FIG. 132.—Diagram showing the principle and construction of the telephone receiver.
The receiver, as has already been explained, consists of a thin iron disk, placed very near but not quite touching the end of a small bar of steel, permanently magnetized, about which is wound a coil of fine insulated wire. The ends of this coil are connected to the wires leading from the transmitter and battery. The varying currents of electricity, produced by the transmitter, generate corresponding changes in the magnetism of the receiving instrument, and thus, by alternately attracting and repelling the diaphragm, cause it to vibrate and emit sounds.
FIG. 133.—The photophone.
Alexander Graham Bell, the ingenious inventor of the telephone, with the aid of Sumner Tainter was the first who achieved success in the attempts to transmit speech without the aid of connecting wires between the source of intelligence and the receptor.
In 1873 Willoughby Smith announced that the element selenium possesses the abnormal property of changing its electrical resistance under the influence of light. Bell and Tainter took advantage of this discovery, and devised selenium cells, in which selenium is formed into narrow strips between the edges of broad conducting plates of brass. The resistance of the cell in the darkness is approximately twice the resistance when illuminated.
FIG. 134.—Photophone receiving apparatus.
FIG. 135.—Photophone transmitting apparatus, using acetylene flame to furnish light.