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GENERATORS 151
contact surface. The curvature of the contact face
makes some slight difference, but not enough to affect results appreciably.
(72) A fresh, new rubber band with a piec tring at each end, previously calibrated as to stretch by lining, say a pound, gives excellent results. The elongation of the rubber when raising the brusli holder finger can readily be measured and is almost exactly proportional to the weight raised. In the example already cited we should expect fourteen ounces to stretch the rubber seven-eighths of an inch if the pound weight elongated it one inch. Or exact results may be had by first calculating the exact pressure the brush should have and then raising an equal weight (measured on a reliable scale) with the rubber, noting the exact amount of stretch.
Care of Commutator
Much depends upon the care the commutator receives.
(73) Unless the commutator is in good condition you will have sparking, which operates immediately to roughen and "pit" the commutator surface, thus making the condition progressively worse.
Cleanliness
The commutator must be kept clean and smooth.
(74) Aside from a daily endwise brushing to remove dust and other deposits from the undercut slot between the communtator bars, there should be little need for cleaning unless too much lubrication, or lubrication of the wrong kind has been applied, or the brushes contain too much paraffin.
(75) Once every ten hours of operation the commutator should be brushed lengthwise, using a brush with bristles as stiff as may be without danger of injuring the surface. This is to remove from the undercut grooves between commutator bars any dust or foreign substance that may have lodged there. Much of the dust is carbon scraped from the brushes and copper worn from the copper bars. This worn copper is, of course, a conductor.