Discussion on “physical limitations in D-C. commutating machinery” (Lamme), San Francisco, Cal., Sept. 16, 1915. (see proceedings for August, 1915)

E. H. Martindale: The point impressed on me with the greatest emphasis was in connection with the use of the soft graphitic brush for high-speed machines and with machines having solid commutators. It is a misnomer. The brush to be used on the solid commutator need not be soft. It must be non-abrasive, but one of the hardest brushes manufactured in this country has no abrasive action and is used very extensively on undercut commutators and on speeds up to 6000 ft. per min. I want to emphasize the distinction between hardness and abrasiveness, and point out the fact that this has no relation to the softness or graphitic nature of the brush.