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CURRENT RECTIFICATION 317
it shows that the brush finger is too tight and the chances are that a good cleaning is in order. The purchase price of the scale is fully justified by the discovery of an undesirable condition even in a single brush holder.
If the brush is, for example one and one-half inch wide by three-eighths of an inch thick its contact area is roughly nine-sixteenths of a square inch, so that the balance reading should be fourteen ounces.
(137) Width times thickness will serve for calculating contact surface. The curvature of the contact face makes some slight difference, but not enough to affect results appreciably.
(138) A fresh, new rubber band with a piece of string at each end, previously calibrated as to stretch by lifting, say a pound, gives excellent results. The elongation of the rubber when raising the brush 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 e*xact amount of stretch.
Care of Commutator
Much depends upon the care the commutator receives.
(139) 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.
(140) 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.