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A History of Wireless Telegraphy (2nd edition, revised), J. J. Fahie, 1901, pages 80-91:
PROFESSOR JOHN TROWBRIDGE--1880.
Such are a few of the early instances noted of the extreme sensitiveness of the telephone, by the aid of which the problem of wireless telegraphy was now to be attacked with a fair measure of success, and advanced a long way towards a practical solution.
Mr J. Gott, then superintendent of the Anglo-American Telegraph Company at St Pierre, was, I believe, the first to suggest the employment of the telephone in this connection. In a brief communication, published in the 'Jour. Inst. Elec. Engs.' (vol. vi. p. 523), he says: "The island of St Pierre is, perhaps, better insulated than most places. Hundreds of yards from the station, if a wire be connected to earth, run some distance, and put to earth again, with telephone in circuit, the signals passing through the cable can be heard."
There are two offices on the island,--one used for repeating the cable business on the short cables between Sydney, C.B., and Placentia, N.F., and operated by the Morse system, with a comparatively powerful battery; the other is the office at which the Brest and Duxbury cables terminate, and is furnished with very delicate instruments--the Brest cable, which is upwards of 2500 miles long, being operated by Thomson's exceedingly sensitive dead-beat mirror galvanometer; whilst on the Duxhury cable the same inventor's instrument, the siphon recorder, is used. The Brest instrument was found seriously affected by earth-currents, which flowed in and out of the cable, interfering very much with the true currents or signals, and rendering it a difficult task for the operator to decipher them accurately. The phenomenon is not an uncommon one; and the cause being attributed to the ground used at the office, a spare insulated wire, laid across the island, a distance of nearly three miles, and a metal plate connected to it and placed in the sea, was used in lieu of the office ground. This had a good effect, but it was now found that part of the supposed earth-currents had been due to the signals sent by the Morse operator into his wire, for when the recorder was put in circuit between the ground at the cable office and the sea ground--three miles distant--the messages sent by the Morse were clearly indicated,--so clearly, in fact, that they were automatically recorded on the tape.
It must be clearly understood that the two offices were in no way connected, nor were they within some 200 yards of each other; and yet messages sent at one office were distinctly read at the other, the only connection between the two being through the earth, and it is quite evident that they could be so read simultaneously at many offices in the same neighbourhood. The explanation is clear enough. The potential of the ground at the two offices is alternately raised and lowered by the Morse battery. The potential of the sea remains almost, if not wholly, unaffected by these, and the island thus acts like an immense Leyden jar, continually charged by the Morse battery and discharged, in part, through the short insulated line. Each time the Morse operator depressed his key he not only sent a current into his cable, but electrified the whole island, and this electrification was detected and indicated on the recorder. 50
As the result of these experiences, Mr Gott gave it as his opinion that "speaking through considerable distances of earth without wires is certainly possible with Bell's telephone, with a battery and Morse signals."
Professor John Trowbridge of Harvard University, America, was, however, the first to systematically study the problem, and to revive the daring project of an Atlantic telegraph without connecting wires, and the less ambitions but equally useful project of intercommunication between ships at sea. 51 In fact, Trowbridge's researches may truly be said to form a new starting-point in the history of our subject, for, as we shall see later on, it is chiefly to him that Messrs Preece, Bell, and probably other experimenters in this field, owe their inspirations. 52 His investigations, therefore, deserve to be carefully followed.
The observatory at Harvard transmits time-signals from Cambridge to Boston, a distance of about four miles, and the regular recurrence of the beats of the clock afforded a good means of studying the spreading of the electric currents from the terminal of the battery which is grounded at the observatory. In all the telephone circuits between Boston and Cambridge, in the neighbourhood of the observatory line, the ticking of the clock could be heard. This ticking had been attributed to induction, but this, according to Prof. Trowbridge, is an erroneous conclusion, as he shows by a mathematical analysis into which we need not enter. The result goes to show that, with telephones of the resistance usually employed, no inductive effect will be perceived by the use of even ten quart Bunsen cells between wires running parallel, a foot apart, for a distance of 30 or 40 feet.
