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MANAGERS AND PROJECTIONISTS 608
Arc Controllers
THERE no longer exists any legitimate excuse or reason for using a hand fed projection arc, no matter whether it be a high intensity, a reflector type or an ordinary arc. There are several excellent arc controllers now available, and the improved results brought about by their use as against the hand-fed arc far more than justifies their cost. Electric-magnetic arc controllers have as their motive power a small direct current motor, which feeds the carbons, both positive and negative, forward intermittently as they are burned awa)', and at a rate which maintains what is, to all intents and purposes, a constant length of arc gap.
As has been said, the carbons are fed forward to the arc intermittently, and not continuously. This action is accomplished by means of a solenoid, or solenoids (some types of controller use one and some two solenoids, but where two are used they are so connected that they act as a unit in the control of the motor circuit) so connected that when the arc voltage rises the solenoid pulls its armature toward itself, against the action of a small spring which holds it, the armature, normally away from the solenoid contact.
This action of the armature of the solenoid has the effect of opening the circuit of the motor which drives the arc controller mechanism, thus stopping the motor and halting the feeding of the carbons forward to the arc.
When the feeding of the carbons has been thus stopped, it is not resumed until the arc has lengthened sufficiently to raise its voltage enough to increase the pull of the solenoid until it again overcomes the pull of the spring, and draws the armature toward it, thus closing the motor circuit.
Reduced to simple terms this means that, in practice, every change in distance between the carbon tips of an arc alters the arc voltage, because it operates to change the resistance of the arc. As the distance between the carbon points increases the arc voltage rises, since a higher voltage is necessary to force the current across the wider opening. Conversely, as the distance between the carbon tips is decreased the arc voltage is reduced.