Limits of injurious sparking in direct-current commutation

In a paper read before the American Institute of Electrical Engineers in December, 1898, the author advanced the hypothesis that the injury done by sparking at the brushes of a direct-current dynamo was not due, as was then commonly supposed, to the current jumping across the gap between the segment and the brush, but was done while the brush was in contact with the segment, the copper of the segment being first melted and then volatilized by the concentration of current energy at one or the other of the segment edges. Perfect commutation was also defined as such a change in the current in the coil under commutation that the current density over the contact surface between the brush and the segment would be constant and uniform, and that perfect commutation would result if the coil impedance were negligible as compared with the brush contact resistance. It was also shown that with perfect commutation the total energy developed over the contact surface would be a minimum and that every part of the contact surface would receive an equal amount of energy. On this principle the question of sparking is simply a question of the temperature of the segment while it is under the brush.