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Telephony

current, telephone, telegraph, speech, variations, diaphragm and tele

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TELEPHONY, Wireless. Prior to the transmission of speech 'without wires by means of ether waves developed electrically several more or less successful wireless methods of transmitting speech were utilized experimentally; For instance, a device termed the photophone was employed by Bell, the inventor of the tele phone, in which light rays were utilized. In this device, Fig. 1, a beam of light is concen trated by means of a lens / upon a small con cave mirror carried on the exact centre of a diaphragm attached to a mouthpiece P. The rays reflected from the mirror are directed upon a receiving apparatus by a double convex mirror P suitably positioned. In the receiving appara tus a small selenium cell is placed in the focus of a parabolic reflector P. This cell is made part of an electric circuit consisting of a tele phone receiver t and an ordinary dry cell b. Selenium possesses the property of varying its electrical resistance with the variations of light to which it is exposed. Hence when speech is uttered at the mouthpiece the diaphragm vi brates in consonance with the sound waves of the voice. These vibrations cause variations in the light rays reflected from the small mirror, which variations are reproduced at the selenium cell. This in turn causes variations in the re sistance and current of the telephone circuit and the speech spoken at the mouthpiece is repro duced in the telephone receiver. , The rate of air vibrations or oscillations due to speech ranges from about 20 to 20,000 per second. As the length of an electric wave or oscillation is equal to the speed of electricity, 186,000 miles per second, divided by the number of oscillations per second, it is evident that the length of electrical oscillations of 20 to 20,000 per second is comparatively great. Hence to attempt to transmit without wires electrical os cillations set up primarily by the voice by means of a telephone transmitter, aerial wires about 15 miles in length would be necessary, which is not feasible.

For reasons that will become obvious it may not be amiss to refer here to the Van Ryssel berghe system of Simultaneous Telegraphy and Telephony which for years has been in serv ice on a large scale in this country. This sys

tem is briefly described in the article on TELEG RAPH Y. Any action of the telegraph sig nals upon the telephone receiver is prevented by placing induction coils in the circuit which makes the rise and fall of the telegraph currents so gradual that the diaphragm of the telephone re ceiver is merely deflected and does not produce an appreciable sound. Upon this gradual rise and fall of the telegraph current is superposed the rapid vibrations of the speech waves set up by the telephone transmitter on the same wire. These minute vibrations do not affect the tele graph instruments. Quoting from Mayer, W., 'American Telegraphy,' p. 347, the the ory of the operation of simultaneous telegraphy and telephony may be briefly outlined as fol lows: Assuming, for example only, the strength of the telegraph current to be 2.000 and that of the telephone current to be 1. If, while the diaphragm of the telephone receiver is attracted or in process of gradual attraction by a telegraph current of positive direction, a tele phone current of similar direction be trans mitted the total current will be suddenly in creased to 2,001 and the diaphragm will be given a sudden minute impulse toward its mag net. Should then a negative telephone current follow, the telegraph current remaining as be fore, the current on the line will be suddenly reduced to 1 999 and the diaphragm by its own tension recedes rapidly from its magnet. In the actual operation of these systems many hun dreds of pulsatory currents might be sent dur ing the time taken to transmit one telegraphic signal, and thus, while the diaphragm is being gradually attracted to or is gradually receding from its magnet owing to variations in the tele graph current, at the same time it may be mak ing hundreds of intermediate forward and backward movements of less amplitude, due to the variations of the line currents caused by the telephone transmitter in transmitting speech waves." In present day language the foregoing would be termed a modulation of the telegraph current in accordance with the variations of current due to voice waves.

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