THE story of how and why the various commercial frequencies came into use and then dropped out again, in most ca ses, is not primarily the story of the fre quencies themselves, but of the various uses to which the alternating current has been applied. ' In other words, fundamental changes in the application of al ternating curTent have led to radical changes in the frequencies. Some of the applications which have had a determining factor on the frequency of the supply system are as follows ;

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... andescent lighting, trans formers, transmission systems, arc lighting, induction motors, synchronous converters, constructional condi tions in rotating machinery and operating conditions. A brief consideration of these items individually, from the present viewpoint, indicates that while some of them had, at one time, very considerable influence in deter mining frequency conditions, yet, in a number of cases, the originai reasons have disappeared through improve ments and refinements, as will be described later. At v' a rious times the following standard frequencies have been in use in this country, namely, 133YJ, 125, 83 YJ, 66%, 60, 50, 40, 30 and 25 cycles per second. These did not appea r chronologically in the ordeJ; given above, and a few odd frequencies in a few special ap plications are omitted.