dynamos and alternators required a source of power to drive their field coils. This could not be supplied by their own generator's output, without some process of 'bootstrapping'. Henry Wilde, an electrical engineer from Manchester, England, developed a combination of magneto and electro-magnet generator, where the magneto was used only to supply the field to the larger alternator. These are illustrated in Rankin Kennedy's work Electrical Installations Kennedy himself developed a simpler version of this, intended for lighting use on ships, where a dynamo and magneto were assembled on the same shaft. Kennedy's innovation here was to avoid the need for brushgear, altogether. The current generated in the magneto is transmitted by wires attached to the rotating shaft to the dynamo's rotating field coil. The output of the dynamo is then taken from the stator coils. This is 'inside-out' compared to the conventional dynamo, but avoids the need for brushgear. The invention of the self-exciting field by Varley, Siemens & Wheatstone removed the need for a magneto exciter. A small residual field in the iron armature of the field coils acted as a weak permanent magnet, and thus a magneto. The shunt wiring of the generator feeds some of its output current back into the field coils, which in turn increases output. By this means, the field 'builds up' regeneratively, though this may take 20–30 seconds to do so fully. Use of magnetos here is now obsolete, though separate exciters are still used for high power generating sets, as they permit easier control of output power. These are particularly common with the transmissions of diesel-electric locomotives. Power generation Magnetos have advantages of simplicity and reliability, but are limited in size owing to the magnetic flux available from their permanent magnets. The fixed excitation of a magneto made it difficult to control its terminal voltage or reactive power production when operating on a synchronized grid. This restricted their use for high-power applications. Power generation magnetos were limited to narrow fields, such as powering arc lamps or lighthouses, where their particular features of output stability or simple reliability were most valued. Wind turbines Small wind turbines, particularly self-build designs, are widely adopting magneto alternators for generation. The generators use rotating neodymium rare-earth magnets with a three-phase stator and a bridge rectifier to produce direct current (DC). This current either directly pumps water, is stored in batteries, or drives a mains inverter that