Telegraphing Across Space - Part 2/2

* Paper read before the Society of Arts, March 30th, 1898.

(Continued from Telegraphy Across Space - Part 1/2.)

II - Induction Methods

One must not close this section without reverting to a most pregnant point of advance made about 1888 or 1889 by Dr. Oliver Lodge. When experimenting upon the oscillatory discharge, he conceived the happy idea of turning two circuits into resonance, or, as he termed it, “syntony” with one another, in such a way that when an oscillating electric spark occurred in one of the circuits, the inductive effect on the other immediately set up in it electric oscillations which manifested themselves by an overflow spark. I call this experiment pregnant because it affords a hint of another possibility - namely, that of signalling inductively from one area to another, and using around those areas, not merely circuits of wires, but syntonic circuits, which, therefore, are necessarily much more sensitive in their response one to the other. Some of Tesla’s high-frequency experiments also have an obvious bearing on this point.

III. - Electric Wave Methods.

After Clerk Maxwell had predicted the existence of electromagnetic waves, and had shown that their speed of propagation is identical with that of light, it required, in reality, very little to demonstrate by experiment the existence of such waves. But that very little was not actually achieved until the year 1888, when the lamented Prof. Heinrich Hertz showed simple methods of producing, detecting, and measuring these waves. It had been known for many years, from the predictions of Kelvin and Von Helmholtz, and confirmed by the experiments of Fedderssen, that in many cases an electric discharge is of an oscillatory character. In the years 1887-8, Lodge, Fitzgerald, and others were investigating the nature of these oscillations, and the manner in which they are guided by conducting wires, when Hertz conceived the idea of investigating the disturbances which such oscillatory discharges set up in the surrounding space. He showed that, given a simple apparatus, which he called an “oscillator,” consisting of two metal plates or conductors, connected by a conductor interrupted at one intermediate point by a “spark-gap,” the oscillator on the appearance of each spark emitted a train of electric waves into the surrounding space. He further showed that if a mere circuit or ring of wire of suitable size, the continuity of which is interrupted at one point by a minute gap, is placed in the path of these travelling waves in a suitable position, the waves as they reach it set up electric surgings in this wire, and if sufficiently energetic cause it to show a small spark in the gap. This simple detecting device he termed a “resonator.” Armed with these apparently primitive pieces of apparatus, he then devoted himself to the task of exploring the propagation of the waves. He found that, like waves of light, they could be reflected by metallic surfaces, could be refracted by prisms, concentrated by lenses, and even could be polarised. He measured their wave-length and velocity of propagation. He found that they could pass readily through walls of wood, stone, or brick, which are opaque to ordinary lightwaves. Metals and other conductors of electricity, on the contrary, absorbed them, and were consequently opaque.

In these researches of Hertz we meet, for the first time,† with the recognition of a true travelling wave. With this immense discovery there was opened out an entirely new field of possibilities. Hitherto there had been inductive actions known which might reach out from wire to wire, only to fall back again when their excitant cause died away. But now the electric wave, once started on its path, did not collapse back into the wire when the spark ceased; on the contrary, it went travelling on. And just as the javelin, which can travel on after the impulse has ceased, can act at greater range than the sword, whose thrust is limited by the length of arm and blade, so the true electric wave, by the very fact that it is a true travelling wave, can carry signals to greater distances than the mere inductive influence that simply extends outwards from a wire or from a coil.

The work which Hertz had begun, was, after his death, carried on by a whole army of investigators. Of these, and of their achievements, the best account that has yet appeared is Prof. Lodge’s little book on “The Work of Hertz and his Successors.” To that book enquirers must be recommended for details. Suffice it here to say much has been done in perfecting both the oscillator and the detector. Notable amongst these matters have been the forms of oscillator designed by Lodge and by Righi; the latter having the spark gap immersed in oil or vaseline between two metal balls. Many forms of detector have been proposed. Very early Lodge produced one under the name of “coherer,” consisting of a metallic point very lightly pressed against a metal plate, and connected in circuit with a galvanometer and a local cell. The light contact constitutes an imperfect joint, which is practically non-conductive until caused to cohere and conduct by the impact of an electric wave; or, perhaps more accurately, by the stimulus of the minute surging electric current which results from the impact of an electric wave. Subsequently, taking a hint from M. Branly, Lodge substituted as a detector a new kind of coherer, consisting of a small glass tube partly filled with loose metallic filings - iron or nickel by preference - joined in the circuit. Such a coherer acts as a species of relay, by means of which an electric wave, incapable in itself of affecting a galvanometer or other instrument, is enabled to do so indirectly by setting into operation a local current. After the coherer has thus operated, it usually remains in the conductive state until subjected to some mechanical jar or shock. Lodge proposed to apply for this purpose a mechanical tapper worked either by clockwork or by a trembling electric mechanism. On several occasions, and notably at Oxford in 1894, he showed how such coherers could be used in transmitting telegraphic signals to a distance. He showed that they would work through solid walls. Lodge’s greatest distance at that time had not exceeded some 100 or 150 yards. Communication was thus made between the University Museum and the adjacent building of the Clarendon Laboratory. For more than 18 months the Rev. F. Jervis Smith, of Oxford, using a carbon powder coherer, has maintained communication between his house and the Millard Engineering Laboratory, over a mile away.

