Telephone magneto

A telephone magneto is a hand-cranked electrical generator that uses permanent magnets to produce alternating current from a 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 activate signals on operator consoles.

The telephone instrument obtained talking current by powering a carbon microphone with a local battery, consisting of "N° 6" zinc–carbon dry cells.

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,[2] but their use continued into the mid-20th century.

Ringing current magnetos were used in the public switched telephone network (PSTN) as late as the 1980s,[3] when they were still used in manual private branch exchanges (PBXs), small business switchboards worked by operators.

Telephone magneto viewed from beneath shows the armature (inset, left) and the horseshoe field magnets, and the gears to drive the rotor
Ericsson Dachshund telephone of 1892 featuring an integrated magneto driven by the handle on the left. The legs of this instrument are actually the permanent magnets for the magneto.