APPLICATION OF THIS METHOD OF
SIGNALLING AT A DISTANCE
TO ACTUAL TELEGRAPHY.
Although the method of signalling to a moderate distance through walls or other non-conducting obstructions by means of Hertz waves emitted from one station and detected by Branly filing tubes at another station was practised by the author and by several other persons in this country, it was not applied by them to actual telegraphy. The idea of replacing a galvanometer, which was preferably a well-damped or speaking galvanometer, by a relay working an ordinary sounder or Morse was an obvious one, but so far as the present author was concerned he did not realise that there would be any particular practical advantage in thus with difficulty telegraphing across space instead of with ease by the highly developed and simple telegraphic and telephonic methods rendered possible by the use of a connecting wire. In this non-perception of the practical uses of wireless telegraphy he undoubtedly erred. But others were not so blind, though equally busy; and notably Dr. Alexander Muirhead foresaw the telegraphic importance of this method of signalling immediately after hearing the author’s lecture on June 1st, 1894, and arranged a siphon recorder for the purpose. Captain Jackson also, at Devonport, made experiments for the Admiralty, and succeeded in telegraphing between ships in 1895 (or 1896). Prof. Popoff’s telegraphic application in 1895 is mentioned on [page 62].
By some chance a knowledge of the coherer method of detecting electric waves did not spread fast in Germany, the many German workers in Hertz waves continuing, for some time after 1894, the older and less efficient, though for metrical purposes often more convenient, mode of detecting them. But, in Italy, the work described in the preceding lecture became well known, and the subject was developed largely, especially by Prof. Righi, of Bologna, in the optical direction. It was also developed in the same direction with many most interesting results by Prof. Bose, of Calcutta, as mentioned in the text. Prof. Righi made a large number of experiments, which he has since described in an Italian treatise, “Opticé Elettrica,” and it appears that it was from him that Signor Marconi learned about the subject, and immediately conceived the idea of applying it to commercial telegraphy. He appears to have worked at the subject for a short time in Italy, aiming at getting the receiver to be more satisfactory and dependable, and improving the early form of Branly filings tube [depicted on page 23] by greatly diminishing its size, bringing the terminals closer together, and replacing the coarse borings by fine filings. He also sealed them up in a vacuum, just as the author did, as related on [page 34]. The only differences, indeed, between his procedure and the author’s during this time were that Signor Marconi preferred nickel filings with a little mercury and a low vacuum, whereas the author adhered chiefly to iron and brass filings and to high vacua. At last he brought it over to Dublin, where he was advised to take it to the Chief of the Government Telegraphs, Mr. Preece, and accordingly he took his, at that time, crude apparatus to the Post Office in a sealed box. There was no point of novelty in it at this stage.
With the powerful aid of the Post Office Signor Marconi proceeded to develop his system of telegraphy on a large scale; and, sometimes failing, sometimes succeeding, gradually increased the distance over which signalling was possible, and especially began to develop from unpromising beginnings his special method for long-distance, viz., the employment of a sending and receiving conducting plate or other small surface, at the top of a lofty pole, connected through what was at that time supposed apparently to be the real radiator, with the earth. This elevated plate, connected as it now is through a simple spark gap with the earth, is an obvious modification of a Hertz vibrator; for it may be regarded simply as a Hertz vibrator with its axis vertical, as Hertz often used it, and with its lower plate replaced by the earth, so as to double the available capacity; but the action of a pair of such elevated plates, connected through the earth conductively and through the air inductively, as now used by Marconi for sender and receiver respectively, is not quite like that of a Hertz vibrator and a Hertz receiver acting on one another by emitted radiation in the ordinary way. If it were not the same earth to which the plates were connected, they would have to act ordinarily by radiation, but since it is the same earth, and that earth conducting (possibly, indeed, with a submerged cable sheath connecting favourably-chosen stations), then the two elevated plates are partially like the greatly separated terminals of a single Hertz vibrator.
Only one of the plates is charged during a sending operation, the other is at zero potential, but some trace of the electrostatic lines from one plate may extend in curved lines to the other, just as they extend to every elevated conductor within hail of the sender in any direction.
Then comes the snap of the spark gap and the sudden discharge, equivalent to the rush of an opposite charge of electricity suddenly into the sending plate, disturbing the electric equilibrium at a distance—at any distance to which any trace of electrostatic field had been able to reach—and giving a kind of what is called in lightning a “return stroke.” The effect of this on the distant plate and conductor must be almost infinitesimal; nevertheless, separating it from the earth is the most sensitive detector to a minute sudden rush or jerk of electricity that can be imagined, or that has hitherto been invented,—the coherer. Accordingly, absurdly minute though the disturbance is, the coherer feels it, instantly increases in conductivity, works the relay, and gives the signal. Every spark at the distant spark gap causes a similar rush in or out of the distant elevated plate, and the receiving plate collects such a fraction of this disturbance as to stimulate the coherer and give a signal every time. Not that it is to be supposed never to miss fire. At the present time a coherer is not a rough instrument that can be left free from expert attention with safety for a long time. There are times when it goes on working for days or even weeks, but there are other times when it gives trouble and needs some form of attention. Let us hope that these latter times will become less frequent, and that the whole thing will become quite dependable before long. The pertinacious way in which Mr. Marconi and his able co-operators have, at great expense, gradually worked the method up from its early difficult and capricious stage to its present great distances and comparative dependableness is worthy of all praise.
Telegraphy by means of Hertz waves, though perhaps chiefly developed in this country, has also been pursued successfully by Prof. Slaby in Germany, who has attained considerable distance over land, with its numerous obstacles, and has published an account of his researches in a book called “Funkentelegraphie”; while like success over land has been attained by M. E. Ducretet, M. Blondel and others in France. M. Ducretet has, indeed, put on the market a compact apparatus whereby beginners can readily try their hands at this mode of signalling; as well as a large-scale apparatus like that employed by Lieutenant Tissot for lighthouse signalling on the coast of Brittany.
The filings tube now chiefly employed by the author is of the following form:—It is a sealed glass tube containing carefully selected iron filings, and exhausted to the highest vacuum. Close together are two little silver globes melted each on its own platinum wire terminal, which are connected with convenient screws on an ebonite stand. The filings are adjusted so as just to cover the two silver globes, and no more; a pocket, or reservoir, however, is sometimes provided whereby more or fewer filings can be easily introduced into the working compartment for experimental purposes. This pocket serves to fix the whole tube to its ebonite body, which is provided with a clamp to attach it to the stiff spring, or movable lever, or other form of support, through which it is to receive the mechanical shocks necessary to restore or decohere it after an electrical stimulus.
The usual plan is to employ an electrical hammer to rap strongly on a stiff brass spring to which the ebonite is clamped, but another plan is to attach the coherer to a lever tilted strongly by an electromagnet after the fashion of a sounder. A rapid succession of gentle taps is often better than one violent one, but experience is the best test of the kind of restoration wanted, for it depends a good deal on the strength of the electrical stimulus. There are methods of dispensing with this decohering operation altogether.
After a fairly strong electric stimulus all the filings are stuck together into a sort of mat, and nothing but a thorough shaking up will pull them asunder again. A still more violent electric shock may indeed have a decohering effect, but it is not a plan to be recommended, for it seems to be a heat effect, akin to the blowing of a fuse.