A brass or copper disk 3 inches or more in diameter and upward of ½ inch thick has its periphery divided by a number of saw cuts, which divisions are often filled in with plugs of hard rubber or fibre. This disk is mounted on a shaft, which latter is either the shaft of an electro-motor, or is provided with a pulley by which it can be rapidly rotated. A strip of spring copper on each side of the disk presses upon the toothed surface, one strip being connected to the coil and the other to the battery or other current source. It will now be seen that when the disk rotates the slits or pieces of hard rubber cause the break in the circuit through the brushes or copper strips, the rapidity of the breaks depending upon the rate of rotation of the disk, and the number of slits in the wheel.
The slits or rubber pieces should be one-half the width of the intervening brass, but must be at least one sixteenth of an inch in width, especially where a high voltage is used in the primary coil.
The shaft of the machine may serve as one point of connection in place of one of the copper brushes; but in this event either a wide journal must be used, or else some conducting substance, as plumbago, replace the lubricating oil in the bearings.
Pole Changing Breaker.
Fig. 22 shows a diagram of a pole changing contact breaker which will allow of rapid alternations of current. It is operated by an electric motor by preference, although any motive power can be applied to it.
Fig. 22.
W a W b are two brass wheels, the peripheries of which are broken by the insertion of insulating blocks I I, shown black in the sketch. S S are the shafts on which the wheels are mounted, the two wheels being necessarily insulated from each other. 1, 2, 3, 4 are four brushes of copper pressing on the rim of the wheel and leading in the current from the battery B. The primary coil is attached to the brass body of the wheel or to the shafts. When the wheel is in the position shown, the coil and battery are on an open circuit; but on the wheel commencing to revolve, the brushes 1 and 2 bear on the brass, and the current flows from the positive pole of the battery to 2 through the wheel W a to the coil P, up through wheel W b and out at 1 back to the battery. The next position of the brushes 1 and 2 will be on the insulations, and 3 and 4 will come into action. Then the positive current will reach W b by means of brush 3, and after traversing the primary coil and wheel W a, emerge at 4 to the battery, thus reversing the current through P as many times as there are sets of segments, which latter can be multiplied according to requirements. The main point to be considered after that of good connections is that the brushes 1 and 3 and 2 and 4 do not at any time touch any part of the brass wheel at the same time, as this would short circuit the battery. This is avoided by making the insulating space longer than the brass surface, and adjusting the brushes as in the sketch, that each pair of them is a fraction further apart than the length of the brass tooth.
Accordingly, a wheel may be constructed with many segments and rotated at a high speed and rapid reversals of current produced, the uses of which are manifold.