The practical telephone handbook and guide to the telephonic exchange (1906)
Poole, Joseph · New York, Macmillan Co. · leaf 211
Fig. 21: Scale TV Fig. 213 which joins to the main exchange. For the working of thesupervisory and calling signals, and for supplying speakingcurrent to lines not connected through to the main, one ormore pairs of power leads are run between the main EXCHANGE SWITCH-BOARDS i87 and branch exchanges.…
Telephone
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