Two lines of small wire nails are driven around the outside of the points, and a German-silver wire of No. 24 B. & S. gauge wound in zig-zag fashion around the nails from one point to the other.
Fig. 314.—A Rheostat.
The rheostat is placed in series with any device it is desirable to control. When the handle is on the point to the extreme left, the rheostat offers no resistance to the current. When the lever is placed on the second point, the current has to traverse the first section of the German-silver wire and will be appreciably affected. Moving the handle to the right will increase the resistance.
If the rheostat is connected to a motor, the speed can be increased or decreased by moving the lever back and forth.
In the same manner, the light from a small incandescent lamp may be dimmed or increased.
A CURRENT REVERSER OR POLE-CHANGING SWITCH
A pole-changing or current reversing switch is useful to the experimenter. For example, if connected to a small motor, the motor can be made to run in either direction at will. A motor with a permanent magnet field can be reversed by merely changing the wires from the battery so that the current flows through the circuit in the opposite direction. If the motor is provided with a field winding, however, the only way that it can be made to run either way is by reversing the field. This is best accomplished with a pole-changing switch.
Such a switch may be made by following the same general method of construction as that outlined on pages 107 and 108, but making it according to the design shown in Figure 315.
Motors such as those illustrated can be made to reverse by connecting to a pole-changing switch in the proper manner.