As is well known, it is electric currents and not sound-waves that are transmitted over a telephone circuit.  The magneto-electric telephone in its simplest form consists of a pair of instruments called respectively the transmitter and the receiver.  We talk into the transmitter and listen at the receiver.  Both transmitter and receiver consist of a permanent magnet of hardened steel around one end of which is placed a coil of insulated wire.  In front of this coil a diaphragm, or thin plate, of soft iron, is so supported as to be capable of freely vibrating towards and from the magnet pole.

The operation of the transmitting instrument is readily understood in the light of Faraday’s discovery.  It is simply a dynamo-electric machine driven by the voice of the speaker.  As the sound-waves from the speaker’s voice strike against the diaphragm, which has become magnetic from its nearness to the magnet pole, electric currents are generated in the coil of wire surrounding such pole, since the to-and-fro motions cause the lines of electro-magnetic force to pass through the wire on the moving coil.  The operation of the receiving instrument is also readily understood.  It acts as an electric motor driven by the to-and-fro currents generated by the transmitter.  As these currents are transmitted over the wire, they pass through the coil of wire on the receiving instrument, and reproduce therein the exact movements of the transmitting diaphragm, since, as they strengthen or weaken the magnetism of the pole, they cause similar motions in the diaphragm placed before it.  Consequently, one listening at the receiving diaphragm will hear all that is uttered into the transmitting diaphragm.  It was thus, by the combination of the dynamo and motor, both of which were given by Faraday to the world, that we have received this priceless instrument, which has been so potent in its effects on the civilization of the Twentieth century.

The electric telegraph had its beginnings long before Faraday’s time.  As early as 1847, Watson had erected a line some two miles in length, extending over the housetops in London, and operated it by means of discharges from an ordinary frictional electric machine.  In 1774, Lesage had erected in Geneva an electric telegraph consisting of a number of metallic wires, one for each letter of the alphabet.  These wires were carefully insulated from each other.  When a message was to be sent over this early telegraphic line an electric discharge was passed through the particular wire representing the letter of the alphabet to be sent; this discharge, reaching the other end, caused a pithball to be repelled and thus laboriously, letter by letter, the message was transmitted.  How ludicrously cumbersome was such an instrument when contrasted with the Morse electro-magnetic telegraph of to-day, which requires but a single wire; or with the harmonic telegraph of Gray, which permits the simultaneous transmission of eight or more separate