Even before this Mr. Nikola Tesla, in a lecture delivered at St. Louis in 1893, had made a further suggestion of great importance. He proposed to transmit electric energy by oscillations to any distance, without communicating wires, by erecting at each end of the stretch a vertical conductor joined at its lower part to the earth, and at its upper to a conducting body of large surface. This constitutes a vertical base line from which to disseminate the oscillating disturbances.

About two years ago a young Italian, Mr. Marconi, came to this country, and succeeded in inducing the British Telegraph Department to give him facilities for experimenting upon wave-method of transmission. First upon Salisbury Plain, and then across the Bristol Channel, he succeeded in transmitting Morse signals to greater distances than anyone had previously attained. He sent signals from Lavernock Point to Bream Down - about nine miles as the crow flies over the open channel. To accomplish this he used as base lines two vertical conductors earthed at their lower ends, and carrying at the top extended surfaces, He used a Righi transmitter. As receiver he employed the special form of Lodge-Branly coherer presently to be described. This was connected in the manner Lodge had recommended in a local circuit, and was tapped by a mechanical tapper operated by a vibrating electric mechanism. The local circuit operated a Post Office relay connected to a Morse instrument signalling the dots and dashes. The coherer was itself included in the vertical base line. So far all is old. The special coherer used in these experiments by Marconi has very fine metallic powder, chiefly of nickel and silver, in a small glass tube exhausted of air. He also applied shunting resistances to the relay contacts, and interposed a fine iron wire closely coiled, as an impedance in the local circuit on each side of the coherer.

In 1897, some further experiments were carried out by Prof. Slaby, of Charlottenburg, on an even larger scale. He abandoned everyone of the novelties introduced by Marconi, and fell back upon the methods previously known. He used a simple Lodge-Branly coherer, employed elevated conductors as base lines, discarded the useless little iron wire impedance coils in the local circuit, and substituted for the Post Office polarised relay one made out of a Weston galvanometer. His success shows that all that is essential can be thus attained. He chose as the scene of his operations the Havel, and set up elevated conductors upon the castle of the Pfaueninsel, and on the campanile of the church at Sacrow. Thus equipped, he transmitted signals, at first about three-quarters of a mile, then three miles across the water. He found trees and masts to interfere with the signals to some degree. He then proceeded, with the aid of the military authorities, to experiment over an open stretch of country - from Rangsdorf to Schöneberg. The elevated conductors were wires raised by means of hydrogen balloons to heights of nearly 1,000ft. Signals were obtained at a distance of 21 km., or over 13 miles. Neither in Marconi’s nor in Slaby’s successful operations were syntonic devices employed. The following table summarises the results of Marconi’s and Slaby’s work.

Commenting on these results, Slaby notes how over open sea a much greater distance appears to be attained from a base line of given length. Assuming Marconi’s best proportion, he calculates the vertical length of base line needed for communicating across the English channel at Dover to be 265ft., while from London to Paris, over land and sea, would require 4,700ft. He even estimates base lines of 6,600ft. as sufficient, were it not for the curvature of the globe, to serve for communication across the Atlantic.

The most recent improvements made towards perfecting this method of transmission are those of Dr. Oliver Lodge, whose labours, continued during the past few months, are still in progress. He has first reorganised the transmitter apparatus so as to make it a more persistent radiator. It emits longer trains of waves. This has been accomplished by introducing in the path of the oscillations, between the spark-gap and the wings, a few turns of stout wire to act as an impedance coil. By this means the oscillations can be accurately tuned. The receiving apparatus is also tuned; in fact, each apparatus is made to operate both as emitter and as receiver, in turn, as required. Lodge has also modified the arrangements of the coherer circuits, to render them more certain of operation, no local current being allowed to pass through the coherer until after it had been affected by the waves. He has, in fact, thoroughly redesigned the sending and receiving instruments upon a rational basis, so that they shall be both less sensitive to stray impulses, and more sensitive to properly attuned waves. The results obtained with these have not yet been made public; but, employing a siphon recorder as the receiving instrument, remarkable precision of signalling has been attained. Further developments in this direction will doubtless be awaited with much interest. Meantime, in other countries - the United States, Russia, and France - other experimenters are at work. Any account given at the present time will, therefore, be necessarily incomplete.

