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536 HANDBOOK Oi/ PROJECTION FOR
age will be very low. This prevents the field from becoming so hot and also prevents overtravel of the voltmeter hand.
THE CHANGE-OVER— When the time approaches for change-over, close the table switch on the idle projector and bring the carbons of its lamp into firm contact. With the arc in the projector you are using adjusted to normal operating voltage, open the short circuiting switch of the idle projector VERY SLOWLY. If, as the blade leaves the contact, there is an arc, CLOSE THE SWITCH AGAIN INSTANTLY. The arc is evidence that the carbons of the lamp are not making good contact, and if you open the switch under such a condition, you will probably break the circuit entirely, thus putting out the arc which is burning, leaving a blank screen until it can be again struck. If the carbons are in good electrical contact, no arc will appear as you open the short circuiting switch.
Immediately after opening the short circuiting switch, separate the carbons, RATHER SLOWLY, and adjust the arc to normal working length, whereupon you will have both arcs burning in series, and your voltmeter will show approximately double w7hat it was before.
IMPORTANT — In closing down the projector which has finished its reel, do NOT merely shove the short circuiting switch shut. That will put out the arc, yes, but not in the best way. It is an unnecessary and rather heavy electrical shock to the generator to have half its load thus yanked off instantly. Instead, bring the carbons of the lamp into contact with each other, not too rapidly, then close the short circuiting switch and open the projector table switch. That is the RIGHT WAY TO DO IT. Merely closing the short circuiting switch, without first bringing the carbons into contact, is VERY BAD PRACTICE.
IMPORTANT — When the motor-generator is running idle you should keep BOTH short-circuiting switches closed, since this prevents the voltage building up to an abnormally high value, thus imposing over-travel of the voltmeter hand and probable heating of the generator field windings.
CARE OF MOTOR GENERATOR. Type SK. Commutator : See General Instruction No. 7, page 502. Brushes are set in the neutral position at the factory and the bracket which holds them is dowel pinned into permanent position, which should not be altered, since it is correct, no matter which di