The telephone system of the British post office. A practical handbook (1901)
Herbert, Thomas Ernest · London, New York, Whittaker and Co. · leaf 97
actically across the cords. Upon the left the front spring and studare used to complete the primary circuit of the operators speaking set.This prevents the battery being worked when the operator is not actuallyspeaking. The back spring and contact are used to restore the indicatorattached to the… … Figure 70. We are now prepared to consider the whole of the operating connections,and these are indicated in Figure 70. It will be seen that each pair ofcords has a self-restoring indicator and a galvanometer bridged perma-nently across it, and that the two ringing keys merely cut off the otherside…
Telephone · Postal service
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