CHAPTER XXXVIII.

Auxiliary Brush Regulation of DmicoT Curkknt Dynamos.

An interesting method devised hv Mr. Tesla for the regula-tion of direct current dynamos, is that which lias come to heknown as the “third brush” method. In machines of this type,devised by him as far back as 1885, lie makes use of two mainbrushes to which the ends of the field magnet coils are connected,an auxiliary brush, and a branch or shunt connection from an in-termediate point of the field wire to the auxiliary brush. 1

The relative positions of the- respective brushes are varied,either automatically or by hand, so that the shunt becomes in-operative when the auxiliary brash has a certain position uponthe commutator; but when the auxiliary brush is moved in itsrelation to the main brushes, or the latter are moved in theirrelation to the auxiliary brush, the electric condition is disturbedand more or less of the current through the field-helices isdiverted through the shunt or a current is passed over the shuntto the field-helices. By varying the relative position upon thecommutator of the respective brushes automatically in propor-tion to the varying electrical conditions of the working-circuit,the current developed can be regulated in proportion to the de-mands in the working-circuit.

Fig. 253 is a diagram illustrating the invention, showing onecore of the field-magnets with one helix wound in the same direc-tion throughout. Figs. 254 and 255 are diagrams showing onecore of the field-magnets with a portion of the helices wound inopposite directions. Figs. 256 and 257 are diagrams illustrating

1. The compiler has learned partially from statements made on severaloccasions in journals and partially by personal inquiry of Mr. Tesla, that agreat deal of work in this interesting line is unpublished. In these inventionsas will be seen, the brushes are automatically shifted, but iu the broad methodbarely suggested here the regulation is effected without any change in theposition of the brushes. This auxiliary brush invention, it will be remem-bered, was very much discussed a few years ago, and it may he of interest thatthis work of Mr. Tesla, then unknown in this field, is now brought to light.