Discussion on “some troubles encountered in the operation of carbon brushes on direct-current generators and motors,” (Martindale), Cleveland, Ohio, March 19, 1915. (see proceedings for March, 1915)

D. B. Rushmore: Every designer of d-c. machines owes a debt of gratitude to Professor Arnold for his clear and careful elucidation of the phenomena of commutation. The phenomena are so exceedingly complicated, when you consider all the e.m.fs. involved, and all of the variables that can enter in the shape of quality and condition of the commutator and the carbon brush, the width, the pressure, distribution, and wear, etc. It was not until we got the little X-ray apparatus, known as the oscillograph, that we could begin to find out what was going on. The development of the electrical industry, and especially the d-c. machinery, is dependent upon this little carbon brush, and upon the peculiar qualities of contact resistance that we get with it.