SIGNALLING ACROSS SPACE
WITHOUT WIRES.
BEING A DESCRIPTION OF
THE WORK OF HERTZ
& HIS SUCCESSORS.
BY
PROF. OLIVER J. LODGE, F.R.S.
THIRD EDITION,
With Additional Remarks concerning the Application
to Telegraphy, and Later Developments.
LONDON:
“THE ELECTRICIAN” PRINTING AND PUBLISHING COMPANY,
LIMITED,
Salisbury Court, Fleet Street.
Copyright.
Works by Dr. O. J. LODGE.
Lightning Conductors and Lightning Guards. A Complete Treatise on the subject of Electric Discharges in general.
Pioneers of Science. A popular Illustrated History of the Early Astronomers and their Work up to Recent Times.
Modern Views of Electricity.
Elementary Mechanics.
Protection of Buildings from Lightning. Mann Lectures to the Society of Arts, 1888.
Secondary Batteries and the Electrical Storage of Energy. Cantor Lectures to the Society of Arts, 1883.
TABLE OF CONTENTS.
(The lines in italics which look like headings of paragraphs are really statements about experiments which were at that place shown.)
| 1. | Royal Institution Lecture on the Work | |
| of Hertz and some of his Successors:— | PAGES | |
| Biographical Introduction | [ 1] | |
| Elementary Explanation about Electrical Radiation | ||
| and Absorption and the Effect of Syntony | [3-8] | |
| Syntonic Leyden Jar Experiment | [ 6], [21] | |
| Side Observations on the Effect of Light | ||
| on Electric Discharge | [9-12] | |
| Various Detectors of Radiation | [13-23] | |
| Physiological Non-Effect of Sufficiently Rapid Alternations | [17] | |
| Boltzmann Gap Detector—the Precursor of the Coherer | [18] | |
| Branly’s Observations ([see also Appendix, page 95]) | [20] | |
| Early Form of Coherer and of Branly Filings Tube | [21-23] | |
| Early Signalling over 40 or 60 yards | [24-25] | |
| Use of Telephone as Receiver | [26] | |
| Experiments showing Syntony | [27-28] | |
| Hypothesis concerning the Nature of Vision | [29-31] | |
| Summary of Various Detectors of Radiation | [30] | |
| Radiation Detected from extremely Weak Sparks, and by | ||
| very compact Receiver with Collecting Wire | [32-34] | |
| Vacuum Filings Tube | [34] | |
| Effect of Metallic Enclosure | [35-38] | |
| Experiments on Reflection, Refraction, and Polarisation | ||
| of Radiation | [39-42] | |
| Note about Electric Waves on Wires | [43] | |
| 2. | Application of Hertz Waves and Coherer | |
| Signalling to Telegraphy | [45-48] | |
| Coherer Behaviour and Tommasina’s Experiment | [49] | |
| Single-Point Coherer | [55] | |
| General Remarks about Patent Law | [50] | |
| Attempts at Syntonic Telegraphy | [52-60] | |
| Syntonic Radiators and Receivers | [51-56] | |
| 3. | Details of other Telegraphic Developments:— | |
| Popoff’s 1895 Demonstration | [60-62] | |
| Slaby’s Overland Experiments | [63-66] | |
| Marconi’s Recent Demonstration | [66-72] | |
| 4. | A History of the Coherer Principle | [73-87] |
| Professor Hughes’ Early Observations | ||
| before Hertz or Branly | [88-94] | |
| Work of Monsieur Branly | [95-108] | |
| 5. | Communications with respect to Coherer | |
| Phenomena on a Large Scale:— | ||
| Professor Elihu Thomson | [109] | |
| Dr. Morton | [111] | |
| 6. | Photo-Electric Researches of Drs. Elster | |
| and Geitel | [115-126] | |
| 7. | Photo-Electric Researches of Professor Righi | [127] |
SIGNALLING THROUGH SPACE
WITHOUT WIRES.
THE WORK OF HERTZ
AND
SOME OF HIS SUCCESSORS.
The following pages (up to page 42) are the Notes of a Lecture delivered by Dr. O. J. Lodge before the Royal Institution of Great Britain on Friday evening, June 1, 1894. These notes have been revised by Dr. Lodge, and prepared for publication in the form here presented. After page 42 an account is given of the later applications of Hertzian wave experiments to wireless telegraphy, and a series of Appendices are also given.
