In the subsequent working of the dynamo it will of course be unnecessary to follow the whole of these proceedings every time the machine is started, as it is extremely unlikely that the machine will be damaged from external causes while working without the attendant being aware of the fact. Adjusting the Brushes.--The _adjustment of the brushes_ upon the commutator requires careful attention if sparking is to be avoided. There are two adjustments to be made: 1. For pressure; The brushes must bear against the commutator segments with sufficient pressure for proper contact. 2. For lead. The brushes must have the proper angular advance (positive or negative, according as the machine is a dynamo or motor) to prevent sparking. Ques. At what point on the commutator should the brushes bear? Ans. The points upon the commutator at which the tips of the brushes (carried by opposite arms of the rocker) bear, should be, in bipolar dynamos, at opposite extremities of a diameter. In multipolar dynamos the positions vary with the number of poles and the nature of the armature winding. Ques. What provision is made to facilitate the correct setting of the brushes? Ans. Setting marks are usually cut in the collar of the commutator next to the bearing. [Illustration: Figs. 664 and 665.--Diagrams illustrating how to set brushes. Some brush holders require brushes set _with_ the direction of rotation of the commutator, and others, set _against_ the direction of rotation. In fig. 664 is shown a brush holder of the first class, which must always be set as indicated by the arrow. If set in the opposite direction, trouble will ensue, as an inspection of the figure will show, because the surface of the commutator and the brush would form a toggle joint, and the brush would tend to dig into the commutator and either break itself or bend the brush rigging. In fig. 665 is shown a brush holder of the second type. This brush is set against the direction of rotation, but an inspection of the cut will show that there is, in this case, no tendency for the brush to dig into the commutator surface. Each type of brush holder, of which there are several, should be adjusted as recommended by the manufacturer to secure proper working.] Ques. How are the brushes set by these marks? Ans. The tips of all the brushes carried by one arm of the rocker are set in correct line with the commutator segments marked out by one setting mark, and the tips of the brushes carried by the other arm or arms are set in correct line with the segments marked out by the other mark or marks.