Nikola Tesla

“ It is known that in machines of great electromotive force—such, for instance, as those used for are lights—whenever one commutator bar or plate comes out of contact with the collecting brush a spark appears on the commutator. ”

“ My invention enables me to prevent the sparking in a simple manner. For this purpose I employ with the commutator-bars and intervening insulating and preferably incombustible material, which I arrange to bear on the surface of the commutator, near to and behind the brush. ”

“ In an electric apparatus in which sliding contacts with intervening insulating material are employed, with the contact springs or brushes, of a solid insulator or bad conductor of electricity, as and for the purposes set forth. ”