 
carriers. Elster and Geitel consider that the magnetic effect 
observed by them supports this latter view, it being known that a 
magnet acts on currents through gases ; and they surmise that 
the impact of light vibrations may directly assist electric inter- 
change between a gas molecule and the surface, by setting up in 
them syntonic stationary vibrations, something like resonant 
Leyden jars. It is to be remembered that phosphorescent 
substances, such as Balmain's paint powder, exhibit marked photo- 
electric effect in daylight. 



. x 




>E 

Fig. D. 

Explanation of Fig. D. A simpler arrangement, like the one above 
(Fig. C), whereby clean liquid alkali metals can be introduced into the 
experimental chamber B, from the preliminary chamber A, through a 
cleansing funnel F which dips its beak into the interior. 

The unilateral character of the electric motion, and the charging 
of neutral surfaces by light, require special hypotheses, concerning 
an E.M.F. at the boundary of gases and conductors, such as- 
Schuster and Lehmann have made. 

Wied. Ann. !$> P- 564- Note on a Neiv Form of Apparatus for 
Demonstrating the Photo-electric Discharging Action of Daylight. 
A vacuum tube suitable for experiments with sodium amalgam 
or pure sodium, or the liquid sodium-potassium alloy, is described, 
with the aid of which a current (shown by the charge of an electro- 
scope) can be maintained by a dry pile through the rarefied gas- 
above the metal when it is illuminated from ordinary windows. 



46 THE WORK OF HERTZ 

Wied. Ann. 43, p. 225. On the Dependence of the Discharging Actiou> 
of Light on the Nature of the Illuminated Surface. 

Experiments also on differently-coloured lights. Summary of 
results. The photo-electrically active metals arrange themselves in 
the following order Pure K, alloys of K and Na, pure Na. 
Amalgams of Rb, K, Na, Li, Mg, (Tl, Zn) ; the same as their 
voltaic order. With the most sensitive term of the series a candle 
six metres off can be detected, and the region of spectral red is not 
inactive. The later terms of the series demand smaller waves, and 
even for potassium blue light gives a much greater effect than red. 
No discharge of positive electricity is observable with these 
substances. 

Wied. Ann. 44> P- ~2. On the Dissipation of Electric Charge from 
Mineral Surfaces by Sunlight. 

Hitherto only Balmain'a paint powder has been observed to be 
active among non-metallic substances. Now they try other phos- 
phorescent bodies, and arrive at the following results : 

Fluor-spar is conspicuously photo-electric, both in sunlight and 
daylight, especially the variety of fluorite called stmkfluss. 

Freshly-broken surfaces discharge much more rapidly than old 
surfaces. 

Blue waves, and not alone the ultra-violet, have a perceptible 
effect on fluor-spar. 

In a vacuum the mineral loses its photo-electric sensibility and 
its conductivity too. Contact with damp air restores its sensibility. 
Moistening with water weakens, but does not destroy, the sensitive- 
ness. On the other hand, igniting the mineral destroys both its 
photo-electric power and its exceptional phosphorescent property. 

Distinct traces of photo-electric power are shown by the follow- 
ing minerals also : Cryolite, heavy spar, celestine, arragonite, 
strontianite, calcspar, felspar, and granite. 

The hypothesis that the power of phosphorescing when illu- 
minated is approximately a measure of the discharging power of 
light has been verified in many cases ; the exceptions can probably 
be explained by the influence which the electrical conductivity of 
the illuminated substance exerts on the rate of discharge of 



AND SOME OF HIS SUCCESSORS. 



47 



electricity from its surface. This agreement confirms the view 
expressed by us on the occasion of experiments with Balmain's 
paint, that, during electrical discharge by light, actions take place 
which are analogous to those of resonance. Messrs. Wiedemann 
and Ebert had previously been led by other considerations to the 
same conclusion. 

AVe are compelled by the results of the present experiments to 
conclude that a more rapid discharge of electricity into the atmos- 
phere takes place in sunlight than in darkness from the surfaces of 




^r^^H^r 



140 Volts 




Fig. E. 

