The practical telephone handbook and guide to the telephonic exchange (1906)
Poole, Joseph · New York, Macmillan Co. · leaf 533
er of pairs of supervisory signals beingfitted directly below the calling signals. The former signalsare said to be indirect or negative, as they are operated all thetime that speaking goes on and restore to normal position whenthe subscribers replace their receivers, and thus give an… … SB 3 bO /^JV buijaMSyy SPECIAL EXCHANGE SYSTEMS 505
Telephone
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