The magneto bell gains its name from the old hand operated magneto (generator) creating an alternating current used by early manual telephone exchange systems. When the telephone exchanges became automatic, the use of alternating current to ring the customer's bell was retained.
The external bell is mounted on a RAC Roadside Box, seen here at Avoncroft Museum, Bromsgrove. The home of the national Kiosk Collection.
The Bell 67A is one of the most ubiquitous external bells found in the U.K. The bell does not have any interrupter contacts unlike a D.C. operated bell as often found in door bells. Instead two coils on poles are placed either side of a permanent magnet. The armature carrying the striker is first attracted to one pole and then the other as ringing current passes through the windings. The A.C. ringing current is low frequency, originally around 16 Hz and laterly 25 Hz.
Designed to be used with fixed telephones (not the post 1980's modern plug in landline phones) this external bell is wired in series with the bell within the telephone.
Photographs courtesy of Phil Hallam.
A switch may be fitted to short circuit the bell at night to prevent its ringing being a nuisance to neighbours.
RingBell : GPO Nostalgia Website