Torsion is the kind of strain produced in a bar or wire when one end is kept fixed and the other is rotated about the axis. The axis itself is in no way changed, but every other line originally parallel thereto takes the form of a helix or screw. The torsion is measured by the inclination of the diameter of any section to the diameter originally parallel to it of the section at unit distance. The moment of the forces which produce the torsion is called the torsional stress or torque; and the coefficient of torsion is the ratio of the stress to the strain. The coefficient of torsion depends on the Rigidity (q.v.) of the material and upon the size and form of the bar. For wires and bars of circular section the coefficient of torsion varies as the fourth power of the radius. That is, to produce the same torsion in two wires whose diameters are as 2 to 1, we must apply, in the case of the thicker wire, a torsional stress 16 times greater than that needed for the thinner wire. By the manufacture of excessively thin quartz fibres Professor Boys has obtained torsional coefficients of very minute magnitude.
Torsion-balance is a form of apparatus in which the torsion of a wire is used for the measurement of various kinds of forces. In this way Coulomb discovered the fundamental laws of electric and magnetic attraction (see ELECTRICITY) and Cavendish measured the density of the Earth (q.v.). By means of his quartz fibres Professor Boys has been able to reduce into a very small compass the whole apparatus for making the Cavendish experiment.
Torsion, in Surgery, is a method of common application for the purpose of checking arterial hæmorrhage in certain cases. The wounded vessel is drawn out and fixed by a pair of forceps a quarter of an inch from the end; the end of the artery is then twisted round till it will not untwist itself. It is especially useful when there are many small arteries wounded in an operation, as, for example, in the extirpation of a large tumour.