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CHAPTER X
CURRENT RECTIFICATION
We shall here deal with current rectification only as it applies to projector, stereopticon and spotlight light sources. Rectification for sound purposes will be dealt with elsewhere.
(1) Current rectification consists of changing alternating current into direct current, or a close approximation thereof, through some instrument other than a dynamo commutator. It may be accomplished by any one of the various devices described below.
(2) With few exceptions, theatres everywhere in the United States and in Canada are served with alternating current, usually at 110 or 220volt pressure, 60 cycles.
Aside from one new type of high intensity, alternating current light source now enjoying considerable favor in the smaller theatres, the alternating current arc is not suited to projection work. It is noisy. It makes it difficult to maintain steady screen illumination. The energy of the current being divided between the two carbon tips, upon each of which a crater is formed, only one of which is available for use, it requires a very much higher amperage to secure screen brilliancy equal to that produced by the d. c. arc.
The usual practice therefore is to rectify the current, and thus have direct current at the projector light source.
(3) This may be done either by means of a mercury arc rectifier or by applying a. c. from the service lines to an a. c. motor coupled to and driving a d. c. dynamo, or by other recognized means.
(4) Whatever the rectifying device may be it serves two important purposes, namely (a) it receives a. c.
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