TORSION, in mechanics, is the twisting or wrenching of a body by the exertion of a lateral force. If a slender rod of metal suspended vertically, and having its up per end fixed, be twisted through a cer tain angle by a force acting in a plane perpendicular to its axis, it will, on the removal of the force, untwist itself, or return in the opposite direction with a greater or less velocity, and, after a series of oscillations, will come to rest in its original position. The limits of torsion within which the body will return to its original state depends upon its elasticity. A fine wire, of 'a few feet in length, may be twisted through several revolutions without impairing its elasticity ; and within those limits the force evolved is found to be perfectly regular, and direct ly proportional to the angular displace ment from the position of rest. If the angular displacement exceeds a certain limit, the particles of the body will be wrenched asunder ; or if the elasticity is not perfect (as in a wire of lead, for ex ample, before disruption takes place), the particles will assume a new arrangement, or take a set, and will not return to their original position on the withdrawal of the disturbing force.
The resistance which cylinders or prisms formed of different substances oppose to torsion, furnishes one of the usual methods of determining the elasti city and strength of materials ; and the property which a metallic wire or thread stretched by a small weight possesses of becoming twisted and untwisted in a series of isoehronous and perfectly regu lar oscillations, has been ingeniously ap plied in the torsion balance to the mea surement of very minute forces, and thereby to the establishment of the fUn dements' laws of electricity and magnet ism, and to the determination of the mean density of the earth. (See BALANCE
ox Tonsiosl.) The laws of torsion have been experi mentally investigated by Coulomb in a variety of substances ; as metallic wires, hairs, fibres of silk, &c. The method which he employed consisted in attach ing a body of given form and dimensions to the extremity of the wire, and, after twisting it through a certain angle, to abandon it to the action of the force evolved, and observe the time of the oscillations. The following general laws were found to hold good : 1. On loading a wire or thread with different weights, it will settle in different positions of stability ; that is to say, an index attached to the weight will point in different directions if the weight be I varied, and the angular deviation may amount even to a whole circumference.
2. The oscillations are isochronous.
8. The time of oscillation is propor tional to the square root of the weight which stretches the wire.
4. The time of oscillation is as the square root of the length of the wire.
5. The time of oscillation is inversely as the square of the diameter of the wire.