Explanation of Fig. E. Arrangement used by Elster and Geitel for ex- 
posing various phosphorescent minerals to daylight, while under inductive 
charge. They were put in powder in the tray P, and the transparent wire- 
gauze N above them was charged positively from a battery. The metal cover 
MM' could be removed and replaced at pleasure, and the effect on a delicate 
quadrant electrometer connected to P observed. By this method con- 
siderable tension can be got up on the mineral surface, notwithstanding 
that it is close upon zero potential. The light effect depends on tension, 
not potential. 

the earth, which is composed of mineral particles charged, as the 
positive sign of the slope of atmospheric potential indicates, with 
negative electricity. 

It seems to us evident that there exists a direct electric action of 
sunlight upon the earth, and that we have given experimental 
evidence in favour of the theory put forward by von Bezold and 
Arrhenius, according to which the sun acts on the earth, not by 
electrostatic or electro-dynamic action-at-a-distance, which would 
involve difficulties of a theoretical character, but through the 



48 THE WORK OF HERTZ 

medium of the electrical forces of light waves. We hope soon to 
establish the consequences of this theory in meteorology in another 
Paper, giving the results of two years' observations on the intensity 
of the most refrangible rays of sunlight and of the slope of atmos- 
pheric potential. 

Wied. Ann., J^8, p. 338. Experiments on the Gradient of Atmos- 
pheric Potential and on Ultra-Violei Solar Radiation. 

Elster and Geitel describe the observations they have made for 
two years on solar radiation, at observing stations of low and high 
altitude, as tested by its electrical discharging power ; and they 
plot curves of such effective radiation for days and months along 
with the curves of atmospheric potential observed at the same 
places. These curves are of much interest, and need study. 
Incidentally they find that, of the whole effective solar radiation, 
60 per cent, was absorbed at altitudes above 3,100 metres ; 23 per 
cent, of the remainder was absorbed in the layer between this and 
a station at 1,600 metres ; and 47 per cent, was absorbed between 
this and 80 metres above sea level. Or, in other words, of 236 
parts which enter the atmosphere 94 reach the highest observing 
station (Sonnblickgipfel), 72 the middle one (Kolm-Saigurn), and 
38 the lowest (Wolfenbuttel). They discuss the question as to how 
far the daily variation of terrestrial magnetism is due to electrical 
currents in the atmosphere excited by sunshine and other metero- 
logical matters. 

[The Paper and plates are worthy of reproduction in full in the 
Philosophical Magazine.] 

Wied. Ann., J/.6, p. 281. On the Behaviour of Alkali Metal 
Cathodes in Geissler Tubes ; On Photo-electric Discharge in a 
Magnetic Field ; and On the Measure oj Photo-electric Currents 
in Potassium Cells by means of a Galvanometer. 

Results : The resistance of a Geissler tube provided with a 
cathode surface of pure alkali metal is diminished by the light from 
the sparks of an induction coil ; especially when the pressure is "1 
to "01 mm. of mercury. The resistance which rarefied gas opposes 
to an electric current in a magnetic field is greatest in the direction 



AND SOME OF HIS SUCCESSORS. 



49 




Fig. F. 

Explanation of Fly. F. A vacuum tube of rarefied hydrogen containing 

alkali metal as cathode, say the liquid K - Na alloy, or solid K or Na. A 

spark gap at S serves as alternative path, and a stream of sparks can occur 

to the plate P in the dark. But when light falls on the surface A, this 

stream of sparks can cease, showing that the resistance of the vacuum tube 

is diminished. 

N 





Fig. G. 
Explanation of Fig. G. Showing position of magnetic poles with respect 
to the vacuum tube discharge. With the poles across the line of discharge, 
as in Fig. on left, excitation of the magnet opposes the leak from the sur- 
face. With the poles as in Fig. on right, the discharge is not much affected 
it is even sometimes slightly increased. 





K, K 2 

Fig. H. 
Explanation of Fig. H. Potassium vacuum bulbs containing ^ milli- 
metre of hydrogen mounted and connected to battery and galvanometer, 
and arranged as a photo-electric photometer. 

E 



50 THE WORK OF HERTZ 

normal to the magnetic lines. The changes of resistance effected 
by any kind of light in a vacuum tube with alkali metal cathode 
can be measured galvanometrically. (A Daniell cell gives 100 
divisions on a Rosenthal galvanometer when coupled up through 
such an illuminated tube, each division meaning about 10 10 ampere). 

Wied. Ann., Jf8, p. 625. On the Photo-electric Comparison of 

Sources of Light. 

