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702 MOTION PICTURE PROJECTION
STARTING FIRST LAMP
When the speed of the machine is up to normal and the starting box or switch is in running position and the rheostat handle set as marked by the white arrow, short-circuit the one lamp by means of its short-circuiting switch. Then close the lamp switch and bring the carbons together so that they barely touch ; then separate them about 1-16 of an inch, gradually increasing the separation as carbons heat up until the proper length of arc is reached. The D. C. arc should be from 5-16 to 3-8 of an inch long or about twice as long as an A. C. arc. Adjust the generator field rheostat until the proper amount of current is flowing. If the carbons are held together too long the machine voltage will be automatically reduced to zero, so that the arc will not have sufficient voltage, and will therefor break when the carbons are separated. Should this occur, keep the carbons apart about 10 seconds until the machine voltage can automatically build up again, then strike the arc as directed above.
The switchboard panel, having instruments mounted on it along with the field rheostat, is very useful, and the proper current can at all times be accurately maintained. As the machine warms up, the handle of the rheostat may have to be moved one or two buttons from the mark to maintain the desired voltage and current. If the circuits are all connected as shown in the diagram, the polarity should be as indicated, the upper carbon being positive. Should the upper carbon be negative and the instrument on the panel board