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378 MOTION PICTURE HANDBOOK
usually be detected by the clicking sound made by the brush in passing over the defective segment. When the segment is low the brush rides in toward the shaft each time the bad bar passes under it. If it is high the brush will jump. The remedy will depend somewhat upon the cause. It may be that the segment has become loose, in which case the bar may be driven back into place by tapping lightly with a wooden mallet, or by using a wooden block and hammering gently, but the armature will probably have to be taken out and sent to the repair shop, unless you yourself can tighten the clamp ring — a rather delicate operation. If the segment is high by reason of the fact that, being of harder material than its mates it has worn down more slowly, then, using a fine file it may, with great care, be dressed down. If, on the other hand, it is low, then the only remedy is to turn down the rest of the bars to match. If the fault is slight this may be done by re'moving the brushes and holding a piece of grindstone which has been turned out to fit the circumference of the commutator to it while it is revolved rapidly. This process is, however, slow. The best way is to put the armature in a lathe and turn it off. The grinding may, in the case of a motor, however, be done with the brushes down and the machine running by its own power, but if this is done it should be done with great caution. When you are through the face of the brushes should be thoroughly cleaned by drawing No. J^ sand paper around the curve of the commutator with the sand side next to the brushes in order to grind off their face, and thus remove any particles of sand which may have become embedded in the brush, since it would scratch the commutator and cause undue wear. It is better to do the grinding with the brushes raised and the machine run from some outside source of power where it is practicable.
(g) A rough or eccentric commutator. This may be caused by improper care, or by the use of defective materials in its construction. A rough commutator may be detected merely by feeling. The mica insulation between the segments will either stand out in ridges, or be worn down so that there is a small groove between the segments. An eccentric commutator may most readily be detected by holding some instruments firmly against the frame opposite the commutator, so that its ends just touch the bars. If the commutator is true it will touch all the way round as the armature is slowly revolved, but if the commutator is eccentric it will, of course, only touch the high spots. If the eccentric