Attempts to make such a potassium cell into a photometer. 

Wied. Ann., 52, p. l^SS. Further Photo-electric Experiments. 

Plates of platinum, silver, copper need exceedingly ultra-violet 
light before they show any photo-electric power ; zinc, aluminium, 
magnesium show it for visible violet and blue light ; the alkali 
metals, in an atmosphere of rarefied hydrogen, advance their range 
of sensibility into the spectral red ; while under the most favourable 
conditions they show a sensibility only inferior to that of the eye 
itself. The Authors now use galvanometric methods of measuring 
the effect, instead of only electrometers, and they arrive at the 
following results : 

(1) The three alkali metals Na, K, Kb, have different sensibility 
for differently-coloured lights. For long waves their order of 
sensibility is Rb, Na, K ; though Rhubidium is far exceeded by the 
other two metals in white light. 

(2) Illumination of a plane alkali-metal cathode surface with 
polarised light causes greatest discharge if the plane of polari- 
sation is normal to plane of incidence ; and least, if the two 
coincide. 

[This is a most remarkable observation. Its probable meaning 
s that the electric oscillations of light are photo-electrically effec- 
tive in so far as they are normal to the surface on which they act ; 
while electric oscillations tangential to the surface are scarcely 
operative. Different angles of incidence must be tried before the 
proof is complete. O. J. L.] 

(3) Electric oscillations of very short period, such as are given 
by a Hertz oscillator, are commutated by illumination in the 



AND SOME OF HIS SUCCESSORS. 51 

presence of alkali metals in rarefied gas, so as to be able to set up 
a constant electric tension in the gas. 

[A Zehnder tube* was used, and the momentary phases of the 
oscillation during which the metal is negatively charged are ap- 
parently taken advantage of by the illumination.] 

(4) The photo-electric dissipation shown by powdered fluor-spar 
is dependent on the colour of the mineral, in such a way that the 
deepest blue violet or green specimens are the most sensitive. 



* See Fig. 13, p. 16. 

k2 



THE WORK OF HERTZ 



APPENDIX II. 



PHOTO-ELECTRIC RESEARCHES OF 
M. AUGUSTE RIGHI.* 

M. Righi has observed the following facts : (1) That ultra-violet 
rays reduce to sensibly the same potential two metals placed near 
each other (plate and gauze parallel and close) ; (2) That several 
photo-electric couples of this kind can form a battery ; (3) That a 
simple metallic plate charges itself positively under the influence of 
radiation ; (4) That a voltaic arc formed with a zinc rod gives the 
strongest effect, while the sun gives none. 

Besides these facts he finds : (a) That certain gases and vapours, 
such as coal-gas and CS 2 , absorb the active rays strongly. 

(b) That if the discharging body is easily movable it recedes like 
an electric windmill. 

(c) A film of gypsum interposed between gauze and plate charges 
itself negatively on the side facing the negatively-charged plate. 

(d) Radiation produces its discharging effect even on non- 
conductors (ebonite and sulphur). With glass, resin and varnishes 
the action is feeble, or nearly nothing. 

(e) If the experiment is made with a copper gauze and a zinc 
plate, the phenomenon nearly disappears on varnishing the gauze. 
His hypothesis is that radiation produces convection of negative 
electricity, the carriers being molecules of air. 

(/) The carrying molecules move along the lines of force, and 
throw electric shadows. To show this he varnishes a zinc cylinder, 
all except a generating line, charges it negatively to 1,000 volts 
with a dry pile, and places it parallel to a large earth-connected 
plane, which has a narrow rectangular portion insulated from the 
rest and communicating with an electrometer. Light only acts on 
the uncovered line of the cylinder, and on turning the cylinder 
round the electrometer is only deflected when it is exposed to 

* Comptes Rendus, vol. 107, p. 559. 



AND SOME OF HIS SUCCESSORS. 53 

some of the (circular) lines of force emanating from the active 
line of the cylinder. 

{()) Radiation charges positively an insulated metal, even when 
it is an enclosure with walls of the same metal ; the metal being 
certainly uncharged at the beginning of the experiment. The same 
occurs with sulphur and ebonite. If there is a feeble initial plus 
charge, radiation increases it. 

