Many people use the telephone daily without having the slightest conception of the principles upon which it operates. The fundamental principle is a comparatively simple one, involving merely the carrying of sound waves by means of an electric current, but in a large city with thousands of telephones and many exchanges, the problem of proper connection and transmission becomes a complicated one.

The transmitting and receiving instruments are identical in nature, each consisting of a coil of insulated wire connected with the line.

In transmitting, the message is spoken into the mouthpiece at one end. The to-and-fro motion thus imparted to the metallic diaphragm attached to the mouthpiece produces induction currents in the coil. These impulses passing over the main line produce similar movements in the diaphragm of the receiving instrument and thus cause the latter to reproduce the message in articulate sound to the one listening.