This device is a variable resistor designed by English physicist Charles Wheatstone (1802 – 1875) in about the 1840s. It is composed of two cylinders, one of brass and the other of boxwood, around which the same conducting wire is wound. The cylinders may be rotated by two cranks, thus winding more wire onto one cylinder or the other. If wire is wound onto the brass cylinder, it is short-circuited and its resistance is negligible; when wire is wound onto the wooden cylinder (which is grooved so that each turn of wire is isolated from those beside it) the wire has a resistance that is directly proportional to its length, and therefore to the number of turns.

Device on display.