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380 MOTION PICTURE HANDBOOK
but the speed and current are increased. A weak field may be caused by (1) a loose joint in the magnetic circuit; (2) heat may lower the insulation of the field winding sufficiently to allow the current to short circuit through it; (3) there may be a metallic short in the field coil. Remedies: With at voltmeter test across each field coil; the one showing the least drop is the defective one. If all read the same, then there is a loose joint in the magnetic circuit.
(p) A shaky foundation, or anything else that causes vibration in the machine will set up commutator sparking. The only remedy is to eliminate the vibration.
Should a ring of fire develop, or something that looks like a ring of fire, around the commutator, it may be caused by (a) a piece of copper pulled across the insulation between two bars: (b) an open circuit in the armature.
In the first instance the ring will not be strong, but just a thin sparkling streak of light around the commutator. The remedy is to remove whatever is causing the short between the bars, which can usually be done by holding a piece of fine sand paper lightly to the commutator, though the right way is to stop the machine and hunt up the trouble, using a magnifying glass if necessary. An open circuit in the armature, however, might be caused by reason of a break in one of the armature wires itself, or in one of its connections with the commutator, and these in turn may be caused by excessive current burning off one of the wires, or a nick in one of the wires may be the seat of the trouble, or the commutator may become loosened and break off one or more of the leads. The defect may be readily located, as the mica will be eaten away from between the commutator segments to which the faulty coil is connected, and the segments themselves will become full of holes and burned at the edges. If this trouble is caught in time the open may be closed and the commutator turned up true. Sometimes, by reason of carlessness, abuse or overload, the armature becomes hot, and this causes the solder on the connections between the coils and* commutator bars to soften, whereupon centrifugal force will throw it out, and there will, of course, be trouble, though there is no complete opening of circuits'. The action, however, so far as the ring of fire be concerned, is the same as if there were, and the commutator bars will become blackened and pitted and their edges burned. But if any of the foregoing faults be caught in time they can be remedied; if not it will be necessary to '