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390 MOTION PICTURE HANDBOOK
projection machine switch of the second lamp and strike its arc. Open the projection machine switch at the lamp which is to be cut out, and then close the change-over switch. By tracing the connections in Fig. 182 and Plate 9 it will be seen that when the change-over switch is opened the current must flow to the lamp which is burning, and must pass through grid resistance, which has the effect of steadying the arc and preventing it from going out at the instant the arc is struck at the second lamp. It is therefore possible to strike the second arc and burn the crater into proper shape while the end of the first reel is still being projected, and to accomplish the effect of dissolving one picture into the next. The steadying resistance is only in circuit when both lamps are burning, and care must be taken that the change-over switch is kept closed when only one lamp is burning. If, for any reason, an increase in current is needed at the arc, or it is necessary to heat up the carbons very quickly, the change-over switch may be opened on one lamp for a few minutes, thus increasing the current in the arc without disturbing the field rheostat setting.
Caution.. — Keep the first arc rather short at the instant the second arc is struck.
If this is done neither arc will go out, or even flutter during the period of lighting the other arc. The ability to handle both arcs perfectly and change over without a flicker in the picture is soon acquired, and if the second arc is started long enough ahead to be perfectly steady there is no difficulty in dissolving one picture into the next sucessfully.
Caution. — Care must be taken that the two lamps are not burned longer than is really necessary, since the compensarc is not intended to carry both lamps continuously, neither has it the capacity to do so.
With one lamp burning the ammeter will show from 35 to 50 amperes, and the voltmeter about 55 volts; when both lamps are burning the ammeter will show approximately 70 to 100 amperes and the voltmeter 70 to 75 volts, the voltage being automatically increased to compensate for the drop in the grid resistance. The voltmeter, as shown in the diagram, Plate 4, is connected across machine terminals 1 and 3 and indicates the machine voltage, which is the same as the arc voltage when the change-over switch is closed.