A switch such as this makes it possible to quickly reverse the running direction of small motors. In order to accomplish this, connect the two brushes on the motor to the two switch levers. Connect the contracts 1 and 3 (the two outside ones) together and lead the wire to one terminal of the battery used for operating the motor. In case the motor is operated by a generator or from the 110 volt current, connect the wire to one terminal of the power source. The middle contact on the switch (marked 2) should be connected to one end of the field winding on the motor. The other end of the field winding should be connected to the remaining battery terminal or power source. Moving the switch back and forth will send the current through the commutator in opposite directions and cause the motor to run in either direction as desired. How to Make a Small Battery Rheostat for Regulating the Speed of Small Motors, Etc. *A Rheostat* is a variable resistance for regulating the amount of current flowing in a circuit. It is a very useful device about the experimental laboratory. It may be used, for regulating the speed of small motors, the amount of current flowing into a storage battery when recharging, the brilliancy of a lamp, etc. [Illustration: FIG. 78.—Details of the Rheostat Base. The lower part of the illustration is a cross section.] Figure 77 shows a form of small rheostat which may be easily built by the experimenter. It consists of a coil of German silver resistance wire wound around a fibre form and set in a groove in a wooden base. A brass lever slides over the edge of the resistance coil so that the portion included in the circuit may be increased or decreased easily at will. The dimensions shown in the several drawings which follow are for a rheostat having a resistance of approximately ten ohms and suitable for general experimental work. It is possible to use the same plans and by doubling the dimensions, make a rheostat which is much larger and may be used to carry heavier currents or which possesses more resistance. [Illustration: FIG. 79.—Looking at the Base from the bottom showing the grooves in which the Wires are laid.] The details of the wooden base are given in Figures 78 and 79. The outside diameter of the base is four inches. It is five-eighths of an inch thick. The lower 7 part of Figure 78 shows a cross section of the base. The groove is three and one-eighth inches in diameter inside, five-sixteenths of an inch thick and five thirty-seconds of an inch wide. The hole in the centre of the base is to accommodate the shaft of the switch lever, Two holes, seven-eighths of an inch apart are drilled at the back of the base to accommodate the binding posts. The binding posts’ holes and also the hole in the centre of the base for the shaft are countersunk at the bottom as shown in Figure 79.