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A History of Wireless Telegraphy (2nd edition, revised), J. J. Fahie, 1901, pages 94-99:
PROFESSOR A. H. DOLBEAR--1882.
Prof. Dolbear of Tuft's College, Boston, was also, about the same time as Graham Bell, engaged on the problem of a wireless telegraph, and produced a very simple and workable apparatus, which he patented in the United States (March, 1882), and of which he gave a description at a meeting of the American Association for the Advancement of Science in the following August. I take the following account from his specification as published in the 'Scientific American Supplement,' December 11, 1886:--
"In the diagram, A represents one place (say Tuft's College) and B a distant place (say my residence).
"C is a wire leading into the ground at A, and D a wire leading into the ground at B.
"G is an induction coil, having in the primary circuit a microphone transmitter T, and a battery f', which has a number of cells sufficient to establish in the wire C, which is connected with one terminal of the secondary coil, an electro-motive force of, say, 100 volts. The battery is so connected that it not only furnishes the current for the primary circuit, but also charges or electrifies the secondary coil and its terminals C and H'. 59
"Now, if words be spoken in proximity to transmitter T, the vibration of its diaphragm will disturb the electrical condition of the coil G, and thereby will vary the potential of the ground at A, and the variations at A will cause corresponding variations of the potential of the ground at B, and the receiver R will reproduce the words spoken in proximity to the transmitter, as if the wires C D were in contact, or connected by a third wire.
"There are various well-known ways of electrifying the wire C to a positive potential far in excess of 100 volts, and the wire D to a negative potential far in excess of 100 volts.
"In the diagram, H H' H2 represent condensers, the condenser H' being properly charged to give the desired effect. The condensers H and H2 are not essential, but are of some benefit; nor is the condenser H' essential when the secondary coil is otherwise charged. I prefer to charge all these condensers, as it is of prime importance to keep the grounds of wires C and D oppositely electrified, and while, as is obvious, this may be done by either the batteries or the condensers, I prefer to use both."
In the article from which I am quoting the author gives some additional particulars which are worth repeating. "My first results," he says, "were obtained with a large magneto-electric machine with one terminal grounded through a Morse key, the other terminal out in free air and only a foot or two long; the receiver having one terminal grounded, the other held in the hand while the body was insulated, the distance between grounds being about 60 feet. Afterward, much louder and better effects were obtained by using an induction coil having an automatic break and with a Morse key in the primary circuit, one terminal of the secondary grounded, the other in free air, or in a condenser of considerable capacity, the latter having an air discharge of fine points at its opposite terminal. At times I have employed a gilt kite carrying a fine wire from the secondary coil. The discharges then are apparently nearly as strong as if there was an ordinary circuit.
"The idea is to cause a series of electrical discharges into the earth at a given place without discharging into the earth the other terminal of the battery or induction coil--a feat which I have been told so many, many times was impossible, but which certainly can be done. An induction coil isn't amenable to Ohm's law always! Suppose that at one place there be apparatus for discharging the positive pole of the induction coil into the ground, say 100 times per second, then the ground will be raised to a certain potential 100 times per second. At another point let a similar apparatus discharge the negative pole 100 times per second; then between these two places there will be a greater difference of potential than in other directions, and a series of earth-currents, 100 per second, will flow from the one to the other. Any sensitive electrical device, a galvanometer or telephone, will be disturbed at the latter station by these currents, and any intermittence of them, as can be brought about by a Morse key in the first place, will be seen or heard in the second place. The stronger the discharges that can be thus produced, the stronger will the earth-currents be of course, and an insulated tin roof is an excellent terminal for such a purpose. I have generally used my static telephone receiver in my experiments, though the magneto will answer.
"I am still at work upon this method of communication, to perfect it. I shall soon know better its limits on both land and water than I do now. It is adapted to telegraphing between vessels at sea.
