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158 HANDBOOK OF KINEM A T O G R APH Y drum of wire is called the armature. The result of such a strange arrangement is that currents or waves of electricity are caused to form in the whirling wire coils, and this electric current is duly led to a circular arrangement of metallic conductors called a commutator, upon which press ' brushes ' usually consisting of blocks of carbon. These collect the current and take it to binding screws or lugs, and from them wires known as ' main leads ' further convey the electrical energy on its mission of usefulness. Sometimes this mission is directly to supply light for the projection arc and the small incandescent lights of the picture hall. Sometimes, on the other hand, the main leads take the current to a series of glass boxes filled with weak sulphuric acid and containing lead plates covered with a paste of lead oxide. These are known as accumulators or secondary storage batteries. When thejf are in use, the current in its passage through the acid and the lead plates converts the paste on the latter into a very unstable form of metallic lead. The accumulator is then said to be ' charged/ for it now has in its turn conferred upon it the property of giving out electricity from its terminals in an amount dependent upon the size of the plates and charging they have received. Thus, we already see that besides electricity being generated as the result of purely mechanical action, it may also be given off in connection with chemical action. It is more usual, however, to take the current direct off the dynamo or generator for supplying the pro- jection arc. Electricity as produced for picture work has a voltage of anything from 60 to 500, the latter being very high for the work intended, and about the greatest potential to be met with in practice. This word ' voltage ' will be explained a little later, together with other electrical terms. In any case, current of the voltage to be met with in picture halls only flows well through metal leads, preferably copper. Where the current is required it is accordingly led by the simple means of connecting a copper wire of suitable diameter (according to the amount of electricity wanted in any particular place) with one terminal of the electrical generator, and bringing the electrical flow back along a second copper wire to the other terminal of the generator. The current is thus supplied with a loop of wire along which it may run out from one terminal (the positive) and back again to the second terminal (the negative). Break this loop by cutting the wire at any point and the current refuses to leap the gap—unless it is a particularly short one. That last simple fact is utilised in practice as the means of check- ing electrical flow when not required. The adaptation of the idea takes the form of the electrical switch. A switch is merely a hinged metallic prong which can be made to grip firmly between conveniently situated metallic jaws, or to fly out of contact with them at will. The prong and the jaws into which it fits form part of the ' circuit' or electrical loop of wire to and from the