without breaking the current through the field helices. My improvement has besides the advantage to facilitate the self exiting of the machine in all cases where the resistance of the field wire is very great comparatively to the resistance of the main circuit at start, for instance on arc-light machines. In this case I place the auxiliary brush c, near to or in preference in contact with the brush b, as shown in Fig. 6. In this manner the part M-1, Figs. 1, 2 and 3 is completely cut out, and as the part M has a considerably smaller resistance then the whole length of the [field] wire the machine excites itself, whereupon the auxiliary brush is shifted automatically to its normal position.
In Figs. 7, 8, 9, 10 and 11 which further illustrate my invention, a, and b are the positive and negative brushes of the main circuit, and c an auxiliary brush. The main circuit D extends from the brushes a and b, as usual and contains the helices M or the field wire, and the electric lamps or other working decices D-1. The auxiliary brush c is connected to the point x of the main circuit by means of the wire c-1. 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