"Some very interesting results were obtained when the static receiver with one terminal was employed. A person standing upon the ground at a distance from the discharging point could hear nothing; but very little, standing upon ordinary stones, as granite blocks or steps; but standing on asphalt concrete, the sounds were loud enough to hear with the telephone at some distance from the ear. By grounding the one terminal of the induction coil to the gas or water pipes, leaving the other end free, telegraph signals can be heard in any part of a big building and its neighbourhood without any connection whatever, provided the person be well insulated."
When we come to speak of the Marconi system, we shall see how near Dolbear got to that discovery, or perhaps I should say how nearly he anticipated it. Comparing the arrangement, fig. 8 (especially when, as stated, a Morse key and automatic interrupter were used in place of the microphonic transmitter), with Marconi's, fig. 40, it will be seen that they are practically identical in principle. Dolbear's acute observation of the heightened effects obtained by projecting into free air the ungrounded terminals of the sending and receiving apparatus is his own discovery; while his use of condensers (answering to Marconi's capacity areas) and gilt kites carrying fine wire was another step in the right direction. Of course he does not use the Branly receiver, or the Righi sparking arrangement shown in fig. 40 (they were not known in 1882), but as regards the latter Marconi has himself discarded it in recent times, using a single spark-gap, which even is not absolutely necessary for the production of waves, leaving the secondary coil "open" alone sufficing. 60
Prof. Dolbear's account of the action of his apparatus is in places a little puzzling, which, perhaps, can hardly be wondered at, for Hertz had not yet come to make clear the way which the American professor saw but as in a glass darkly. There can, however, be little doubt that he was using very long electric waves in 1882 (that is, five or six years before Hertz), and in much the same way as Marconi does now. When, for instance, he whistled into his microphonic transmitter, making it vibrate say 4000 times per second, did he not in effect start electric (now called Hertzian) waves 186000/4000 = 46½ miles long? We can easily see this now, but in 1882 the results were not so well understood Dolbear was inclined to attribute them to some kind of ether action, obscure cases of which were then cropping up and attracting attention in the electrical world. 61
Others thought that the results were "only extraordinary cases of electro-static induction." Thus Prof. Houston, who saw some of Dolbear's experiments and had himself repeated them, says: "The explanation of the phenomenon as I understand it would appear to be this--One of the plates of the receiver (that is, of the electro-static telephone) being connected through the body of the experimenter to the ground, partakes of the ground potential, while the other plate is en rapport with the free end of the sending apparatus by a line of polarised air particles. The experiment is simply an exceptional application of the principles of electro-static induction, and I am not at all sure that it is not susceptible of a great increase in delicacy, in which case it would become of considerable commercial value." 62
Prof. Dolbear's friends in America are now claiming for him the discovery of the art of wireless telegraphy à la Marconi. They argue that Marconi arranges and works his circuits in the way substantially shown in Dolbear's patent of 1882; that he employs Dolbear's transmitting devices (induction coil, battery, and Morse key), as well as his aerial and ground connections on the sending and receiving apparatus. Dolbear emitted electric waves of many miles long, and received them on his electro-static telephone; Marconi, by using the same means, emits waves of many feet long, and receives them on a Branly coherer. Where, they ask, is the difference? Marconi's receiver is admitted to greatly extend the signalling range, but this does not affect the principle of the art, only its practical value, as to which they recall the fact that Graham Bell's telephone, as patented in 1876, was practically inoperative, yet the patent secured to him the honour and profit of the invention, as it was held that the principle was there, though in an imperfect form. All this is true, and I hope that Dolbear's early and for the time extraordinary experiments will always be remembered to his credit, but this, I think, should be done without detracting from the merit due to Marconi for his successful and, as I believe, entirely independent application of the same principle. But of this more anon.
59 The diagram, which we have carefully copied, does not show how this is done, but the practical reader will easily supply the necessary connections.
60 Broca, 'La Télégraphie sans Fils,' p. 89.
61 See, for example, 'Telegraphic Journal,' February 15, 1876, p. 61, on The "Etheric" Force.
62 'Scientific American Supplement,' December 6, 1884. At first, Dolbear's estimate of distance was modest-"half a mile at least," but it is said that recently he has worked his apparatus up to a distance of thirteen miles.
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