The strength of the field is proportional to the swing of the galvanometer-needle, and, when the galvanometer is calibrated, can be expressed in C.G.S.
If the looped lines are both in good condition and free from leakage, the current sent out on line r 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.
Two receiving instruments, a siphon recorder and a mirror galvanometer, are shown; one only is absolutely necessary, but it is convenient Cable to have the galvanometer ready, so that in case of accident to the recorder it may be at once switched into circuit by the switch s.
If any sensible current flows through this insulator the galvanometer will show a deflection.
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.