In practice it may be considerably less, owing to leakage at the valves and between the piston and cylinder.
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.
The advantage of the high conducting power which copper possesses Over- is of especial value in moist climates (like that of the United Kingdom), since the effect of leakage over the surface of the damp insulators is much less noticeable when the conducting power of the wire is high than when it is low, especially when the line is a long one.
Connexion is made into the office (or to the underground system, as is often the case) from the aerial wire by means of a copper conductor, insulated with gutta-percha, which passes through a " leading in " cup, whereby leakage is prevented between the wire and the pole.
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.