Electroplating requires DC and so the usual AC magneto is unworkable. Woolrich's machine, unusually, has a commutator to rectify its output to DC. Arc lighting 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. Lighthouses De Méritens is best remembered today for his production of magneto generators specifically for lighthouses. These were favoured for their simplicity and reliability, in particular their avoidance of commutators. In the sea air of a lighthouse, the commutator that had been used previously with dynamo generators was a continual source of trouble. The lighthouse keepers of the time, usually semi-retired sailors, were not mechanically or electrically skilled enough to maintain these more complex machines. The de Méritens magneto generator illustrated shows the 'ring wound' armature. As there is now only a single rotor disk, each horseshoe magnet comprises a stack of individual magnets, but acts through a pair of pole pieces. Self-exciting dynamos Both