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INVENTIONS OF NIKOLA TESLA.

helices. The arrangement has, besides, the advantage of facilitatingthe self-excitation of the machine in all cases where the resis-tance of the field-wire is very great comparatively to the resis-tance of the main circuit at the start—for instance, on arc-light

Pig. 256.

machines. In this case the auxiliary brush a is placed near to, orbetter still in contact with, the brush b, as shown in Fig. 258.In this manner the part m' is completely cut out, and as the partm has a considerably smaller resistance than the whole length ofthe field-wire the machine excites itself, whereupon the auxiliarybrush is shifted automatically to its normal position.

In a further method devised by Mr. Tesla, one or more auxili-ary brushes are employed, by means of which a portion or the-whole of the field coils is shunted. According to the relative po-sition upon the commutator of the respective brushes more orless current is caused to pass through the helices of the field, andthe current developed by the machine can be varied at will byvarying the relative positions of the brushes.

In Fig. 259, a and b are the positive and negative brushes ofthe main circuit, and e an auxiliary brush. The main circuit u

Fig. 258.

extends from the brushes a and b, as usual, and contains thehelices m of the field wire and the electric lamps or other work-ing devices. The auxiliary brush c is connected to the point -sof the main circuit by means of the wire . h is a commutator