File:The practical telephone handbook and guide to the telephonic exchange (1906) (14775977453).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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Text Appearing Before Image:
aratively 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 a cup-shapedhollow made in each, in which the carbon pencil a is looselyheld; c, c are attached to a sounding-board of thin deal, wiresbeing connected to c, c for joining up the battery and receiver.
Text Appearing After Image:
Fig. 41 The complete instrument is shown in Fig. 42, and forms theparent of a very numerous class of telephone transmitters;indeed, nearly all transmitters in use are but modificationsor amplifications of some form or other of Hughes micro-phone. Edisons Loud-Speaking Receiver.—This was a very inter-esting instrument, and was much used at one time. It wasbased on the fact discovered by Edison, and utilised by himfor the construction of a telegraphic instrument, that thefriction between a metal and a substance subject to electro-lytic action varies in proportion to the strength of a currentpassing through the points of contact. The telephone re- 5* PRACTICAL TELEPHONE HANDBOOK ceiver constructed on this principle is shown in Fig. 43. Adiaphragm of mica, m, is mounted over an opening in a box,as shown, and a strip of platinum, c (only the end of whichis shown), is attached to its centre, and projects at right anglesfrom it. The end of c lies flat on the surface of a chalkcylinder, a,

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