For this and other reasons, he says, it is impossible to hear telephonic messages by induction from one wire to another, unless the two run parallel and very close to each other for a long distance. This distance generally exceeds the limit at which the ordinary Bell telephone ceases to transmit articulate speech. The effects which have usually been attributed to induction are really, he says, due to the earth connections and to imperfect insulation.
Having determined in this manner that the echoes of the time-signals observed on the telephone lines were not due to induction, but to leakage from the clock circuit, Prof. Trowbridge proceeded to study the extent of the equally electrified or equi-potential surfaces of the ground surrounding the clock battery. His method of exploration was to run a wire 500 or 600 feet long to earth at each end, including a telephone of 50 to 60 ohms resistance. Evidence of a current in this exploratory circuit was plainly shown by the ticking sound which making and breaking the circuit caused in the telephone, and the time-signals could be distinctly heard in a field 220 yards from the observatory where one earth of the time-signal wire is located. At a distance of a mile from the observatory, and not in the direct line between that place and the Boston telephone office, the time-signals were heard by connecting through a telephone the gas-pipes of one building with the water-pipes of another only 50 feet apart. In another experiment at the Fresh Pond lake in Cambridge, signals sent from Boston to Waltham (ten to twelve miles) were heard by simply dipping the terminal wires of the telephone in the lake, and some distance apart, where they must have been far away (?four miles) from the battery earth.
Prof. Trowbridge performed a large number of similar experiments, varied in every way, all going to prove (1) that a battery terminal discharging electricity to earth is the centre of waves of electrical energy, ever widening, and ever decreasing in strength or potential as they widen; and (2) that on tapping the earth in the way described at two points of different potentials (not very distant, if near the central source, and more removed the farther we recede from the source) we can obtain in the telephone evidence of their existence. Prof. Trowbridge then goes on to say:--
"In a discussion on the earth as a conductor, Steinheil says: 'We cannot conjure up gnomes at will to convey our thoughts through the earth. Nature has prevented this. The spreading of the galvanic effect is proportional . . . to the square of the distance; so that, at the distance of 50 feet, only exceedingly small effects can be produced. . . . Had we means which could stand in the same relation to electricity that the eye stands to light, nothing would prevent our telegraphing through the earth without conducting wires.' 53
"The telephone of Prof. Bell, though far from fulfilling the conditions required by Steinheil, is nevertheless our nearest approach to the desideratum.
"The theoretical possibility of telegraphing across the Atlantic without a cable is evident from the survey which I have undertaken. The practical possibility is another question. Powerful dynamo electric machines could be placed at some point in Nova Scotia, having one end of their circuit grounded near them and the other end grounded in Florida, the connecting wire being of great conductivity and carefully insulated throughout. By exploring the coast of France, two points on surface lines not at the same potential could be found; and by means of a telephone of low resistance, Morse signals sent from Nova Scotia to Florida could be heard in France. Theoretically, this is possible but practically, with the light of our present knowledge, the expenditure of energy on the dynamoelectric machines would be enormous." 54
Professor Trowbridge has suggested the applicability of this method to the intercommunication of ships at sea. Let, he says, a steamer be provided with a powerful dynamo. Connect one terminal of the dynamo with the water at the bow of the steamer, and the other to a long wire, insulated except at its extreme end, dragging over the stern, and buoyed so as not to sink. The current from the dynamo will thus pass into the water and spread out over a large area, as before explained, saturating, so to speak, the water with electricity. Suppose this current be interrupted by any suitable means, say one hundred times a second. Let the approaching steamer be provided with a telephone wire, the ends of which dip into the water at her bow and stern respectively. On entering the saturated area the telephone will respond to the interruptions of the dynamo by giving out a continuous buzzing sound. If now in the dynamo circuit we have a manipulating arrangement for breaking up the electric impulses into long and short periods, corresponding to the Morse alphabet, one ship can speak to the other. It is hardly necessary to add that by providing each steamer with a dynamo circuit and a telephone circuit reciprocal correspondence could be maintained, it being only necessary for the steamer desiring to listen to stop and disconnect the dynamo. The success of this method of communicating between ships in a fog depends upon the distance between the ends of the dynamo circuit and upon the strength of the current, or electrical impulses imparted to the water.
