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664 MOTION PICTURE PROJECTION
with the 1.5 ohms resistance of lamp No. 1, with the result that with a 75 volt line the 0.4 ohm circuit attempts to take 187.5 amperes which, added to the 50 in lamp No. 1, makes 237.5 amperes, which at 75 volts is a 17.8 K.W. load, which is a 350 per cent overload on the 3.75 K.W. machine. At such an overload, this equipment cannot maintain its voltage constant, i. e., the voltage will decrease, with the result that the amperes and the light from lamp No. 1 will decrease to such an extent that it may go out entirely.
Fig. 276 again shows a paralleled two lamp circuit, and while the current supplied in this is also from a 50 ampere capacity generator, it is at 110 volts. The machine's capacity is 5.5 K.W., and assuming again we are using lamp No. 1 with 50 amperes and carrying the normal 55 volts arc length, the voltage to be lost in the ballast is 110 less 55, or 55, and with 50 amperes flowing, the required ballast resistance is 55/50 or 1.1 ohm, but the total resistance is 110/50 or 2.2 ohms. Now, when we bring together the carbons of lamp No. 2 we will have paralleled a 1.1 ohm of ballast with the 2.2 ohms resistance of No. 1 lamp circuit, with the result that we get 110/1.1 or 100 amperes in lamp No. 2 circuit. This, with the 50 amperes used in the other lamp, makes a total of 150 at 110 volts or 16.5 K.W., which on the 5.5 K.W. machine is only a 200 per cent overload. As this overload is much smaller than before, this machine will not only come nearer to maintaining its voltage constant, but there is less necessity for its doing so, because the 1.1 ohms of ballast used in this case has a much greater stabilizing effect