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Additional Equipment
Resistances
The external mains resistance has as its principal function the lowering of voltage supplied to the projector, and it may be used on either A.C. or D.C. mains supply. It is essential to connect it according to the maker's instructions, and adjustments should be checked before the start of each show.
It has no working parts requiring operation during projection, though it is possible on certain types of variable resistances to adjust the voltage whilst in use. Those on which no adjustment can be made are known as fixed resistances.
The variable types may have either a series of tapping-points marked with the different voltages obtainable, or be fitted with a sliding knob and pointer which can be moved along a graduated voltage scale. The latter type may be known also as a rheostat, and with many of the older types of projector it was supplied as a means of controlling the speed of the motor (in the absence of an automatic governor).
You will notice that the exterior resistance gets quite hot when in use, and it is best placed under the projector-stand where its heat will not affect the films and records, and also, in a position where there will be a free flow of air around it.
The output voltage-reading should be taken after the resistance has had time to warm up sufficiently to produce a steady current. After final adjustments, it should be left alone until the end of the show unless, of course, you are using it to "fade" the lamp on and off, which will lengthen its life.
Transformers
Transformers carry out the same function as resistances, but do not give off any appreciable heat. They must not be used on
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