Torsion
Torsion, the stress produced in a body when it is twisted. The twist in the body is produced by torque. A body such as a cylindrical rod or a structural beam is in torsion when one end is held stationary while the other end is rotated at right angles to a line (the longitudinal axis) running the length of the rod or beam. Torsion also results when the two ends of a body are twisted in opposite directions.
A body will, within limits, tend to resist being in torsion; it will tend to snap back to its original condition when the torque is removed. Devices such as helical (spiral) springs, and toy airplanes powered by twisted rubber bands, are based on this characteristic. Torsion bars, which act as springs in automobiles, are also applications of torsion.
When torsion exceeds a body's ability to withstand it, the body will shear or break. Structural beams, gear shafts, and other objects that are, or may be, subjected to torque must be designed to withstand the torsion that will be produced in them.
A torsion balance is a device used to measure extremely small forces, such as the gravitational attraction between two small objects or the strength of small electrostatic forces. A simple torsion balance consists of a wire with a horizontal bar suspended from it. When gravitational attraction, for example, is measured, an object of known mass is brought near one end of the bar. The bar rotates slightly and the wire is placed in torsion. The amount of force applied to the bar is calculated by the amount of torsion in the wire, as shown on a graduated scale.
The torsion balance was invented independently in the late 18th century by Charles Augustin de Coulomb, a French physicist, and John Michell, an English scientist.