Paul Zerrahn

The materials for making this rheostat are: A piece of hard black rubber 4x5 in. and | in. thick; ten short round-head brass screws; six inches of 1/4 in. bore, hard black rubber tubing, two binding posts, a quantity of No. 24 gauge iron wire, a small strip of spring brass and four round-bead machine screws 1 1/2 in. long with nuts.

First cut out apiece of the rubber 2 in. square. Then cut out a bottom piece with projections the shape shown in Fig. 1. To know where to place the screws c c c c, draw four diagonal lines from corner to corner. This will give the center of the piece of rubber, as well as the points for the screws. Measure in on these lines 1/4 in. from the corners, marking the points so obtained. On these four points drill 1/8 in. holes, and similar holes on the other piece of hard rubber. The points e e, are round head screws, with their points filed down until they are a trifle over 1/8 in. long under the head.

With a compass draw a circle, using the center obtained as above. Divide one-half the circle into seven parts, spaced 3/8 in. apart as in Fig. 1. and drill holes on these points just large enough to hold the screws e e, etc., firmly. Then put in the screws with heads on the bottom of the piece of rubber. The filed ends should come just through on the top.

A Small Rheostat 271

Fig. 1..

From the iron wire cut lengths about one foot long, and turn up spirals by twisting around a pencil or other cylindrical shape. Connect the points e e, by soldering to them the ends of the wire spirals, with the exception of point /, which has no spiral connection. Next attach the two binding posts, b. Connect one of the binding posts with last right-hand point e. The movable arm consists of a piece of spring brass with a hole atone end for the screw, by which it is attached to the center of the rubber piece having the points e e, and long enough to allow the outer end to make full contact with the points, e e. A knob a can be made from a small porcelain picture knob, or wooden knob such as are used on the covers of cooking utensils. To ensure the arm turning with the knob, a nut is placed on the machine screw binding the arm firmly to the knob. The screw is then put through the center hole drilled in the rubber and another nut added, fastening same with a drop of solder at the same time the wire is soldered to the screw connecting it to the other binding post.

A Small Rheostat 272

Fig. 2.

The rubber tubing is then cut into four pieces 1 1/4 in. long, the long machine screws are put through the corner holes in the top rubber piece, the tubing slipped over them, the bottom piece of rubber put in place, and the nuts then tightened up, holding all parts in position, as in Fig. 2. The resistance here described can be used for regulating the current from batteries to small motors, miniature lamps, etc.