The practical telephone handbook and guide to the telephonic exchange (1906) (14755828402)

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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communication with each other. It is evident that more than one pair of wires may be inuse at the same time, and also that a number of, or all, thestations may be connected together at once. Secrecy Systems.—A great drawback with these instrumentsis that conversations can be easily overheard, it being onlynecessary to connect the switch at any station on to the lineof any engaged number to enable one to overhear all that is said.Many ingenious arrangements have been devised to obviate INTERCOM M UNICA TION TELEPHONES 141 this defect, and so produce a secrecy system. One of themost promising of these was known as Slopers in which,by the use of a special switch, the ordinary connection to thereturn wires of two engaged stations were cut off by mutualarrangement, and the two ordinary lines of the two stationsused to form a metallic circuit line when secrecy was required.Even this, however, was not perfectly successful, as thestatical capacity of the lines not being perfectly balanced,
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and as the lines were not twisted in proper pairs, it was stillpossible to faintly overhear the conversation. Sloper & Parsons System.—This is an improvement on thelast-mentioned, which appears to fulfil the secrecy require-ments with great success. Fig. 154 gives the connections oftwo instruments on a 6-line system. It will be seen that thehome line is connected to the receiver hook, and the switchhandle to the ringing button. A relay is connected as a shuntto the microphone and primary of an induction coil, the second-ary and receiver forming an independent circuit. Themicrophone circuit is disconnected at the relay contact untilthe circuit of the speaking battery is complete through the 142 PRACTICAL TELEPHONE HANDBOOK relay, and this circuit is only completed when some otherstation connects on to the home line. To call, the switch is moved to the number of the stationwanted, and the ringing key or button pressed. This ringsthe bell by means of current from the special common