The New Student's Reference Work/Telephone
Tel′ephone, an instrument for reproducing speech at a distance from its source. Perhaps the simplest solution of this problem is to connect two stretched membranes by a stretched string. The pulsations of the human voice near one membrane will set it into vibration; while the string will transmit these vibrations to the second membrane, which by its vibrations reproduces the original sound for a listening ear. This is the so-called mechanical or acoustic telephone; but it is limited to such short distances — less than a mile — that we shall not discuss it.
The first electric telephone was undoubtedly invented by Philip Reis in Germany in 1861. To him we owe also the name.
Reis' instrument, like all others of this class, consisted of three parts: one which transmitted the message; the line or wire over which the message was carried; and a receiver which finally transformed the message into sound-waves for the listening ear. His transmitter was very simple, being merely a diaphragm carrying a metallic point which alternately made and broke an electric current when the diaphragm was set in vibration by the human voice. The line was an ordinary metallic wire. The receiver was an electromagnet mounted on a sounding-box. As the iron core of the magnet was alternately magnetized and demagnetized, it emitted a sound — the well-known Page effect, discovered by Dr. Page, an American, in 1837. In this manner Reis was able to transmit the proper pitch and intensity; but he was totally unable to convey the correct quality. (See Acoustics for the three distinguishing features of a sound.) The proper method of transmitting the true quality of a note is the joint discovery of the late Elisha Gray and of Graham Bell.
The essential point in the solution — and, therefore, the essential discovery of these gentlemen — is the use of continuous electric currents instead of the interrupted ones employed by Reis.
The instrument which Bell patented and perfected, in universal use to-day as a receiver, consists of a soft-iron diaphragm, held by its periphery, in front of an electromagnet. When an electric current, even a very minute one, is passed through the coil of the electromagnet, the iron disk is attracted. If the current is a pulsating one, the disk is set into forced vibration, accurately following the variations of the current.
In figure 1 the diaphragm is shown at D and the electromagnet at A. The line wires which convey the message to the receiver are attached at E.
For a transmitter, a microphone of the carbon type is almost universally employed. Two forms are in very common use: one, the Blake transmitter, is made up of a piece of gas carbon resting against a platinum pin; the other, devised by Henning, consists of a layer of hard carbon granules. The accompanying diagram, figure 2, shows at D the diaphragm which is set in vibration by the human voice. Between the carbon blocks, B and E, are placed the carbon granules which act as a microphone. See Microphone.
Telephone lines' construction differs essentially from that of telegraph and of electric-light lines. For this large subject, however, the reader is referred to a special treatise. The attention of the student is called, however, to the recent improvements introduced into submarine telephony by Professor Pupin of Columbia College. Hitherto the maximum working-distance of the telephone for under-water lines has been about 25 miles. Dr. Pupin has extended this to 250 miles. For Pupin’s account see Transactions of the American Institute of Electric Engineers, Vol. XV (1899) and succeeding volumes.