Part 1

Transcriber’s Notes:

Underscores “_” before and after a word or phrase indicate _italics_ in the original text. Equal signs “=” before and after a word or phrase indicate =bold= in the original text. Small capitals have been converted to SOLID capitals. Illustrations have been moved so they do not break up paragraphs. Typographical and punctuation errors have been silently corrected.

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.

[1] _Phil. Mag._, XXVI., pp. 229, 230, August, 1888; or “Lightning Conductors and Lightning Guards,” pp. 104, 105; also _Proc. Roy. Soc._, Vol. 50, p. 27.

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.

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.

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.

[2] 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.

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.

_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]

[3] See _Nature_, Vol. XLI., p. 368, where I first described this experiment; or quotation in J. J. Thomson’s “Recent Researches,” p. 395.

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.

_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.

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.

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.

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).