Rheostat

Rheostat is an arrangement whose electrical resistance can be readily varied, and is used for adjusting the strength of electric currents. The instrument originally invented by Wheatstone consisted of two cylinders, one of wood and one of brass, fitted in a frame so that they could be turned by handles. A wire was wound in an open spiral partly on one and partly on the other, and springs were provided which rubbed on the ends of the wire to enable it to be introduced into a circuit. The part of the wire wound on the brass cylinder was short-circuited and so cut out; but that on the wooden cylinder was interposed in the circuit. By winding the wire from the brass to the wooden cylinder the resistance could be increased, and vice versa. In rheostats of modern construction a number of coils of wire of any required resistance are fixed in a frame, and their ends are connected to brass blocks. By means of a switch lever touching the blocks any of the coils can be introduced into the circuit. In another form a pile of carbon plates is provided with copper connections at the ends, and arranged to be compressed by a screw; the resistance interposed at the points of contact of the plates diminishes as the pressure is increased. A trough of liquid (zinc sulphate, dilute acid, mercury, etc.), having movable connection plates, is also sometimes used as a rheostat.