A telephone magneto is a hand-cranked

electrical generator In electricity generation, a generator is a device that converts motive power (mechanical energy) or fuel-based power (chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, ...

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, nicke ...

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 p ...

, magnetos were used to power instruments, while in

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

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 The printing telegraph was invented by Royal Earl House in 1846. House's equipment could transmit around 40 instantly readable words per minute, but was difficult to manufacture in bulk. The printer could copy and print out up to 2,000 words per ...

instruments. Manual telegraphy with keys and reception by either a needle instrument or a

syphon recorder The syphon or siphon recorder is an obsolete electromechanical device used as a receiver for submarine telegraph cables invented by William Thomson, 1st Baron Kelvin in 1867. It automatically records an incoming telegraph message as a wiggling in ...

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

pole Pole may refer to: Astronomy * Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with th ...

s of a stationary horseshoe magnet.

Telephony

Many early manual

telephone A telephone is a telecommunications device that permits 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 efficiently the human voice, into e ...

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 telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...

. 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) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...

(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