The telephone system of the British post office. A practical handbook (1901)
Herbert, Thomas Ernest · London, New York, Whittaker and Co. · leaf 217
flow from the positive pole of the battery through the clearing lampLj, 90o> resistance coil to the third point of the peg, and thence through thebody of the switch-spring S,, through the cut-off relay C R to earth, and so backto the negative pole of the battery. This causes the lamp L^ to glow…
Telephone · Postal service
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