CHAPTER 2
TRANSMITTERS AND RECEIVERS
17.
Introduction to Telephony.
a. Historical background of telephone.
(1) The combination of principles on which the operation of the telephone is based was discovered in 1875 by
Alexander Graham Bell. At once, Bell started a series of experiments to perfect practical instruments for the transmission
of sound over wires. After 9 months, the first complete sentence was transmitted, over an indoor line extending a distance
of about 150 feet. By 1877, an outdoor line from Boston to Cambridge, a distance of about 2 miles, was in use. The early
instruments were crude and not too effective. They operated on the principle that a diaphragm, vibrating in a magnetic
field, can induce an electric current in a wire. The same device was used as both transmitter and receiver. The strongest
magnets and best diaphragms then available would not permit transmissions over long distances.
(2) One year after the invention of the original telephone, however, the perfection of the Blake transmitter
operating on the principle that the vibration of a diaphragm can vary the strength of an already existing electric current
was completed. Immediately, the problem was presented of establishing a means to connect the lines of different
subscribers, whenever they wished to talk. This problem was overcome in 1878 with the opening of the first central
office, or exchange, in New Haven. By 1900, means were evolved for the telephone user and exchange to signal (ring)
each other when calls were to be initiated or completed. Present-day telephone systems provide vast improvements over
those of earlier design and construction in the distances over which satisfactory transmission can be accomplished,
dependability of established plant facilities, and the quality of the reproduced signals.
b. Basic functions of telephone system.
(1) By means of the telephone, conversations may be held over great distances. To accomplish this, the
sound waves of speech must be converted into a form of energy that can be transmitted efficiently over wires.
The conversation is effected by electrical waves (current) in the transmitter of the speaker's telephone set. There,
electrical waves are created which correspond to sound waves both in waveform and frequency. The electrical waves are
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