If during the course of service or repairs, the commutation appears unsatisfactory and warrants immediate attention, the source of poor operation may be attributed to several causes. The connections should be checked to insure good electrical contact and the brushes replaced if they are worn dangerously short. Care should be exercised to use the same grade and size of carbon brush, as these characteristics were deter- mined when the machine was designed and are important contributing factors for satisfactory operation. If there is excessive sparking in the machine, the brushes may be improperly positioned on the commutator resulting in unbalanced comnutation, This can be remedied by shifting the brush yoke to where there is the least amount of sparking. A rough or a dirty commutator, high mica, high or low bars, burned bars, and flat bars represent the chief causes producing chattering and noise. It may be necessary to clean the commutator with fine grain sandpaper applied to the surface with moderate pressure. The brushes must first be lifted from the commutator surface, and, if possible, the machine should be driven without load to provide a constant rotation of the armature at normal speed. Emery paper and emery cloth should never be used on a commutator because emery is an electrical conductor and particles are likely to become imbedded in the mica segments between the bars and cause short circuits. While sandpapering offers the most satisfactory method for removing dirt and small surface irregularities, it may become necessary to resurface a commutator which is badly out of true, by grinding or turning with a tool having a rigid support. The commutator connections should first 3-56