Motion picture handbook; a guide for managers and operators of motion picture theatres ([c1916])

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348 MOTION PICTURE HANDBOOK date, and ought to be entirely discontinued. Motor-generator sets and mercury arc rectifiers have been brought to a high state of perfection, so there is now no good reason why alternating current should be used for projection purposes, nor is its use efficient, when viewed from the standpoint of curtain brilliancy. By this I mean that, whereas it is possible to secure practically as excellent an illumination with alternating current as with direct current, still it will take practically double the amperage to do it. In fact, for a picture of given size a better screen result will be had with 25 amperes D. C. than with 50 amperes A. C., and in order to get the same result in illumination as that produced by 40 amperes D. C. it would be necessary to use fully 80 amperes A. C. Therefore, even allowing that the transformer has a higher efficiency than the rectifier or motor-generator set, still if equal screen illumination is had it will cost more to use A. C. Where the coils are wound around opposite legs of the core, as in Fig. 155, the transformer is called a "core" transformer; where the coil is wound around the central leg of the core (inside the outer legs of the core) it is known as the "shell" type, Fig. 157. Please let it be understood that I am not entering into all the details of transformer construction. The details of construction have much to do with the efficient performance of a transformer, but all I seek to accomplish in this article is to give the operator a fairly comprehensive understanding of the theory upon which the transformer works — not to give him instructions enabling him to build one. That calls for very careful calculations and experiment, which can only be made by a duly qualified electrical engineer. Transformers may be built to deliver current to a threewire secondary from which two voltages may be had; for instance, 220 and 110. This is illustrated in Fig. 160. As a matter of fact usually alternating current three-wire systems are two-wire circuits up to the transformer, and beyond the transformer become three-wire systems merely by either the peculiarity of construction of the transformer or the method of hitching up two transformers. The operator is as a general proposition only interested in the theory of the transformer and the practical operation of the low voltage transformer commonly called "compensarc," "economizer," "inductor," which ordinarily takes 110