Motion picture handbook; a guide for managers and operators of motion picture theatres ([c1916])

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FOR MANAGERS AND OPERATORS 375 variations in speed, and this will, in itself, cause sparking, since it has the effect of producing heavy fluctuations in the voltage. The remedy, of course, is to tighten the belt, or use a belt dressing, and, in this connection, ordinary black printer's ink is as good an article as I know of to stop belt slipping, and ten cents worth obtained at any printer's will last for a month or more. Plate 1. Plate 2. Figur 180. (b) Brushes not set correctly, that is to say, the rocker arm too far one way or another; also the brushes may be too close together or too far apart. In the first case the remedy is to move the rocker arm until the neutral position is found, whereupon sparking will either cease or be reduced to a negligible quantity. If this fails to remove the trouble I would then see if the brushes themselves are the correct distance from each other. In a two-pole machine they should bear on the commutator at diametrically opposite points. That is to say, the distance from brush-point to brush-point should be exactly the same when measured both ways around the commutator; in other words, distance A should equal distance B, ,as per 1, Fig. 180. If it be a four-pole machine * with two positive and two negative brushes (four altogether) the correct distance to set them is one-fourth of the circumference of the commutator between the points of adjacent brushes, that is, distances marked X should all be equal, as per 2, Fig. 180. If it be a mc*chinv, with more than two positive and two negative brushes (more than