A couple of cells of almost any make such as Daniells or Le Clanche are quite enough for working a short telephone line, and for longer distances three or four at the outside are sufficient, for, an excess of electromotor force produces a crackling sound in the receivers, which may be loud enough to seriously inconvenience conversation. For electric bells the number of cells required depends upon the length of the line and the resistance of the bells used, and the right number is usually found by experiment.

If a telephonic line is to be established near a telegraph line, it becomes necessary to use a metallic wire for the return, because the effects of induction, due to the making and breaking of the telegraph current, when an earth return is used by both circuits, becomes sometimes great enough to spoil all telephonic communication.

As a matter of fact it is very seldom that an earth return can be used for telephone lines, for the circuit is then always liable to external disturbances, such as leaks from electric light and power circuits, telegraph currents, earth currents, etc. Even with twin wires the greatest attention must be paid to good insulation of the line, otherwise the above-mentioned disturbances may make their influence felt. When all such stray currents have been eliminated from the system, the length of a telephone line may be greatly increased; conversation has been carried on quite distinctly at distances of 800 and 900 miles.