George Bartlett Prescott

“ In Fig. 21 the plates are arranged as in the ordinary form of a condenser. An initial pressure is put upon them by a screw passing through a portion of the solid frame of the instrument. The diaphragm in vibrating varies the distance between the plates. This alters their static charge, and affects also the electric tension of the line. The resistance of a conductor is dependent upon its shape. If an isometric block of metal be drawn out into a wire, its resistance may be indefinitely increased. This fact lies at the basis of several ingenious telephones invented by Mr. Edison. ”

“ When used in connection with his articulating transmitter, which was developed at a later date, articulate words have been received upon it; and when a duplicate of the instrument is inserted in a closed circuit, which includes a galvanic battery, it becomes a speaking telephone capable of acting both as a transmitter and as a receiver. Mr. Gray did not know at that time, however, that he could use it as a transmitter, although he had fully demonstrated its ability to receive sounds of varying quality. ”

“ Edison has succeeded better with those telephones which produce a variation in the resistance of the circuit than with those which depend for their action upon a variation of the electro-motive force or static charge. An instrument very similar to the carbon-transmitting telephone is shown in Fig. 1, the essential difference being that the carbon is replaced by bibulous paper moistened with water. This semi-conductor, like the carbon, changes its resistance under the influence of varying pressure. ”

“ Edison proposes to compress air with the harmonic engine, and use it as a motor for propelling sewing-machines and other light machinery. It appears to be considerably in advance of other electric engines, and through its agency electricity may yet become a valuable motive-power. ”

“ In his specification he states that he has invented a new art of telegraphically transmitting vocal sounds whereby the tones of the human voice can be transmitted through a telegraphic circuit, and reproduced at the receiving end of the line, so that actual conversations can be carried on by persons at long distances apart, and that he has devised an instrument capable of vibrating responsively to all the tones of the human voice, and by which they are rendered audible. ”

“ Another conception which occurred to him at this time was that of applying the invention to a printing telegraph, so that each type would be actuated by a tone of a particular pitch. Being well conversant with the facts, so far as they were then known in the science of electricity and magnetism, he was fully prepared to avail himself of what had already been done in that line. ”

“ Gray undertook to secure letters patent for some of his conceptions, and with that purpose in view had models made, illustrating the idea of a series of transmitters, each tuned to a different pitch, showing a method of receiving musical or other sounds telegraphically, through the medium of animal tissue. In June, 1874, he filed another case, substituting for the animal-tissue receiver an electro-magnet combined with a hollow box of tinned iron, having an opening in one side, while the other was held over the poles of the magnet at such a distance from it as would produce the best effect. ”

“ Edison's explanation of its mode of action is, that the degree of pressure with which the carbon rests against the plates is varied during the vibration. Thus, after a movement toward the right, the diaphragm suddenly stops and the carbon presses, in virtue of its inertia, on the plate P.
An advantage which the magneto-telephone has over the carbon-telephone is that its diaphragm does not touch anything, and can therefore vibrate with perfect freedom. On the other hand, the diaphragm of the carbon-telephone presses with considerable force upon the carbon. In the form shown in Fig. ”

“ The vibrations thus imparted were to be transmitted through an electric circuit to the receiving station, in which circuit was included an electro-magnet of ordinary construction, acting upon a diaphragm to which was attached a piece of soft iron, and which diaphragm was stretched across a receiving vocalizing chamber, similar to the corresponding vocalizing chamber at the transmitting end. ”

“ Another form, acting on much the same principle, is illustrated by Fig. 6. It is called the inertia-telephone, though it is hardly certain that its action is to be attributed solely to inertia. The carbon, C, is placed between two metallic plates, one of which is fastened to the diaphragm, and the other is held by a screw, bearing in a framework attached to the diaphragm by insulating supports.
When vibrating, the whole system moves, instead of the plate P alone, as in the ordinary carbon-transmitter. Mr. ”

“ Small branches extend from the fork-arms into a box containing a miniature pump having two pistons, one attached to each arm. Each stroke of the pump raises a very small quantity of water, but this is compensated for by the rapidity of the strokes Mr. ”