File:EB1911 Telegraph - Method of testing Circuits.jpg

English: Electrical diagram of a circuit used to test telegraph wires. The method consists in looping the wires in pairs between two testing offices, A and B; a current is sent from a battery, E, through one coil of a galvanometer, g, through a high resistance, r, through one of the wires, 1, and thence back from office B (at which the wires are looped), through wire 2, through another high resistance, r′, through a second coil on the galvanometer, g, and thence to earth. If the looped lines are both in good condition and free from leakage, the current sent out on line 1 will be exactly equal to the current received back on line 2; and as these currents will have equal but opposite effects on the galvanometer needle, no deflection of the latter will be produced. If, however, there is leakage, the current received on the galvanometer will be less than the current sent out, and the result will be a deflection of the needle proportional to the amount of leakage.