The earliest telephones showed a striking degree of similarity in terms of their operating principles, a fact which is all the more remarkable when we realize that they were developed simultaneously by different people who were all largely working in secret.
Bell's telephone, in a modified form, is the standard of today. It is now used as a receiver only, a more efficient transmitter, depending upon entirely different principles, having been invented.
A very interesting fact, and one which might have changed the entire commercial status of the telephone industry is that in 1868 Royal E. House of Binghamton, N. Y., invented and patented an " electro-phonetic telegraph," which was capable of operating as a magneto-telephone, in the same manner as the instruments subsequently devised by Bell. House knew nothing of its capabilities, however, unfortunately for him.
The instrument is provided with a sounding diaphragm of pine wood stiffened with varnish, mounted in one end of a large sound-amplifying chamber so formed as to focus the sound waves at a point near its mouth, where the ear was to be placed to receive them. The electromagnet adapted to be connected in the line circuit had its armature connected by a rod with the center of the wooden diaphragm.
By this means any movements imparted to the armature by fluctuating currents in the line were transmitted to the diaphragm, causing it to give out corresponding sounds ; and any movements imparted to the diaphragm by sound waves were transmitted to the armature, causing it to induce corresponding currents in the line. Two of these instruments connected in a circuit would act alternately as transmitters and receivers in the same manner as Bell's instruments.
IT has been shown that in order to transmit speech by electricity it is necessary to cause an undulatory or alternating current to flow in the circuit over which the transmission is to be effected, the strength of this current must at all times be in exact accordance with the vibratory movements of the body producing the solid.
Bell's transmitter was used as the generator of this current ; as a dynamo, in fact, the energy for driving which was derived from the sound waves set up by the voice. The amount of energy so derived was, however, necessarily very small and the current correspondingly weak, and for this reason this was not a practical form of transmitter, except for comparatively short lines.
Elisha Gray devised a transmitter which, instead of generating the undulatory current itself, simply served to cause variation in the strength of a current generated by some separate source. He accomplished this by mounting on his vibrating diaphragm, a metal needle, extending into a fluid of low conductivity, such as water. The needle, formed one terminal of the circuit, the other terminal being the metal pin, extending up through the bottom of the containing vessel. The vibration of the diaphragm caused changes in the resistance of the path through the fluid, and corresponding changes in the strength of the current.
Bell also used a liquid transmitter in which a conducting liquid was held in a conducting vessel forming one terminal of the circuit. The other terminal was a short metallic needle carried on the diaphragm and projecting slightly into the liquid, so that the area of contact between the liquid and the needle would be varied to better advantage by the vibration of the diaphragm than if the needle were immersed a greater distance into the fluid.
These instruments, unlike that of Reis, simply caused variations in the resistance of the circuit, and thereby allowed a continuous but undulatory current to pass over the line, the variations in which were able to reproduce all the delicate shades of timbre, loudness, and pitch necessary in articulate speech.
Gray embodied in his apparatus the main principle upon which all successful battery transmitters are based, but it was not long before a much better means was devised for putting it into practice.
In 1877 Emile Berliner of Washington, D. C, applied for a patent on a transmitter depending upon a principle previously pointed out by the French scientist, Du Moncel, that if the pressure between two conducting bodies forming part of an electric circuit be increased, the resistance of the path between them will be diminished, and conversely, if the pressure between them be decreased, a corresponding increase of resistance will result.
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