It is probable that a dynamo capable of maintaining one hundred incandescent lamps could establish a sufficient difference of potential between the water at the bow and at the end of a trailing wire, half a mile long, to affect a telephone on an approaching ship while yet half a mile distant.
In a discussion on Prof. Graham Bell's paper, read before the American Association for the Advancement of Science, 1884, Prof. Trowbridge described another plan, using instead of the telephone circuit a sensitive galvanometer connected up to a cross-arm of wire, whose ends dip into the water at each side of the ship. When one vessel comes within the area electrically saturated by another, the galvanometer will show how the equipotential lines are disturbed, and if a map of these lines be carefully traced we can fix the position of the approaching ship. He adds: "The method could also be applied to saturating the water around a rock, and you could take electrical soundings, so to speak, and ascertain your position from electrical maps carefully made out."
In a later paper published in the 'Scientific American Supplement,' February 21, 1891, Prof. Trowbridge discusses the phenomena of induction, electro-magnetic and static, as distinguished from leakage or earth conduction, and with reference to their employment in wireless telegraphy.
The hope, he says, that we shall be able to transmit messages through the air by electricity without the use of connecting wires is supposed by some to indicate its realisation at a future day. Let us examine how near we are at present to the realisation of this hope.
He supposes that the chief use of any method by which connecting wires could be dispensed with would be at sea in a fog. On land for considerable distances it is hardly probable that any electrical method could be devised in which air or the ether of space could advantageously replace a metallic conductor. The curvature of the earth would probably demand a system of frequent repetition, which is entirely obviated by the use of a wire. If, however, an electrical or magnetic system could be made to work through the air even at the distance of a mile, it would be of very great use at sea in averting collisions; for any system of signals depending upon the use of fog-horns or fog-whistles is apt to mislead on account of the reflection of the sound from layers of air of different densities and from the surface of the water. The difficulty of ascertaining the direction of a fog-horn in a thick fog is well known. The waves of sound, even if they are carefully directed by a trumpet or by parabolic reflectors, diverge so rapidly that there is no marked difference in the intensity between a position in the direct line and one far to one side.
The most obvious method of signalling by electricity through the air is by electro-magnetic induction. Suppose we have a coil of copper wire consisting of many convolutions, the ends of which are connected with a telephone (fig. 7). If we place a similar coil, the ends of which are connected to a battery through a key, within a few feet of the first and parallel to it, each time the current is made and broken in the battery coil instantaneous currents are produced by induction in the other coil, as can be heard by the clicks in the telephone.
To illustrate induction at a distance, Prof. Joseph Henry placed a coil of wire, 5½ feet in diameter, against a door, and at a distance of 7 feet another coil of 4 feet diameter. When contact was made and broken with a battery of eight cells in the first coil, shocks were felt when the terminal wires of the second were placed close together on the tongue.
In all such methods the wires or coils which produce an electrical disturbance in a neighbouring coil are never more than a few feet apart. Now let us suppose that a wire is stretched ten or twelve times, to and fro, from yard-arm to yard-arm of a steamer's foremast, and connected at the ends either with a powerful battery or dynamo, or with a telephone, as may be required either for signalling or for listening. Let an approaching steamer have a similar arrangement. If now the current on one vessel be interrupted a great number of times per second, a musical note will be heard in the telephone of the other vessel, and vice versâ. The sound will be strongest when the two coils are parallel to each other. If, therefore, the coils be movable the listener can soon find the position of greatest effect, and so fix the direction in which the signalling steamer is approaching.