Introductory.—1894.
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 achievement 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 sown 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.
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, and no less than eighteen Papers were contributed to German periodicals by him, in addition to the papers incorporated in his now well-known book on electric waves.
In closing these introductory and personal remarks, I should like to say that the enthusiastic admiration for Hertz’s spirit and character felt and expressed by students and workers 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 exalt 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 intention 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 1888,[1] 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; and the simplest way will be for me hastily to summarise our knowledge of the subject before the era of Hertz.
Fig. 1.—Oscillations of Dumb-bell Hertz Vibrator
(after Bjerknes).
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.
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 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.
Fig. 2.—Oscillation of Ring-shaped Hertz Resonator
excited by Syntonic Vibrator (after Bjerknes).
Fig. 3.—Oscillation of Ring Resonator not quite syntonic with Radiator.
(For method of obtaining these curves [see Fig. 14].)
Conversely, if the source is a persistent vibrator, correct tuning is essential, or it will destroy at one moment ([Fig. 3]) motion which it originated the previous moment. Whereas, if it is a dead-beat or 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 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 etherial 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.
This is the function of the telegraph wire; it is to the ether 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 Electrophorus.
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.[2] 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.
All this was known, I say, known with varying degrees of confidence; 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 themselves by minute electric sparks.
Fig. 4.—Experiment with Syntonic Leyden Jars
([cf. page 21]).
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.[3]
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 ¼λ 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 must be equal, whereas in a ring circuit the magnetic energy 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 thus 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 persistent 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 Sparking 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 is depicted in [Fig. 2]. I 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 verification of what had already been predicted about the waves, and by laborious and difficult interference experiments he ascertained that the previously calculated length of the waves was thoroughly borne out by fact. These interference experiments in free space are his greatest achievement.
Fig. 6.—Circular Resonator.
(The knobs ought to nearly touch each other.)
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.
He worked out every detail of the theory splendidly, separately analysing the electric and the magnetic oscillation, using language 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 disentangling all their main relations most harmoniously.
Holtz 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: glass cuts it off.
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 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 IV., Fig. 59]), that if the B knobs were subjected to steady strain instead of to impulsive rush—e.g., if 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, Branly, and others have extended, that a freshly-polished zinc or other oxidisable surface, if charged negatively, is gradually discharged by ultra-violet light.
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 of electricity begins directly the glass plate is withdrawn.
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;
Quartz 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 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 fluorspar 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[4] ([Fig. 9, next page]).
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[5] with illumination.
Fig. 9.— Zinc Rod discharging Negative Electricity in the very Ultra-violet Light of a Spectrum formed by a Quartz Train. The best discharging light is found far beyond the limits of the visible spectrum.
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. It is made of four large copper sheets soldered together two and two, strung up by thin rope to a gallery, and each pair connected with the other by several yards of No. 0 pure copper rod, interrupted by a pair of sparking knobs ([Fig. 10]).
Large Hertz Vibrator in action; Abel’s Fuse Detector;
Vacuum Tube Detector; 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 nevertheless, each time it is excited, sparks can be drawn from most of the reasonably elongated conductors in the theatre of this Institution, and indeed from wire fencing and iron roofs outside this building.
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 suspensions of the theatre lamps were altered. They had at first been held in position by wire guides, which served as collectors of the Hertz waves or impulses.
Fig. 10.— Hertz Oscillator on reduced scale,
⅒th 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 ([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 myself introduced and 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. 11]) without air gap, which must be excited by a positive spark at one end and a negative spark at the other, 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,[6] and, therefore, well adapted for interference and diffraction experiments. But, indeed, spheres can be made to vibrate longer than 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]).
Fig. 11.— A Small Dumb-bell Form of Radiator for Impulsive Rush.
Fig. 12.— Dr. Lodge’s Hollow Cylindrical Radiator, arranged horizontally against the outside of a Metal-lined Box containing the Spark-producing Apparatus. Half natural size. Emitting 3 in. waves.
