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462 RICHARDSON'S BLUEBOOK OF PROJECTION
(6) The transformer consists, for all practical purposes, of a coil of wire (Wire A) placed close to another coil (Wire B). In transformers used in the projection room these two coils will be wound upon the same iron core. For some purposes, radio receivers, for example, iron cannot be used in a transformer, by reason of the fact that reversal of direction of the current is so rapid that the motion of the molecules of the iron (which of course is magnetized) cannot keep pace with it. But for all light source control purposes and in most sound equipment iron-core transformers are the rule. They are able to handle vastly more power than would be possible if the iron were not present.
Transformers and Voltage Change
(7) In a transformer, the winding of wire just spoken of as Wire A, which carries the magnetizing current, is called the primary winding. Wire B represents the secondary winding.
(8) A one-volt drop across one complete turn of primary wire will induce a one-volt drop across one complete turn of secondary ■ wire, therefore if the primary consists of one turn of wire carrying one ampere of curent at one volt, and the secondary consists of two turns of wire, the current in the secondary will be J/2 ampere at two volts for the secondary as a whole. In the same way, if the primary power is 110 volts, the secondary power can be made 220 volts at one-half the current. Or, by connecting the transformers the other way around, the secondary output can be made 55 volts, at twice the current. Any voltage "step-up" or "step-down" can be secured by means of a transformer. A transformer can also be made with one primary and a number of secondaries, each delivering a different voltage.
Since transformers, as explained above, cannot work with direct current unless it be pulsating, these convenient voltage changes are unfortunately impossible with the smooth d. c. drawn from a battery or a commercial power line. In some sound systems, where it is neces