Torsion Balance, device that measures very small electric, magnetic, or gravitational forces by measuring the angle through which an arm turns before the resisting force of torsion brings it to rest. It was originally designed by the English geologist John Michell and improved by the British chemist and physicist Henry Cavendish. The apparatus was reinvented by the French physicist Charles Augustin de Coulomb, who used it to study electrical and magnetic attraction.
A torsion balance consists of two small spheres. Each sphere usually has a mass equivalent to 1 g (0.03 oz), attached to the ends of a horizontal rod that is suspended at its centre by a fine wire or, in more recent experiments, a quartz fibre. If, for example, two larger lead spheres are placed so that they are adjacent to, but on opposing sides of, the smaller spheres, the smaller masses will be attracted to the larger spheres-causing the wire or fibre to twist. The degree of twisting is measured by the movement of a beam of light reflected from the rod onto a scale. The gravitational force between the two different masses may be thus ascertained. In electrical experiments, charged conductors are substituted for the small masses. To study magnetic effects, two magnets are used.
