Wireless Telegraphy and Telephony
Forfatter: Alfred P. Morgan
År: 1917
Forlag: The Norman W. Henley Publishing Company
Sted: New York
Udgave: Third Edition, Fully Illustrated
Sider: 33
UDK: 621.396.1 Mor
A practical Treatise on Wireless Telegraphy and Telephony, giving Complete and Detailed Explanations of the Theory and Practice of Modern Radio Apparatus and its Present Day Applications, together with a chapter on the possibilities of its Future Development
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Il6 WIRELESS TELEGRAPHY
ends of this coil are connected to the wires leading from
the transmitter and battery. The varying currents of elec-
tricity, 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 approxi-
mately twice the resistance when illuminated.
This property of the cell was immediately applied to the