Nearly every possible loss of power about a dynamo has been investigated more or less thoroughly and efforts made to eliminate such loss by careful design of the part in which the loss occurs. The losses at the brushes due to friction, electrical resistance, etc. have been investigated several times but the results of these tests do no always agree with each other and in so far as they do agree are not generally known. It is usual, in finding the losses of a machine, either experimentally or by calculation, to include the brush losses with the load losses. The actual value of the losses at the brushes is seldom known. Consideration of the lack of certain knowledge of this subject, led us to attempt this investigation. It was our original intention to investigate the voltage drop and contact resistance of several kinds of brushes at different current densities, different brush pressures, different speeds of rotation of the commutator or collector rings, different frequencies of alternating current, etc.