Many of these senders will do for receivers too, giving off sparks to other insulated bodies or to earth; but besides the Hertz type of receiver, many other detectors of radiation have been employed. Vacuum tubes can be used, either directly or on the trigger principle, as by Zehnder ([Fig. 13]),[7] 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 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.[8] This was the method he used in this theatre four years ago. Blyth used a one sided electrometer, and V. Bjerknes has greatly developed this method ([Fig. 14]), abolishing the need for a spark, and making the electrometer metrical, integrating and satisfactory.[9] 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 Langley, and applied to the detection of electric waves in wires by Rubens and Ritter 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.
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 quadrants 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 damped, though the open plate radiator does not.
Going back to the physiological method of detecting surgings, Hertz tried the frog’s leg nerve-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. Ritter has succeeded; but, in my experience, failure is the normal and proper result. Working with my colleague, Prof. Gotch, at Liverpool, I too have tried the nerve and muscle preparation of the frog ([Fig. 15]), and we find that an excessively 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 needles in direct contact with the nerve.
All that such oscillations do, if continued, is to produce a temporary 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.
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 (namely, the terminals of a derived circuit from a wire connecting the outer coats of a pair of discharging Leyden jars), 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. The C D terminals do not stimulate the nerve, though at very high alternative potentials, but they gradually and temporarily paralyse it, so that the test terminals A B produce no effect for a 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 stimulus ever reaches them at all. By dissecting out a nerve and muscle from a deceased frog after the historic manner of physiologists, and applying the stimulus direct to the nerve, at the same time as some other well known ¹/₁₀₀th 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 ¹/₁₀₀th-volts stimulus above is gradually throttled 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.]
Air Gap and Electroscope charged by Glass Rod and
discharged by the Wave Impulse from a moderately
distant Sphere excited by Coil.
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.” The electroscope being charged to the verge of overflow, the impact of weak electric waves collected by a bit of wire sticking up from the left-hand binding screw precipitates the collapse of the leaves.
Among trigger methods of detecting electric radiation, I have spoken of the Zehnder vacuum tubes; another method is one used by Boltzmann.[10] A pile of several hundred volts is on the verge of charging an electroscope through an air gap just too wide 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 completely with any gentle rise of potential. Bring excited glass near it, the leaves diverge gradually and then suddenly collapse, 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. Moreover, it is a detector for 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 handkerchief 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 consequent radiation, especially the radiation of high frequency, the higher 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” long-period Hertz waves any knobs will do, but to generate the “ultra-violet” short-period waves 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 vibrations. 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 work in connection with photo-electricity is now becoming known, and who has devised an instrument more sensitive to radiation than even Boys’ radiomicrometer, in that it responds to the radiation of a star while the radiomicrometer does not, found some years ago that some of his light-excitable cells lost their sensitiveness capriciously on tapping, 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.[11]
The sensitiveness of selenium to light is known to everyone, and Mr. Shelford Bidwell 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 or ebonite, diminished its resistance enormously, from some millions to some hundreds of ohms, when it was exposed to the neighbourhood, even the distant neighbourhood, of Leyden jar or coil sparks. He likewise found that a tube of metallic filings behaved similarly, and that both recovered their original resistance on shaking or tapping. 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.[12]
With me the matter arose somewhat differently, as an outcome of the air gap detector employed with an electroscope by Boltzmann ([Fig. 16]). 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; conducting an ordinary bell-ringing current if a single voltaic cell was in circuit; and, if there were no such cell, exhibiting an electromotive force of their own sufficient to disturb a low resistance galvanometer vigorously, and sometimes requiring a faintly perceptible amount of force to detach them. The experiment was described to the Institution of Electrical Engineers in 1890,[13] and Prof. Hughes said he had observed the same thing.
Fig. 16a.— Receiver in Syntonic Jar Experiment,
with Knob Coherer and Tapper-back ([cf. Fig. 4]).