It may not even be necessary to connect the telephone with the coil, for it has been found that if a telephone, pure and simple, be held to the ear and pointed towards a coil in which a current of electricity is rapidly interrupted, the makes and breaks will be heard, and this even when the wire coil of the telephone is removed, leaving only the iron core and the diaphragm. 55
Nothing could seem simpler than this, but, unfortunately, calculation shows that under the best conditions the size of the coils would have to be enormous. Prof. Trowbridge has computed that to produce an audible note in the telephone at a distance of half a mile, a coil of ten turns of 800 feet radius would be necessary; but it is evident that a coil of this size would be out of the question. Instead, however, of increasing the size of the coil beyond the practical limits of the masts and yard-arms, we could increase the strength of the current so as to be effective at the distance of half a mile; but, again, calculation shows that this strength of current would be beyond all practical limits of dynamo construction, unless we discover some method of tuning, so to speak, two coils so that the electrical oscillations set up in one may be able to evoke in the other sympathetic vibrations. 56
Since, then, we have little, apparently, to hope for from electro-magnetic induction in signalling through a fog, cannot we expect something from static induction? This form of induction can be well illustrated by an early experiment of Prof. Henry. An ordinary electrical machine was placed in the third storey of his house, and a metal plate 4 feet in diameter was suspended from the prime conductor. On the first floor or basement, 30 feet below in a direct line, was placed a similar plate, well insulated. When the upper plate was charged by working the machine, the lower plate showed signs of electrification, as was evidenced by its effect on the pith-ball electroscope. 57
The distance to which this electrical influence can be extended depends upon the charging power of the machine and the dimensions of the plate. If we could erect an enormous metal plate on a hill, insulated and powerfully charged, it is probable that its electrical influence could be felt at the distance of the horizon; but here, again, the question of practical limits comes in as a bar, so that, at the present time (February 1891), this method of signalling without wires seems as little practicable as the others.
After following me in this study of Prof. Trowbridge, the reader may well begin to despair, for while the learned Professor's investigations are extremely interesting, his conclusions are very disappointing. But the darkest hour is just before the dawn, and so it is in this case.
50 See now Salvá's curious anticipation in 1795 of this phenomenon, ante. The peculiarity, due to geological formation, is not confined to St Pierre; it is often met with in practice, though usually in lesser degrees. See some interesting cases, noted by C. K. Winter and James Graves, 'Jour. Inst. Elec. Engs.,' vol. i. p. 88, and vol. iv. p. 34.
51 Mr H. C. Strong of Chicago, Illinois, claims to have suggested in 1857, in a Peoria, Ill., newspaper, the possibility of communication between ships at sea by means of a wireless telegraph then recently invented by his friend Henry Nelson of Galesburg. See Mr Strong's letter in the New York 'Journal of the Telegraph,' August 15, 1877.
52 See infra. Professor Trowbridge's researches are given at length in a paper, "The Earth as a Conductor of Electricity," read before the American Academy of Arts and Sciences in 1880. See also 'Silliman's American Journal of Science,' August 1880, which I follow in the text.
53 See ante.
54 A writer in the 'Electrician' (vol. v. p. 212), commenting on this passage, says: "Prof. Trowbridge seems to overlook the advantage of employing large condensers between the dynamo machines and the earth. They would prove of great service in exalting the earth potentials at the terminal stations."
55 Mr Willoughby Smith was, I believe, the first in recent times to observe these effects. See his paper on "Volta-Electric Induction," 'Jour. Inst. Elec. Engs.,' vol. xii. p. 457. But exactly similar effects, mutatis mutandis, were described by Page in 1837, to which he gave the name of Galvanic Music, and which he found to be due to the fact that iron when magnetised and demagnetised gave out a sound. De la Rive, in 1843, rightly traced this sound to the slight elongation of iron under the magnetic strain--a fact which, in its turn, was first observed by Joule in 1842. For Page's discovery see the 'Magazine of Popular Science,' 1837, p. 237.
56 Prof. Oliver Lodge is now engaged on this very problem. See 'Jour. Inst. Elec. Engs.,' No. 137, p. 799.
57 See an excellent account of Henry and his work in the New York 'Electrical Engineer,' January 13, 1892, and succeeding numbers, from the pen of his daughter, Mary A. Henry. Abstracts of these papers are given in the 'Electrician,' vol. xxviii. pp. 327, 348, 407, 661.
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