A telephone magneto is a hand-cranked

electrical generator In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an extern ...

that uses permanent

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

s to produce alternating current from a rotating armature. In early

telegraphy Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas pi ...

, magnetos were used to power instruments, while in

telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...

they were used to generate electrical current to drive electromechanical ringers in telephone sets and activate signals on operator consoles.

Telegraphy

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 A horseshoe magnet is either a permanent magnet or an electromagnet made in the shape of a horseshoe (in other words, in a U-shape). The permanent kind has become the most widely recognized symbol for magnets. It is usually depicted as red a ...

Telephony

Many early manual

telephone A telephone, colloquially referred to as a phone, is a telecommunications device that enables two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most ...

s had an attached hand-cranked magneto that produced an alternating current (AC) at 50–100 V for signaling to ring the bells of other telephones on the same (party) line, and to alert an operator at the local

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

. These were most common on long rural lines served by small manual exchanges which did not use a common battery circuit. 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, Kennedy, Electrical Installations, 1903, pp. 123, facing 128 but their use continued into the mid-20th century. Telephone magnetos featured a large gear rotated by hand with a handle, that drove a much smaller gear on the armature rotor, providing a high gear-ratio to increase the rotational speed of the magneto armature. A mechanical switch on the output terminals engaged only when the rotor was turning, so that the magneto was normally disconnected from the telephone circuitry. Ringing current magnetos were used in the

public switched telephone network The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...

(PSTN) as late as the 1980s, when they were still used in manual private branch exchanges (PBXs), small business switchboards worked by operators. Many such systems obtained ringing power from an extra wire pair from a central office, rather than having a motor generator for such a small installation, which generated noise and required maintenance. Magnetos were used when such a facility was not available, or as a backup source when the facility failed. Unlike in the public telephone network, which has a standard ringing cadence (the repeating pattern of ringing and silence), the ringing cadence when using a magneto depended on the skill of the operator. When ringing local extensions, some switchboard operators used local codes of ringing to indicate internal, external or urgent calls. Linesman's test sets also included a magneto, for use when ringing out to either the exchange or the subscriber, from anywhere along the line. Their use extended into the 1980s.

References

{{reflist Electrical generators History of the telephone

Telegraphy

Telephony

See also

References