Telephone magneto
Encyclopedia
A magneto is an electrical generator
that uses permanent magnet
s to produce alternating current. Owing to their simplicity and reliability, they were widely used in early telephone
systems.
instruments. Manual telegraphy with key
s and reception by either a needle instrument or a syphon recorder could be powered by batteries. With the development of automatic and printing instruments such as the Wheatstone ABC telegraph, these required greater currents. This current was supplied by a hand-cranked magneto: a hand-crank with a belt drive to increase speed rotated a pair of coils past the pole
s of a stationary horseshoe magnet that supplied the field.
s had a hand cranked "magneto" generator to produce a (relatively) high voltage alternating signal to ring the bells of other telephones on the same (party) line and to alert the operator. These were usually on long rural lines served by small manual exchanges, which were not "common battery". The telephone instrument was "local battery", containing two large "No. 6" zinc-carbon
dry cells. By around 1900, large racks of motor-generator sets in the telephone exchange supplied this ringing current instead and the local magneto was no longer required.
A gear mechanism was used to speed-up the crank rotation to the speed of the magneto armature. Telephone magnetos also incorporate a switch that only engages when rotating, so that the magneto is normally out of circuit.
The last survival of ringing current magnetos in the public telephone network (PSTN) was as late as the 1980s, where they were still used with PMBX (Private Manual Branch Exchange, a small business switchboard, worked by operators). Rather than providing a motor generator set for such a small installation, along with its noise and need for mechanical maintenance, a hand magneto was used. Unlike the public telephone network and its standard ringing cadence (two pulses, then a gap, in the UK) the cadence of a manual ringer depended on the operator. When ringing local extensions, some switchboard operators used local codes of ringing to indicate internal, external or urgent calls.
Around the same time, the linesman's test set still included a magneto, for use when ringing out to either the exchange or the subscriber, from anywhere along the line. These lasted into the 1980s.
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
that uses permanent magnet
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, and attracts or repels other magnets.A permanent magnet is an object...
s to produce alternating current. Owing to their simplicity and reliability, they were widely used in early telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
systems.
Telegraphy
Telegraphy pre-dated telephony and magnetos were used to drive some of the early printing telegraphPrinting telegraph
The Printing Telegraph was invented by Royal Earl House in 1846.The device was made by linking two 28-key piano-style keyboards by wire. Each piano key represented a letter of the alphabet and when pressed caused the corresponding letter to print at the receiving end. A "shift" key gave each main...
instruments. Manual telegraphy with key
Telegraph key
Telegraph key is a general term for any switching device used primarily to send Morse code. Similar keys are used for all forms of manual telegraphy, such as in electrical telegraph and radio telegraphy.- Types of keys :...
s and reception by either a needle instrument or a syphon recorder could be powered by batteries. With the development of automatic and printing instruments such as the Wheatstone ABC telegraph, these required greater currents. This current was supplied by a hand-cranked magneto: a hand-crank with a belt drive to increase speed rotated a pair of coils past the pole
Pole piece
A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name....
s of a stationary horseshoe magnet that supplied the field.
Telephony
Many early manual telephoneTelephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
s had a hand cranked "magneto" generator to produce a (relatively) high voltage alternating signal to ring the bells of other telephones on the same (party) line and to alert the operator. These were usually on long rural lines served by small manual exchanges, which were not "common battery". The telephone instrument was "local battery", containing two large "No. 6" zinc-carbon
Zinc-carbon battery
A zinc–carbon dry cell or battery is packaged in a zinc can that serves as both a container and negative terminal. It was developed from the wet Leclanché cell . The positive terminal is a carbon rod surrounded by a mixture of manganese dioxide and carbon powder. The electrolyte used is a paste of...
dry cells. By around 1900, large racks of motor-generator sets in the telephone exchange supplied this ringing current instead and the local magneto was no longer required.
A gear mechanism was used to speed-up the crank rotation to the speed of the magneto armature. Telephone magnetos also incorporate a switch that only engages when rotating, so that the magneto is normally out of circuit.
The last survival of ringing current magnetos in the public telephone network (PSTN) was as late as the 1980s, where they were still used with PMBX (Private Manual Branch Exchange, a small business switchboard, worked by operators). Rather than providing a motor generator set for such a small installation, along with its noise and need for mechanical maintenance, a hand magneto was used. Unlike the public telephone network and its standard ringing cadence (two pulses, then a gap, in the UK) the cadence of a manual ringer depended on the operator. When ringing local extensions, some switchboard operators used local codes of ringing to indicate internal, external or urgent calls.
Around the same time, the linesman's test set still included a magneto, for use when ringing out to either the exchange or the subscriber, from anywhere along the line. These lasted into the 1980s.