The practical telephone handbook and guide to the telephonic exchange (1906)
Poole, Joseph · New York, Macmillan Co. · leaf 524
CIAL EXCHANGE SYSTEMS 495 Each instrument is fitted with two press buttons, one ofwhich when pressed earths the a ! wire, and the other one the b wire, of the loop. Each button actuates a calling-lamprelay—the lamp in one case being fitted on the a divisionboard and in the other on the B division.… … £*WW HiHWi^L Fig. 462.— Connections of ; Divided Eoard fitted at St Petersburg This device minimises a very serious difficulty inherent in thisclass of board—viz. that any necessity for selection thrownupon subscribers, no matter how simple it may appear, alwaysleads to a great deal of trouble to…
Telephone
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