(A) While the discharging power of radiation for negative elec- 
tricity is strongest with zinc and aluminium, and slower with copper 
and gold, following the Volta series ; the E.M.F. set up by radia- 
tion, when it charges things positively, is greatest with gold and 
carbon, and less with zinc and aluminium ; again following the 
Volta series, but inversely. 

0) If radiation falls on an insulated metal plate connected with 
an electrometer, in an enclosure of the same metal, the positive 
electrification shown by the deflection of the electrometer is greater 
as the plate is further from the walls of the enclosure. The action 
stops when the metal has attained a certain electric density, con- 
stant for a given metal ; so the potential of a plate is naturally 
higher as its capacity is less. It is thus established that radia- 
tion acts on the particles of gas in contact with a conductor ; 
they go away with a negative charge, leaving plus on the con- 
ductor, until an electric density sufficient to balance this action is 
attained. 

(j) It is probable that if the solar rays do not produce an effect 
it is because of the absorbing action of the atmosphere. In fact, if 
one places a tube whose ends are glazed with selenite between the 
source of light and the metals being experimented on, the effects 
become sensibly stronger when the tube is exhausted. 



54 THE WORK OF HERTZ 



APPENDIX III. 



ELLIPTICALLY POLARISED ELECTRIC RADIATION. 

Since the delivery of my lecture to the Royal Institution, on 
June 1st, Herr Zehnder has published* a mode of getting elliptically 
and circularly polarised electric radiation. He takes a couple of 
plane-polarising grids, such as are depicted in Fig. 21, page 33, 
and places them parallel to each other at a little distance apart 
with their wires crossed. 

If the two grids are close together they will act like wire-gauze, 
reflecting any kind of polarised radiation equally ; but if the warp 
and woof are an eighth-wave length apart, and the plane of the inci- 
dent radiation is at 45 to the wires, the reflected radiation will be 
circularly polarised. A change in the circumstances will, of course, 
make it elliptical. Such a pair of grids acts, in fact, like a 
Babinet's Compensator. 



* Berichte der Naturforschenden GescUschaft zu Freiburg i. B., Bd. IX. 
Heft. 2, June 21, 1894. 



AXD SOME OF II IS SUCCESSORS. 55 



APPENDIX IV. 



ON MAGNETISATION PRODUCED BY HERTZIAN 
CURRENTS; A MAGNETIC DIELECTRIC:* 

BY M. BIRKELAND. 

"Two years agof it was proved by conclusive experiments that 
Hertzian waves travelling along an iron wire magnetise transversely 
the very thin layer into which the alternating current penetrates, 
and whose thickness does not exceed some thousandths of a milli- 
metre. Once proved that alternate magnetisation can be produced 
with such rapidity, other questions present themselves. One asks, 
for instance, if it is not possible to demonstrate in magnetic cylinders 
stationary magnetic waves analogous to the electric stationary waves 
along metallic wires." 

The author finds that the conductivity of massive iron makes 
it an unsuitable substance, and uses instead a mixture of iron filings, 
or of chemically-obtained iron powder, with paraffin, to which he 
sometimes adds powdered quartz. This he moulds into cylinders, 
and inserts as the core of a spiral in an otherwise ordinary Hertz 
resonator. 

The figure shows emitter and receiver drawn to scale ; the mag- 
netic cores are introduced into the spiral A, and their effect on the 
length of the resonator spark is observed. With this arrangement 
of exciter the electric effect of the spiral is negligeable, since it is well 
removed from electrostatic disturbance, and subject only to mag- 
netic. The spiral is of 12 well-insulated turns, the spark-gap is a 
micrometer with point and knob, and a pair of adjustable plates 
to vary the capacity for purposes of tuning. 

* Abstracted from Comptes Ecndus, June 11, 1894, and communicated 
by Dr. Oliver Lodge. 

t Why two years ago ? It was practisally proved by Savart early in the 
century, and has been observed over and over again since. However, it is 
true that experiments have been more numerous and conclusive of late, 
and have been pushed to very high f requencies. 0. J. L. 



56 



THE WORK OF HERTZ 



He employed 12 different types of cylinder, all about 20 centi- 
metres long, and 4 centimetres diameter. 

1. A massive cylinder of soft iron. 

2. A bundle of fine iron wires embedded in paraffin. 

39. Six cylinders of the agglomerate of chemically-reduced iron 
in powder and paraffin, containing respectively 5, 10, 15, 20, 25, 
and 50 per cent, of iron. 

