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602 HANDBOOK OF PROJECTION FOR
former that instead of the regular stock transformer having a range from 40 or 50 to 60 or 65 amperes, as is the present practice, the purpose would be very much better served if such devices had a range of from 50 to 80, because 50 is as low an alternating current amperage as ought to be used for the projection of modern motion pictures, and 80 amperes is none too high for good work.
ADJUSTMENT. — The change of secondary amperage usually is accomplished by means of "tapping in" on the primary coil. This is illustrated in Fig. 200, in which A-B-C are wires connecting with the primary coil at various points, and D the adjusting switch of the transformer. Remembering that the E. M. F. and the amperage of the secondary is dependent upon the relative number of turns of wire in the primary and scondary, it will readily be understood that the altering of the position of switch D would alter the voltage and amperage of the secondary, since it would add to or reduce the active turns of wire in the primary coil. We believe no further explanation of this point is necessary, since the drawings and what has already been said, should make the matter sufficiently clear. Change in secondary amperage may also be accomplished by altering the position of the primary and secondary coils with relation to each other.
WIRE SIZES. — Where the projection circuit is served by an Inductor, Economizer, or Compensarc of 60 amperes secondary capacity, the usual custom is to install primary circuit wires of only sufficient capacity to carry the primary current, which will be decidedly less than the secondary current. We would, however, recommend that the primary and secondary circuit wires be of equal size, with a capacity of, say, 70 amperes, because in case something goes wrong with the low voltage transformer it may become necessary to temporarily install a rheostat, and if this be done, certainly one would not wish to pull less than 60 amperes, under which condition the wires of the primary circuit would be too small.
PERMISSIBLE TEMPERATURE^-See Page 511.
THE CHOKE COIL.— The choke coil, also called a "reactance" coil, is diagrammatically illustrated in Fig. 201. It represents what might be called magnetic resistance. If an iron core consisting, in practice, of thin sheets of metal, be built up, and one of the insulated wires of an alternating circuit be wrapped a number of times around it, as shown.