If one or more of the brushes in a set be out of line with their setting mark, it will be necessary to adjust the brushes up to this mark by pushing them out or drawing them back, as may be required, afterwards clamping them in position. When adjusting the brushes, the armature should always be rotated, so that the setting marks are horizontal. The rocker can then be rotated into position, and the tips of both sets of brushes conveniently adjusted to their marks. In those brush holders provided with an index or pointer for adjusting the brushes, the setting marks upon the commutator are absent, length of the pointer being so proportioned that when the tips of the brushes are in line with the extreme tips of the pointers, the brushes bear upon the correct positions on the commutator. [Illustration: Fig. 666.--Method of soldering cable to carbon brush. Drill a hole in the end, also in the side of the brush, as shown in the sketch, and after thoroughly tinning the "pig-tail," place it in the end hole and fill the holes up with solder through the side hole. Another method is to drill a hole through the carbon so that the cable will just slip through, countersink the edge of the hole a little, clean the cable thoroughly and pass it through the hole. Then with any good flux and solder, fill the countersunk part on both sides.] Ques. What should be done after adjusting the brushes to their correct positions upon the commutator? Ans. Their tips or rubbing ends should be examined while in position to see that they bed accurately on the surface of the commutator. In many instances it will be found that this is not the case, the brushes sometimes bearing upon the point or toe, and sometimes upon the heel, so that they do not make contact with the commutator throughout their entire thickness and width. The angle of the rubbing ends will therefore need to be altered by filing to make them lie flat. Ques. How is the proper brush contact secured? Ans. When the brushes do not bed properly they should be refitted to secure proper contact. Ques. How is the pressure adjustment made? Ans. This is effected by regulating the tension of the springs provided for the purpose upon the brush holders. Ques. With what pressure should the brushes bear against the commutator? Ans. The tension of the springs should be just sufficient to cause the brushes to make a light yet reliable contact with the commutator. The contact must not be too light, otherwise the brushes will vibrate, and thus cause sparking; nor must it be too heavy, or they will press too hard upon the commutator, grinding, scoring and wearing away the latter and themselves to an undesirable extent, and moreover, giving rise to heating and sparking.