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MOTION PICTURE PROJECTION 665
than the 0.4 ohms ballast used with the low voltage machine.
Carrying the illustration one point further. Suppose the voltage of the generators were 220, calculating in the same manner with a 50 ampere arc the capacity of the machine will be 11 K.W., the rheostat required would have a voltage drop of 220 less 55 or 165 and a consequent resistance of 165/50 or 3.3 ohms.
On starting the second arc it would take 220/3.3 or 67 amperes, a total for the two arcs of 67 plus 50 or 117 amperes and an overload of 134 per cent.
However, when the parallel system of arc operation is used, the usual practice is to have generator voltage about 80 to 85 volts. Then in order to minimize the overload and its disturbing effect when striking an arc, an adjustable high resistance ballast is provided for each lamp circuit, as shown in Fig. 277.
The entire resistance of the rheostat to be in the circuit when striking an arc, after which the operator must either make adjustment of the resistance or with a switch connected as shown on lamp No. 2, Fig. 277, short-circuit the greater portion of it, in order to get the amperes required for projection. This is not a convenient, but with many MG sets of that type is a necessary, arrangement where such low voltage supply is used with parallel arcs.
The figures given above with reference to constant voltage generators supplying current to motion picture arcs, will apply to any constant voltage source of direct current whether that