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4 MOTION PICTURE HANDBOOK negative. For reasons which will be fully explained later, the positive carbon must always be above and the negativ below when using direct current. Alternating current, on the other hand, flows first in o^ direction and then in the other, this being caused by t peculiar construction of the generator (dynamo). The ci rent flows one way for a time varying from 1-25 to 1-275 a second. It then is reversed, flowing in the opposite diri tion for the same period of time. These two periods of flo are termed a "cycle." To make this term more clear: sup- pose you walk across a room, then turn and walk back again. When you got back where you started from you would have completed what would correspond to a "cycle" in alternatin . current. Therefore when you hear "60 cycle" current spok' of it means that the current reverses its direction 120 tim ^ per second. If it be 132 cycle the reversals are at the ra of 264 per second, etc. In a two-pole dynamo the curren ' flows in one direction during the time the armature makes one-half of a revolution and in the opposite during the other half, so that one complete turn of the armature completes one "cycle." Modern dynamos, however, have more than two poles (some of them a great many more), thus reduc- ing the necessary armature speed required to produce-a given current frequency. ("Frequency" means the number of cy- cles per second.) All dynamos, direct or alternating, are constructed to produce a certain voltage, and the machine designed to produce, for instance, no volts could not and would not produce 220 volts or any other pressure than that named. With alternating current each carbon of the lamp is alter- nately positive and negative. This has a very decided effect on projection light, as will be explained later on. Many operators claim that as good results cannot be obtained from alternating as with direct current, but this is not true, though it is somewhat more difficult and requires more cur- rent (amperes) to produce results equal to those possible with direct current. Alternating current is preferred by power, and many lighting companies, mainly for the reason that it can be generated at high tension (voltage) transmitted to the place where it is to be used and there transformed (reduced to