The telephone system of the British post office. A practical handbook (1901)
Herbert, Thomas Ernest · London, New York, Whittaker and Co. · leaf 75
Figure 47. Figure 48. thin sheets of Iron, in order to prevent lipes of force from straying outinto the surrounding air. This prevents induction from one indicator toadjacent ones. If this were not done it would be possible to overhearconversations from one circuit to another whose indicator was…
Telephone · Postal service
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