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EVER since the advent of a-c systems in aircraft, designers have been increasingly led to consider methods of obtaining constant frequency from variable-speed sources, as the aircraft engine is usually at least two to one in speed range. The first device to obtain constant speed from a variable-speed source was under development for many years, and it is only recently that it has been considered reliable enough for general use. Fortunately, the timing has been good, since the very large increase in aircraft electrical loads has coincided almost exactly with the availability of this device. Of course, the large increase in electrical loads has led to the proposal of many devices either to provide constant speed to drive a generator, or to obtain constant frequency directly from a generator driven at a variable speed.