Fig. 255.
It is true that auxiliary brushes have been used in connection with the helices of the field-wire; but in these instances the helices receive the entire current through the auxiliary brush or brushes, and these brushes could not be taken off without breaking the circuit through the field. These brushes cause, moreover, heavy sparking at the commutator. In the present case the auxiliary brush causes very little or no sparking, and can be taken off without breaking the circuit through the field-helices. The arrangement has, besides, the advantage of facilitating the self-excitation 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 the start—for instance, on arc-light machines. In this case the auxiliary brush c is placed near to, or better 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 part M has a considerably smaller resistance than the whole length of the field-wire the machine excites itself, whereupon the auxiliary brush is shifted automatically to its normal position.
| Fig. 256. | Fig. 257. |
In a further method devised by Mr. Tesla, one or more auxiliary brushes are employed, by means of which a portion or the whole of the field coils is shunted. According to the relative position upon the commutator of the respective brushes more or less current is caused to pass through the helices of the field, and the current developed by the machine can be varied at will by varying the relative positions of the brushes.
Fig. 258.
In Fig. 259, 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 of the field wire and the electric lamps or other working devices. The auxiliary brush c is connected to the point x of the main circuit by means of the wire c'. H is a commutator of ordinary construction. It will have been seen from what was said already that when the electro-motive force between the brushes a and c is to the electromotive force between the brushes c and b as the resistance of the circuit a M c' c A is to the resistance of the circuit b C B c c' D, the potentials of the points x and y will be equal, and no current will pass over the auxiliary brush c; but if that 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. In the first case the current through the field-helices will be partly neutralized and the magnetism of the field magnets will be diminished. In the second case the current will be increased and the magnets gain strength. By combining with the brushes at a b c any automatic regulating mechanism, the current developed can be regulated automatically in proportion to the demands of the working circuit.
In Figs. 264 and 265 some of the automatic means are represented that maybe used for moving the brushes. The core P, Fig. 264, of the solenoid-helix S is connected with the brush a to move the same, and in Fig. 265 the core P is shown as within the helix S, and connected with brushes a and c, so as to move the same toward or from each other, according to the strength of the current in the helix, the helix being within an iron tube, S', that becomes magnetized and increases the action of the solenoid.