File:The practical telephone handbook and guide to the telephonic exchange (1906) (14569464338).jpg

Identifier: practicaltelepho00pool (find matches)
Title: The practical telephone handbook and guide to the telephonic exchange
Year: 1906 (1900s)
Authors: Poole, Joseph
Subjects: Telephone
Publisher: New York, Macmillan Co.
Contributing Library: Northeastern University, Snell Library
Digitizing Sponsor: Boston Library Consortium Member Libraries

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stance, the variation would then represent /oth, or 5 per cent.,of the whole resistance. But suppose, now, it is directly inthe circuit of a long line the total resistance of which is 1000ohms, a variation of only i-oVoth part, or Ol per cent., wouldthen be obtained, and the effect on the receiver would becomparatively feeble. If the resistance of the transmitter 56 PRACTICAL TELEPHONE HANDBOOK were increased1 to, say, 100 ohms, and the variation was in-creased in the same proportion to 20 ohms, then in a 1000-ohm circuit we should have a variation of -gV^hj or 2 per cent.,which is twenty times as great. This latter is the principlefollowed when a common battery is used for exchange working,the current for working the subscribers transmitters beingsent from the exchange. By maldng use of an induction coil,the resistance of the circuit containing the transmitter can thusbe kept very low, and the relative variation made very large.By making a secondary coil of a great number of turns of
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Fig. 40 wire, the currents induced in it by the variations in the primarycoil will have a high E.M.F., and will be able to overcomemuch resistance in the line and instruments at the other endwith comparatively little loss of strength. Microphones.—The next step in the direction of improve-ment was the discovery by Prof. Hughes of London in 1878of the fact that any loose contact between conductors wouldact as a telephonic transmitter, owing to the variations of re-sistance caused between them by the impact of the soundwaves. The simple means he employed caused much aston-ishment. Three nails arranged as in Fig. 40, and joined upwith a battery and a Bell receiver, were found to be sufficient HISTORY 57 to respond to speech, and were even so sensitive as to renderaudible the most minute sounds, such as those caused by thewalking of a fly, etc. The best effect was obtained withcarbon in one shape or another, that given in Fig. 41 being oneof the best forms. Two carbon blocks, c, c, have

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