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470
HANDBOOK OF PROJECTION FOR
so that no current will pass through the lamp. They should be connected ahead of the projector table switch. If this is done the idle lamp may be trimmed without opening the circuit.
The following are the dimensions data of the steel panel:
RATING OF COMPENSARC
AMMETER SCALE MARKING
DIMENSIONS IN INCHES
Height
Width
Depth
Two 35-amp. lamps Two 50-amp. lamps Two 70-amp. lamps
0 30 35 40 0 40 50 60 0 60 70 80
11 11 11
il
i
Two 100-amp. lamps Two 125-amp. lamps
0 90-100-110 0-110-125-135
17 17
88
&/4
When one arc is burning the process of starting the other is as follows: Close the projector table switch and freeze the carbons of the idle lamp. Next open the short circuiting switch of the idle lamp and separate the carbons in the usual way, except that it should be done very slowly. As the second arc is sprung, the machine automatically increases its voltage until it has double the voltage required to force the current against the resistance of one arc ; in other words, until it is sufficient to force the current against the resistance of the the two arcs operating in series. Ordinarily this will be between 105 and 120 volts, varying with the amperage. See Page 400.
To strike either arc alone it is only necessary to open the short circuiting switch of the lamp you are going to use, close the other short circuiting switch and strike the other arc in the usual way. Under this condition the dynamo will automatically generate only that voltage necessary to force the current across the resistance of the single arc.
To extinguish either arc when both are burning, slowly feed the carbons of the lamp together until they are in contact, whereupon close the short circuiting switch of the lamp it is desired to extinguish. An arc may, of course, be extinguished merely by closing its short circuiting switch, but this would give the machine no chance to gradually decrease its voltage to meet the new condition. The bringing of the carbons of the lamp into contact with each other slowly, operates to lower the voltage of the generator gradually, so that there is no shock to it when the short circuiting switch is closed.