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. [Illustration: 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. The two outside points or contacts (marked _D_ and _D_) should both be connected to one of the brushes on the motor. The middle contact, _C_, is connected to the other brush. One terminal of the field is connected to the battery. The other terminal of the field is connected to the lever, _A_. _B_ connects to the other terminal of the battery. [Illustration: Fig. 315.—A Pole-Changing Switch or Current Reverser. The Connecting Strip is pivoted so that the Handle will operate both the Levers, A and B.] When the switch handle is pushed to the left, the lever _A_ should rest on the left-hand contact, _D_. The lever _B_ should make contact with _C_. The motor will then run in one direction. If the handle is pushed to the right so that the levers _A_ and _B_ make contact respectively with _C_ and _D_ (right-hand), the motor will reverse and run in the opposite direction. A COMPLETE WIRELESS RECEIVING SET Many experimenters may wish to build a wireless receiving set which is permanently connected and in which the instruments are so mounted that they are readily portable and may be easily shifted from one place to another without having to disturb a number of wires.