Then for control experiments : 

10. A cylinder of agglomerate of zinc powder in paraffin, with 40 
per cent, of zinc. 

11. A cylinder of brass filings in paraffin, 20 per cent, of metal. 




>DULaMJiM2flMffl2/ , 



zsssmsmssasmsL'- 



1m 



Fig. I. 



12. A tube of glass, 4 '5 centimetres diameter, filled with 
various electrolytes. 

The manner of observing was as follows (the experiments were 
done in the laboratory of Hertz) : 

The resonator, with its spiral empty, was syntonised with the 
exciter, and the maximum spark measured. It was between 4 
and 9 millimetres long in these experiments. Then one or other of 
the above cylinders was introduced and the spark-length measured 
afresh. 

Cylinder 1 did not affect the maximum spark-length. Cylinders 
2 4 reduced the maximum spark to ^th of its former value ; 7 and 
8 to iJnth, and No. 9 to 770gth of its former value (viz., from 9 



AND SOME OF HIS SUCCESSORS. 57 

millimetres to '05 millimetre). Nos. 10 and 11 had but a feeble 
action, they reduced the spark from 8 to 7 millimetres. 

Tube No. 12, filled with distilled water, scarcely affected the 
spark length ; the period of the secondary increasess a little, but the 
maximumspark is the same as before, once syntony is re-established. 
Filled, however, with dilute sulphuric acid containing 10, 20, or 
30 per cent., the tube reduced the spark considerably, in each 
case about the same, viz., from 9 to 1'3 about. (Currents induced 
by Maxwellian radiation in electrolytes had been already observed 
by J. J. Thomson). 

While trying to re-establish syntony between primary and 
secondary, I found that the period of the resonator was consider- 
ably increased by the cylinders 2 4, but that the maximum 
spark length was much diminished. With the cylinders Nos. 5 9 
in the spiral, it was no longer possible to establish syntony, "a fact 
which is certainly due to their considerable absorption of energy. 
Take, for example, cylinder 9 : electro-magnetic energy must con- 
verge rapidly towards it in order to be transformed, and the space 
rinds itself empty of energy as air is exhausted of vapour in presence 
of an absorbing substance. 

This absorption is probably due to hysteresis in the ferruginous 
cylinders ; the development of Joulian heat, so typically shown by 
cylinder 12, being undoubtedly of the same order in cylinders 3 9 
as in Nos. 10, 11. 

It is probably by reason of this absorption that 1 have not 
succeeded in establishing stationary magnetic waves in a circuit of 
ferro-paraftin." 

If one of the cylinders, 2 9, is wrapped in tinned paper before 
introducing it into the spiral A, its action is completely stopped. 
(These conducting cores diminish the period of the resonator ; it is- 
much as if the spiral A were partially shunted out ; but the maxi- 
mum spark returns as soon as syntony is re-established.) To 
examine this further he enclosed the cylinder in drums of cardboard 
having fine wires either along generating lines, or along circular 
parallels. The latter suspended the action of an interior ferruginous 
cylinder, the former did not. 

To find to what depths the magnetism penetrated, Birkeland 
inserted hollow ferruginous drums into A, measured their effect, 



58 THE WORK OF HERTZ, 

and then plunged solid cylinders into them to see whether the effect 
increased. , 

He thus found that the magnetisation easily traversed 7 milli- 
metres thickness of the 10 per cent, ferro-parraffin, and 5 milli- 
metres of the 25 per cent. 

The substance is comparable to a dielectric on the theory of 
Poisson-Mossotti. 

" The results obtained with our magnetic dielectric invite to new 
researches " such as the mechanical force excited by electric waves 
on a delicately-suspended ferro-paraffin needle, and the rate of 
propagation of Maxwellian waves through such a substance. 



PLEASE DO NOT REMOVE 
CARDS OR SUPS FROM THIS POCKET 

UNIVERSITY OF TORONTO LIBRARY 



QC Lodge, (Sir), Oliver 

16 Joseph 

H4L6 The work of Hertz and 

1894 some of his successors 



P&A Sci, 












9 19 V 

1 mm 

a HI 


H 

HHHT 

B 



SB H BfH *i 




' B "' 



3 H 

IMF