The experiment of the syntonic Leyden jars can be conveniently shown with the double-knob or 1889 coherer. The pair of knobs are arranged to connect the coatings of the receiving jar (a large condenser being interposed to prevent their completing a purely metallic circuit), and in circuit with them is a battery and a bell. Every time the receiving jar responds syntonically to the electric vibration of the other jar, the knobs cohere (if properly adjusted) and the bell rings. If the bell is free in air it continues ringing until the knobs are gently tapped asunder; but if the bell stands on the same table as the knobs, especially if it rests one foot on the actual stand, then its first stroke taps them back instantly and automatically, and so every discharge of the sending jar is signalled by a single stroke of the bell. Here we have in essence a system of very distinctly syntonic telegraphy, for the jars and their circuits must be accurately tuned together, if there is to be any response. A very little error in tuning, easily made by altering the position of the slider ([Fig. 4]), will make them quite unresponsive, unless the distance between them is reduced.
Fig. 17.— Early Form of 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, whose end is bent at right angles and passed through into the hollow circular wooden box, of which the upper figure shows the top and general appearance, and the lower figure shows the inside.
At the maximum distance of response the tuning required is excessively sharp. But, certainly, for these closed and durably-vibrating circuits, the distance of response is small, as has been said before. [Fig. 16a] shows the syntonic Leyden jar experiment arranged with the double knob coherer, instead of with the spark gap of [Fig. 4].
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 astonishingly 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 discontinuous and more permanent manner, than an air gap. Branly’s tube of filings, 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 many respects easier to work with, and except for very 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].)
Fig. 18.— Early Form of Iron Borings Tube. One-half natural size, with solid brass cylinder terminals in each end of the tube, making contact with the borings.
Experiment showing Filings Tube responding to Sphere, to Electrophorus, and to a Quasi-“Spark” from the Discharge of an ordinary Gold-leaf Electroscope.
I picture the action as follows: Suppose two fairly clean pieces of metal in light contact—say two pieces of brass or of iron—connected to a single voltaic cell; a film of what may be called oxide intervenes between the surfaces so that only an insignificant current is allowed to pass, because a volt or two is insufficient to break down the insulating film, except perhaps at one or two atoms.[14] If the film is not permitted to conduct at all, it is not very sensitive; the most sensitive condition is attained when an infinitesimal current passes, strong enough just to show on a moderate galvanometer.
Now let the slightest surging occur, say by reason of a sphere being charged and discharged at a distance of forty yards; the film 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.
The incipient cohesion electrically caused can be mechanically destroyed. Sound vibrations, or any other feeble mechanical disturbances, 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-a-mile was nearer the limit of sensitiveness for this particular apparatus as then arranged. However, this is a rash statement not at present verified.[15] 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 2 in. or 3 in. 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. Signals were obtained across the full width of the college quadrangle, and later, with larger apparatus, between the college tower and another high building half-a-mile away.
Fig. 19.— Radiator used in the library of the Royal Institution, exciting the Coherer ([Fig. 17]) on the lecture table in the Theatre. I also used a radiator with two or with three large spheres between two knobs, and described it in Nature, Vol. 41, p. 462, 1890. This is the radiator which Prof. Righi has improved and made in a compact form with oil between the two middle spheres.
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, 8 in. 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.
Added later.—My assistant, Mr. E. E. Robinson, early noticed that when a telephone was used as receiver, say with a single-point coherer ([see illustration on opposite page]), which is a very sensitive arrangement, every disturbance of the coherer due to received waves is accompanied by a crackle or tick in the telephone, without any tapping back being necessary. This is, indeed, the easiest mode of receiving signals, and we often practised it. If a suitable, well-damped galvanometer, such as a Thomson marine speaking-galvanometer, is included also in the circuit (a more sensitive one is sometimes necessary—and we frequently used a D’Arsonval—but it must be well damped), the meaning of these ticks is recognised; each represents a minute change in the resistance of the coherer—not at all the full change usually employed, but little subsidiary changes, sometimes up and sometimes down, barely sufficient to affect a galvanometer, but quite adequate (being so sudden) to disturb a telephone. This method of receiving, which at first is very sensitive, after a time becomes less so; the point shows signs of fatigue, probably due to too perfect cohesion having been gradually established, and a mechanical tap back is desirable to restore it to its original condition.
If all the signals received were precisely of the same strength, I doubt if these superposed crimples of resistance would occur; but signals depending on quality of sending spark never are of the same strength, and accordingly the sudden slight variations of resistance do occur. Usually an ordinary high resistance telephone was employed, and it was joined to the coherer circuit through one of the usual small transformers—a plan which has many obvious advantages.