In passing finally from a review of that which has already been attained to that which may reasonably be contemplated as within reach of attainment in the near future, I have no wish to assume the rôle of the prophet. Still less would I desire to emulate the example of the imaginative litterateur who, whether his name be Jules Verne or H. G. Wells, stimulates the public curiosity by amazing speculations, and in doing so renders the dis-service that the public so stimulated is made less capable than before of distinguishing between that which is and that which is not within the bounds of reasonable possibility.

It has been shown that there are three general methods of transmitting electric signals across space. All of them require base lines or base areas. The first - conduction - requires moist earth or water as a medium, and is for distances under three miles the most effective of the three. The second - induction - is not dependent upon earth or water, but will equally well cross air or dry rock. The third - electric wave propagation - requires no medium beyond that of the ether of space, and is, indeed, interfered with by interposed things such as masts or trees. Given proper base lines or base areas, given adequate methods of throwing electric energy into the transmitting system, and sufficiently sensitive instruments to pick up and translate the signals, it is possible, in my opinion, so to develop each of the three methods, that by any one of them it will be possible to establish electric communication between England and America across the intervening space. It is certainly possible, either by conduction or by induction; whether by waves I am somewhat less certain. Conduction might very seriously interfere with other electric agencies, since the waste currents in the neighbourhood of the primary base line would be very great. It is certainly possible either by conduction or induction to establish direct communication across space with either the Cape, or India, or Australia (under the same assumptions as before), and at a far less cost than that of a connecting submarine cable. I doubt very greatly whether the wave method can be made applicable at all to these so-distant parts of the globe. But whether by conduction, by induction, or by waves, I am firmly convinced that the immediate road to commercial success lies in two things. Firstly, we must frankly recognise that there is no such thing as telegraphing without wires - that the base line, or the base area surrounded by wires, is a fundamental necessity. Secondly, we must look to establishing real syntony between the sending and the receiving parts of the apparatus to render it, as far as possible, sensitive and independent, without which conditions such systems will become too costly and too unmanageable for commercial ends.

[The paper was illustrated by numerous slides illustrating the methods and instruments used by Hertz, Lodge, Righi, Marconi, and Slaby in their investigations, and the newest syntonic apparatus of Lodge. Experiments were also shown illustrating the transmission of electric waves and their reception and detection. A small Lodge apparatus, constructed by Mr. Miller, was also exhibited in operation.]

Discussion

The Chairman said no doubt all present had come with great expectations, anticipating much pleasure in hearing of the latest developments of one of the most interesting and valuable applications of modern science to useful purposes - electric telegraphy. But whatever their expectations they must have been more than realised by the exceedingly lucid exposition by Prof. Thompson of a most intricate and difficult subject; so lucid, in fact, had it been that probably few realised how intricate it was. He felt with Prof. Thompson that perhaps in the immediate future the application of wireless telegraphy to practical purposes was not quite so wide as some might have anticipated and hoped; but at the same time there were purposes to which they might reasonably hope it might be applied, such, for example, as communication between the shore and lightships, and possibly between ship and ship. It was satisfactory to learn that means were being sought for, and had been to some extent found, of differentiating one telegraphic signal sent through space from another by tuning. That was to him a particularly interesting point, and the explanations which had been given of the methods adopted by Prof. Oliver Lodge for obtaining the transmission of a particular message, and the receipt of that message by the particular person intended to receive it were specially valuable. Obviously it would be very inconvenient if messages sent through space were indifferently receivable by everyone who chose to play the part of an eavesdropper. That condition of things would somewhat resemble that described in one of Hans Christian Andersen’s stories, where the fumes coming from a pipkin revealed to everyone who chose to smell them what each particular person was having for dinner. It was not very desirable that that kind of curiosity should be gratified in connection with telegraphy, and it seemed to him that the uses of telegraphy through space would be very much limited if this sort of thing could not be prevented. Prof. Lodge’s line of experiment, however, seemed to tend in that direction, and to show the means of confining a message to the person intended to receive it. He was sure Prof. Thompson would be pleased to answer any questions on any point that had not been made clear if there were any such, any questions which could arise having been already answered in anticipation. If no one had any such query to put he would conclude by proposing a hearty vote of thanks to Prof. Thompson for his paper.

The vote of thanks was carried unanimously.

† Many years before, Prof. Joseph Henry had transmitted induced electric sparks from one circuit to another in different floors of a building. Doubtless, these were oscillatory; but it is impossible, at this time, to determine whether the arrangements were such as to produce true travelling waves, or whether the action was (like Lodge’s later experiment of the two syntonic circuits) merely one of electromagnetic induction.