Hawkins electrical guide ... : questions, answers & illustrations : a progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications ; a practical treatise (1914)
Hawkins, N. (Nehemiah), 1833-1928 · New York : T. Audel & Co. · leaf 52
listening key isthrown normal. Operator calls extension in usual way on any cord except special andrings extension special bell no matter how listening key be thrown. Since there is no listen-ing key on special cord, operator must use any other cord for connection between P.B.X.and extension…
Electrical engineering
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