Take an ordinary lead pencil and cut seven notches at equal intervals on the pencil down to and around the lead, leaving it bare. A seven-point switch is constructed on a board of suitable size making the points by using screws that will go through the board. A small piece of tin or brass will do for a switch and is fastened as shown. The connections are made on the back side of the board as shown by the dotted lines. This will reduce 40 to 50 volts down to 5 or 10 volts for short lengths of time.--Contributed by Roy Newby, San Jose, Cal.
Illustration: Simple Rheostat
In a board 7 in. long and 5 in. wide bore holes about 1/4 in. apart, in a semicircle 2 in. from the bottom, and cut notches in top end to correspond with the holes. From a piece of brass a switch, C, is cut with a knob soldered on at the end. Nails for stops are placed at DD. Two binding-posts are placed in board at A and B. With about 9 ft. of fine iron wire attach one end to the bottom of post A and run through first hole and over in first notch to back of board and then through second hole and over second notch and so on until E is reached, where the other end of wire is fastened. Connect switch to post B. --Contributed by Edmund Kuhn, Jr., East Orange, N. J.
A spring from an old roller is mounted on a board 4 in. wide, 9 in. long and 3/8 in. thick. A binding-post is fastened to this board at each end, to which is attached the ends of the spring, as shown in Fig. 1. The temper of a small portion of each end of the spring will need to be drawn. This can be accomplished by heating over an alcohol lamp or in a fire and allowing it to cool slowly. The ends are then shaped to fit the binding-posts. A wire is connected to one of the binding-posts and a small square piece of copper is attached to the other end of the wire, as shown in Fig. 2. When this device is placed in a circuit the current can be regulated by sliding the small square copper piece along the spring. --Contributed by H. D. Harkins, St. Louis, Mo.
Illustration: Battery Rheostat