torsion balance, instrument used to measure small forces. It is based on the principle that a wire or thread resists twisting with a force that is proportional to the stress. The torsion balance consists essentially of a wire or thread attached at one end and arranged in such a way that a force applied at the other, or free, end tends to twist it out of shape. The force is measured by the extent to which the wire or thread is so twisted. Torsion balances are used to measure small electric, magnetic, and gravitational forces. One type is used to measure small weights. The invention of the torsion balance is commonly credited to the English geologist John Michell, who made his instrument c.1750, and to the French physicist Charles A. de Coulomb, who independently devised such a balance c.1777.

torsion, stress on a body when external forces tend to twist it about an axis. See strength of materials.

Rod or bar that resists twisting (see torque) and has a strong tendency to return to its original position when twisted. In an automobile, a torsion bar is a long spring-steel element with one end held rigidly to the frame and the other end twisted by a lever connected to the axle. It thus provides a spring action for the vehicle.

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Device used to measure the gravitational acceleration at the Earth's surface. It consists essentially of two small masses at different elevations that are supported at opposite ends of a beam. The latter is suspended from a wire that twists because the masses are affected differently by the force of gravity. When the wire is twisted, an optical system indicates the angle of deflection, and the torque, or twisting force, can be calculated. The torque is correlated with the gravitational force at the point of observation. Torsion balance may also refer to a device used in weighing, a type of equal-arm balance.

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