Simplest Receiving Arrangement: a Telephone in Circuit with Single-point Coherer without Tapper-back. B a needle resting against a watch-spring A adjusted by screw C.
Syntonic Sender and Receiver used in the experiments plotted on [page 28]. The switch enables the coherer K to be connected either to the tuned resonator M L N or to the detecting circuit E F. Weak impulses are felt when the switch is C E, D F; strong impulses when the switch is C A, D B; provided the coil L is similar to the coil of the radiator above. The impulses are plotted in the diagram Fig. 19A.
October 27, 1897.
Fig. 19a.— Current through Coherer after successive Electrical Stimuli, without any mechanical tapping back. The sudden rises are obtained when the circuits are syntonised. Weaker stimuli cause the descents.
The fluctuations of resistance of a coherer dependent on various strengths of stimulus are instructively shown in some metrical experiments made by Mr. Robinson, and a plotting of which I showed to the Physical Society of London in 1897. This plotting is here reproduced, and it shows the singular fact that, whereas a stronger electrical stimulus usually decreases the resistance, as is natural, a weaker subsequent stimulus usually increases it again: so that alternately strong and weak stimuli send the curve zigzagging up and down, until it gets into a condition demanding rejuvenation by a mechanical tap back.
Sometimes a decidedly strong electrical stimulus knocks down the conductivity of the coherer as if it had been tapped back. This is almost certainly due to a burning of the delicate contacts—a blowing of a fuse as it were,—and the effect of this electrical burn back is quite different from the effect of a mechanical tap back, inasmuch as it leaves the coherer insensitive. A shaking up is necessary to restore it.
I now call your attention to the [Table on next page] of various kinds of detector for electric radiation distributed in groups.
Selenium is inserted in this table in the microphone column, because it is 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 inserted with a query, to indicate that its position in the table is not certainly known. It may possibly belong to some other column.
Electrical Theory of Vision.
And I want to suggest that quite possibly the sensitiveness of the eye is of the coherer 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 powder 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, though I hope it is obvious what I intend to suggest.
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 instructive, 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 physiologically not the mere cessation of light. Both are positive sensations, and both stimuli are necessary; for until the filings are tapped back vision is persistent. In the eye model the period of mechanical tremor should be, say, ⅒th second, so as to give the right amount of persistence of impression.
| DETECTORS OF RADIATION. | |||||
|---|---|---|---|---|---|
| Physiological. | Chemical. | Thermal. | Electrical. | Mechanical. | Microphonic. |
| Selenium.(?) | |||||
| Eye. | Photographic Plate. | Thermopile. | Spark. (Hertz.) | Electrometer (Blyth and Bjerknes.) | Impulsion Cell. (Minchin.) |
| ˟Frog’s Leg (Hertz and Ritter.) | Explosive Gases. | Bolometer. (Rubens and Ritter). | Telephone; Air-gap and Arc. (Lodge.) | Suspended Wires. (Hertz and Boys.) | Filings. (Branly.) |
| Photoelectric Cell. | Expanding Wire. (Gregory.) Thermal Junction. (Klemencic.) | Vacuum Tube. (Dragoumis.) Galvanometer. (Fitzgerald.) Air-gap and Electroscope. (Boltzmann.) Trigger Tube. (Warburg and Zehnder.) | Coherer. Hughes and Lodge. | ||
˟ The cross against the frog’s leg indicates that it does not appear really to respond to radiation, unless stimulated in some secondary manner. The names against the other things are unimportant, but suggest the persons who applied the detector to electric radiation.
The interrogation mark against Selenium indicates that its position in the microphonic column may be doubtful.
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, indeed, 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. It must be clearly understood that electric jerks, due to the make-and-break of an ordinary current, are quite adequate to electrically stimulate a coherer in their neighbourhood. It is constantly to be noticed that a coherer responds best to excessively short sparks of a certain sharp quality.
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 draw 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: indeed, these small sparks are often more effective than bigger ones.
Demonstration of Ordinary Holtz Machine
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 disturbance caused by the violent flashes of a Holtz or Wimshurst or Voss machine it is blind to. The loud sparks have no effect on it. They are like infra-red radiation to the eye. If the knobs of the machine are well polished the coherer begins to respond again, evidently by reason of 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 that clean sphere.
