Nikola Tesla

“ H is a commutator of ordinary construction. When the electro-motive force between the brushes a and b, as the resistance of the circuit a, M, c-1, c, A, to the resistance of the circuit b, B, c, c-1, D, the potentials of the points x and y, will be equal and no current will pass over the auxiliary brush c, but if said brush occupies a different position relatively to the main brushes, the electric condition is disturbed and current will flow either from y to x, or from x to y, according to the relative position of the brushes. ”