Rheostat: a Small Ring Rheostat for Toy Motors (Boy Mechanic, 1915)

Smooth a strip of thin fibre about 1/16 in thick, 1/2 in wide and 10 in long. Wind it with No. 22 cotton-covered resistance wire, close turns from 1/4 in from one end to 1/4 in from the other, securing the ends through small drilled holes and leaving 3 to 4 in free. Bend the strip into a ring round a round object, winding outward, and note its inside and outside diameters.

Round the corners and top edges of a seasoned hardwood block 1/2 x 4 1/2 x 4 1/2 in, mark the ring's two diameters from the centre, and saw out the circles so the ring fits between them. Screw this onto a second board 1/4 x 4 3/4 x 4 3/4 in from below with countersunk screws. Drill a hole for one wire end, fit two back-connected binding posts in the corners, and cut grooves underneath for the leads; one post goes to the winding, the other to the centre bolt.

Cut the arm from 1/16 in sheet brass, 2 in long, 1/2 in wide at one end and 1/4 in at the other. Under the narrow end solder thin spring brass to bear on the ring; fit a small handle on top. Mount it on a 1/8 in brass bolt about 1 in long through the centre, with washers under it, the base hole countersunk underneath 1/2 in wide and 1/4 in deep. Solder a short brass strip to the nut, set in a recess, and lock with a second nut or a spring washer.

Set the ring in its groove. Sand the insulation off the top edge of the winding so the spring contact touches the wire. Give the wood a coat of shellac.