Telephone magneto

A telephone magneto is a hand-cranked electrical generator that uses permanent magnets to produce alternating current from the rotating armature. In early telegraphy, magnetos were used to power instruments, while in telephony they were used to generate electrical current to drive electromechanical ringers in telephone sets and on operator consoles.

Telegraphy pre-dated telephony and magnetos were used to drive some of the early printing telegraph instruments. Manual telegraphy with keys and reception by either a needle instrument or a syphon recorder could be powered by batteries. The later automatic and printing instruments, such as the Wheatstone ABC telegraph, required greater currents that could be delivered by a hand-cranked magneto. A hand-crank was used to rotate a belt drive that increases the rotational speed of an armature with a pair of coils between the poles of a stationary horseshoe magnet.

Many early manual telephones had an attached hand-cranked magneto that produced an alternating current (AC) at 50-100V for signaling to ring the bells of other telephones on the same (party) line and to alert an operator at the local telephone exchange. These were most common on long rural lines served by small manual exchanges which did not use a common battery circuit. The telephone instrument contained a local battery, consisting of two large "N° 6" zinc-carbon dry cells, to provide the necessary current for the transmitter. By around 1900, large racks of motor-generator sets in the telephone exchange could supply this ringing current remotely instead and the local magneto was often no longer required, but their use continued into the mid-20th century.

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