A systematic investigation with the aid of an oscillograph into the behaviour of brushes running on a short-circuited commutator is described. Each brush was divided into two or more parts insulated from each other so that the distribution of current across the face of the brush could be examined. The difficult problem of reversible running received special consideration. The commutator contained a slightly high bar and two flats in order to determine the relative merits of different brush and brush-holder arrangements on an imperfect commutator. A distinction is drawn between the effect of “sudden” irregularities as exhibited by high bars, and of a “gradual” irregulalrity such as eccentricity of the commutator. The advantages of using a pressure finger of high inertia and the importance of low inertia in the brush itself are demonstrated. Finally, suggestions are made concerning the lines along which further investigation should proceed.