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The trunk line is cut straight through and all bridge circuits or by-paths from it are cut off.

The conversation was made possible by the use of the vacuum tube and the bridge circuit.

At Station A the ordinary talking set is shown in simplified form, consisting merely of a receiver, transmitter, and hook switch in a single bridge circuit across the line.

When the impulse from the central office ceases, the spring 1 resumes its normal position, thus breaking the bridge circuit through the bell at that station.