page_title: Magneto
section_title: Arc lighting
hierarchical_section_title: Magneto / History / Arc lighting
caption_attribution_description: Fig. 199 Auguste de Méritens magneto generator, showing the 'ring wound' armature that forms a continuous hoop.
context_page_description: A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, although it is usually considered distinct from most other alternators, which use field coils rather than permanent magnets.
  Hand-cranked magneto generators were used to provide ringing current in telephone systems. Magnetos were also adapted to produce pulses of high voltage in the ignition systems of some gasoline-powered internal combustion engines to provide power to the spark plugs. Use of such ignition magnetos for ignition is now limited mainly to engines without a low-voltage electrical system, such as lawnmowers and chainsaws, and to aircraft engines, in which keeping the ignition independent of the rest of the electrical system ensures that the engine continues running in the event of alternator or battery failure. For redundancy, virtually all piston engine aircraft are fitted with two magneto systems, each supplying power to one of two spark plugs in each cylinder.
context_section_description: Most early dynamos were bipolar and so their output varied cyclically as the armature rotated past the two poles.
  To achieve an adequate output power, magneto generators used many more poles; usually sixteen, from eight horseshoe magnets arranged in a ring. As the flux available was limited by the magnet metallurgy, the only option was to increase the field by using more magnets.  As this was still an inadequate power, extra rotor disks were stacked axially, along the axle. This had the advantage that each rotor disk could at least share the flux of two expensive magnets. The machine illustrated here uses eight disks and nine rows of magnets: 72 magnets in all.
  The rotors first used were wound as sixteen axial bobbins, one per pole. Compared to the bipolar dynamo, this did have the advantage of more poles giving a smoother output per rotation, which was an advantage when driving arc lamps. Magnetos thus established a small niche for themselves as lighting generators.
  The Belgian electrical engineer Floris Nollet (1794–1853) became particularly known for this type of arc lighting generator and founded the British-French company Société de l'Alliance to manufacture them.
  The French engineer Auguste de Méritens (1834–1898) developed magnetos further for this purpose. His innovation was to replace the rotor coils previously wound on individual bobbins, with a 'ring wound' armature. These windings were placed on a segmented iron core, similar to a Gramme ring, so as to form a single continuous hoop. This gave a more even output current, which was still more advantageous for arc lamps.
caption: de Méritens' 'ring wound' armature and single pole piece
exif: {}