in an ac generator of a motor vehicle , having a positive polarity brush from which current flows into a corresponding slip ring and a negative polarity brush into which current flows from the corresponding slip ring , the brushes or the springs of the brushes are configured respectively differently , to achieve substantially identical rates of wear for the brushes . this is done for example by increasing the area of contact between the negative polarity brush and its slip ring , lengthening the negative polarity brush , or decreasing the amount of force applied by the spring of the negative polarity brush .