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MANAGERS AND PROJECTIONISTS 493
Current Rectification
ONLY a small percentage of the theatres of the United States and Canada have an available direct current supply. The rest have A. C. supply only. We are not sufficiently conversant with the situation in this respect with regard to other countries in which this book is used to attempt an approximation of the percentage of theatres having A. C. supply only, but no theatres anywhere have a current supply the voltage of which is suitable for use at the arc.
It is a well-known fact that an alternating current projection arc is not only very much more difficult to handle, but also is likely to be more or less noisy (though special A. C. carbons have largely reduced the element of noise) also that very nearly double the amperage is required at the arc to secure an effect equal to a direct current projection arc. In other words, to equal the effect of a 40-ampere D. C. projection arc, an A. C. arc operating at about 80 amperes would be necessary. Why this is so is explained on Page 392.
Due to these facts, the almost universal modern practice in large theatres, and the very general practice in smaller theatres, is to "rectify" the alternating current supply (change it to D. C. and to arc voltage) either by means of a motor-generator set or a mercury arc rectifier, either of which receives alternating current from the line and delivers direct current at the arc, in most cases at arc voltage. These machines have been brought to such a state of perfection with regard to mechanical construction, efficiency, operation and ease of manipulation, that there is now absolutely no legitimate excuse for the continued use of alternating current at the projection arc. True, the exhibitor may offer in excuse the fact that the installation cost of the motorgenerator or mercury arc rectifier, plus the item of deterioration, is a considerable sum, but this in fact is no excuse at all, because he will get all that and very much more besides back in increased patronage of the box office by reason of improved screen results.
This latter is, of course, in a few instances modified by the fact that there is an occasional projectionist who is sufficiently expert in the handling of the alternating current