Motion picture handbook; a guide for managers and operators of motion picture theatres ([c1916])

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FOR MANAGERS AND OPERATORS 349 or 220 volts supply from the line and delivers secondary current at arc voltage, and this is the type of transformer to which we will devote our attention. As has already been remarked, volts times amperes taken from the line equals volts times amperes delivered on the secondary, less the loss in the transformer which, as I have already remarked, may run anywhere from 10 to 20 per cent. In operating a projection arc lamp it is necessary at times to vary the amperes. Now the secondary of a transformer works against a slight resistance of the secondary circuit wires and a considerable resistance of the projection arc, therefore the current flow against this particular fixed resistance can either be increased or decreased by increasing or decreasing the voltage of the secondary. As has already been remarked, the voltage of the secondary will depend upon the relative number of turns in the primary and the secondary coil. The greater the number of turns in the primary with relation to the number of turns in the secondary the less the voltage of the secondary. It is a known fact that the best voltage across an alternating current arc is approximately 35 volts. In order to keep the arc burning steadily it is always necessary to have a steadying resistance or a reactance in the arc circuit. One advantage of reactance for steadying the arc is that there is very little power lost in the reactance, whereas with resistance all of the steadying effect is turned into heat, and, therefore, means a lot of lost power. Reactance can be obtained only on alternating circuits. In order to change the current at the arc it is necessary, therefore, to change the steadying reactance when using a MAM Figure 161.