Transactions of the Society of Motion Picture Engineers (1924)

216 Transactions of S.M.P.E.. May 1924 scene only if the light source in the lamp house operates at a constant value. To produce uniform light emission from the carbon electrodes of an arc lamp, it is necessary to maintain a constant rate of direct current flow through the arc. As the carbons burn away at the ends, the gap between them gradualh^ increases which requires a proportionally increasing impressed voltage to sustain the same arc current. Obvioush^, therefore, if the electrical equipment is designed so that as the arc current decreases when the arc gap and voltage increases, it will be necessary for the operator to readjust his carbon frequently to keep the conditions appreciabh^ constant. It is apparent therefore that if the generator supplying power for the arc is designed so that it will deliver practically a constant current over a wide range of voltage, then the carbons need onh^ be adjusted sufficiently frequently to keep the corners of the picture from becoming discolored. The volt ampere characteristic of such a generator is shown by Fig. 1. In this case the machine is designed to develop about 65 to 67 volts at 75 amperes and also 130 to 134 volts at the same current value, so that two projection lamps may be operated in series during the transition period and each lamp will receive the same current. At the end of the transition period the operator turns the adjusting knob until the carbons touch in the projection machine which is to be discontinued and then closes a switch which short circuits this lamp entirely. By this scheme of operation the current through the arc being used remains practicalh^ constant and the picture uniformly illuminated at all times. Fk Fig. 2 X) illustrates a motor generator built for this service. By especially designing the magnetic circuit of the generator we can prevent the open circuit voltage from rising above the value in