There are two very distinct methods of transmitting the voice possible in telephonic systems : electro magnetic transmitters for use without batteries, and transmitters on the microphone principle requiring some external source of current. The instruments for receiving the voice are called receivers, and are always based on the principle of the electro-magnetic transmitter.

The first practical telephone was invented by Graham Bell, and patented by him in 1877; it was also invented independently about the same time by Elisha Gray. The principle of this telephone is based on the laws of electro-magnetic induction.

A complete apparatus for telephonic communication consists: at the transmitting station, of a microphone, battery, and induction coil, and a bell-push which rings a bell at the receiving station to draw attention; at the receiving end, of Bell telephone receiver and the call-bell.

Apparatus for the Transmission of Sound by Electricity.