Rheostat

Rheostat, the name given by Wheatstone to an instrument for varying an electric resistance between given limits. Many forms have been suggested and used by Pouillet, Jacobi, Poggen dorff, Wheatstone, and others. The most serviceable is perhaps Sir W. Thomson's modification of Wheatstone's double-cylinder rheostat. In it a platinum or platinoid wire is wound round two parallel cylinders, one of which is metal and the other of some insulating material. In any position the part of the wire which is effective as a resistance is the part that is on the insulating cylinder up to where it comes in contact with the metal cylinder. By means of a gearing of toothed wheels and screw shaft the two cylinders are turned simultaneously in one or the other direction, while at the same time a nut travels to or fro and guides the wire as it leaves the one cylinder and coils itself round the other. See ELECTRICITY.