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376 MOTION PICTURE HANDBOOK
four brushes all told), divide the number of commutator segments by the number of poles, or field coils of the machine; the result will equal the distance, in commutator bars, the brushes should be apart.
(c) Dirty brushes or dirty commutator may cause sparking, and may even prevent the generator from picking up its load at starting, and will sometimes cause a badly fluctuating arc. Some of the causes of dirty brushes and dirty commutator are as follows: Carbon brushes contain a small amount of paraffine. When the carbon gets warm this paraffine, if excessive in quantity, is likely to ooze out and coat the commutator, thus partially insulating it in spots, or the paramne may mix with dust and coat the end of the brush with a semi-insulating compound. If copper brushes be used they may become clogged with a mixture of oil and dust; the obvious remedy is to clean the dirty parts. To clean the commutator, use a brush stiff enough to remove any foreign matter which may cling to the surface of the commutator, yet not stiff enough to injure the surface. If the brush will not remove the deposit,, then use 00 sand paper (never use emery paper or emery cloth on a commutator) applying the same while the commutator is revolving, but with just barely enough pressure to clean the metal. After having cleaned the surface, put a few drops of light oil on a cloth, or use the pad already described and hold it lightly to the commutator as it revolves. Don't get much oil on the surface of the commutator — just a "suspicion," as it were. If it is a carbon brush which is dirty, or which does not fit the curve of the commutator, raise it just enough to slip a piece of fine sand paper (^ or No. 1) between the brush and commutator, with the sand side against the brush, and pull it back and forth around the curve of the commutator until enough of the brush has been ground away to clean the surface, or to make it fit the commutator. Be sure and always clean the commutator thoroughly after doing this, since if carbon dust is left adhering to its surface it may work into the insulation and cause a local short circuit between two bars. If the brush is made of metal take it out and clean it thoroughly with gasoline, trimming the edges and corners off with a file if necessary.
(d) The brush not making proper contact with the commutator, which may be due to (1) tensioij spring not being strong enough; (2) tension spring having lost its temper; (3) brush stuck in its holder; (4) brush not fitting the curve