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6 MOTION PICTURE HANDBOOK This is very simple, when one stops to consider the enormous speed of the electric current. This speed has been variously stated, some authorities placing it as high as that of light. With this in mind it will readily be seen that, in- finitesimal as is the time required for an alternation the current will pass hundreds, if not thousands, of times entirely around a circuit many miles in length during that period. Electric speed is entirely beyond the power of the human mind to grasp and comprehend. Careful experiment has proven that a low frequency cur- rent is most economical for power purposes, 25 to 30 cycle being best. On the other hand, the high frequency current is best for light. Current with as low frequency as above named will not do for light at all, as the illumination fur- nished by it is flickery and unsteady. It has been found that 60 cycle current answers fairly well for both purposes and that is the standard being adopted by practically all new commercial plants designed to furnish both light and power. Even the old plants originally furnishing very high or very low frequency current are changing their generators to 60 cycle machines, so that is the alternating current you will now most frequently meet with. Direct current needs no detailed explanation. Being gen- erated in the dynamo it flows out on one (the positive) wire, performs its work and returns to the dynamo via the other ("negative) wire. You are not likely to have to cut into any high tension direct current systems and will almost invariably find direct current to be at a pressure of no volts, except where the three-wire system is used. From this sysetm (as will be explained further on) you can get either no or 220 volts, according to the way you "hitch up" to it. In handling steam we use the term "pound' to denote pressure. In handling water its volume or quantity is ex- pressed in "gallons." Electricity has pressure just exactly as has steam in a boiler, but this pressure is expressed in "volts" instead of, as in steam, in pounds. Electric current has volum.e, or quanti- ty, just as has flowing water, and this quantity is measured, or expressed, in "amperes," instead of in gallons, as is the case with water.