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 electrophorous lid. Spark it to a knuckle, and it does very little. Spark it to a clean knob held in the hand and it works well. But now spark it to an insulated sphere, there is some effect. Discharge the sphere, and take a second spark, without recharging the lid; do this several times; and at last, when 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 electrophorous 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. It is surprising how far these impulses can be felt along an ordinary uninsulated wire or other conductor.
In fact, when thus connected to gas pipes one day when I tried it, the spot of light could hardly keep still five seconds. 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 ordinary well-known waves by a blackboard or other sufficiently opaque substance.
[I did not succeed in this, for a sensitive coherer in an outside shed unprotected by the thick walls of a substantial building cannot be kept quiet for long. I found its spot of light liable to frequent weak and occasionally violent excursions, and I could not trace any of these to the influence of the sun. There were evidently too many terrestrial sources of disturbance in a city like Liverpool to make the experiment feasible. I don’t know that it might not possibly be successful in some isolated country place; but clearly the arrangement must be highly sensitive in order to succeed.]
We can easily see the detector respond to a distant source of radiation now, viz., to a 5 in. 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 ([Fig. 19].)
Fig. 19b.— A Portable Detector, B the Collecting Wire.
Also I exhibit ([Fig. 19b]) a small complete detector made by my assistant, Mr. Davies, which is quite portable and easily set up. The essentials (battery, galvanometer, and coherer) are all in a copper cylinder, A, three inches by two. A bit of wire, B, 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 to be sensitive to electric waves, if it be made of any oxidisable medal (not of carbon),[16] and we have an instrument which might be called a micro-something, but which, as it appears to act by cohesion, I at present call a coherer. Perhaps some of the capriciousness of an anathematised bad contact was sometimes due to the fact that it was responding to stray electric radiation. ([See Appendix III]., pp. 109 and 111.)
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 persistent 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 uniting effect of an electric jerk.
Receiver in Metallic 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 galvanometer coil and coherer, all shut up in a copper box ([Fig. 19c]). The galvanometer 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 magnetic action of the current in 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 the sheet copper.
Fig. 19c.— Protected Detector. A is an occasional wire passing through shuttered aperture. E is a lead tube enclosing leading wires, as in [Fig. 21].
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 copper lid on to a flange in six places was not enough. But by the use of pads of tinfoil and tight clamping, 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, not connected to anything at either end, provided it is insulated where it passes through, the waves will utilise it as a speaking-tube, and run blithely in. And this happens 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 the two must be encased in a metal tube ([Figs. 19c] and [21]), and this tube must be well connected with the metal of both enclosures, if nothing is to get in but what is wanted.
Fig. 20.— Spherical Radiator for emitting a Horizontal Beam, arranged inside a Copper Hat, fixed against the outside of a metal-lined Box, which contains induction coil and battery and key. One-eighth natural size. The wires pass into the box through glass tubes not shown.
Similarly when definite radiation is desired, it is well to put the radiator in a copper hat open in only one direction ([Fig. 20]), 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 the sending apparatus in a packing case lined with tinfoil, to the outside of which the sending hat ([Fig. 20]) is fixed, and to pull the key of the primary exciting circuit by a string from outside, so that not even key connections shall protrude, else exact optical experiments are impossible.
Fig. 21.— General arrangement of experiments with the Copper “Hat,” showing Metal Box on a Stool, standing outside the Theatre. The Box is not exactly represented, but inside it the Radiators were fixed with a graduated series of apertures; the Copper Hat containing the Coherer is seen on the Table with the Metal Box on the left of the Table containing Battery and Galvanometer Coil connected to it by a compo pipe conveying the wires, as in [Fig. 19c]; the Lamp and Scale barely indicated at one side of the Table; a Paraffin Prism; and a Polarising Grid of copper wires stretched on a frame. (This figure is from a thumbnail sketch by Mr. A. P. Trotter, taken at the Lecture in 1894.)
Even then, with the lid of the hat well clamped on, something gets out, but it is not enough to cause serious disturbance of qualitative 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. (Tinplate is quite opaque. Lead paper lets a little through.)
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, as at A, [Fig. 19c].
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 in tinfoil, and a gas-lighter is powerless.