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38 RICHARDSON'S BLUEBOOK OF PROJECTION
(27) One of the first duties of the projectionist upon taking charge of a projection room is to determine whether or not the various circuits have sufficient capacity to convey the current without excessive drop in
voltage. .
(28) Up to a certain point resistance ot copper circuits change very little with increase or decrease of current flow. It is when the wires are loaded to a point where temperatures rise above normal that waste begins because with every increase in temperature resistance rises and power must be consumed to overcome it.
Rheostatic Resistance
Resistance already has been explained in general terms. We will now deal with it in connection with rheostatic resistance. (29) This is one way to reduce voltage to the precise pressure necessary to force a required number of amperes through resistance. (30) The rheostat transforms the pressure into heat, which is absorbed by the atmosphere immediately surrounding the resistance coils or grids. The power thus consumed represents waste if more pressure is broken down than is necessary to stabilize the arc. .
The voltage of a circuit used in projection is presumed to be at a fixed pressure. It is and, for reasons we will explain, must be considerably higher than is required tj force the requisite number of amperes through the arc resistance. This excess voltage must be reduced or "broken down" to the amount required to perform the work This work is performed by a rheostat— an instrument composed of several resistance elements connected in series with each other, mounted on a frame from which it is thoroughly insulated and the whole encased in a suitable sheet metal cover, perforated thoroughly to provide means of ventilation.
The supply voltage may be almost anything from W to 250 volts, though ordinarily, when taken from power lines it is either 110 or 220 volts. Amperage requirements on the other hand vary widely, depending upon the kind of arc employed and the amount of light