This term is applied to the twisting or wrenching of a body by the exertion of a lateral force. If a slender rod of metal be suspended vertically, and, having its upper end fixed, be twisted through a certain angle by a force acting in a plane perpendicular to its axis, it will, .on removal of the force, untwist itself,'and return with greater or less velocity, and after a series of oscillations, will come again to a state of rest. The limits of torsion within which a body will return to its original state, depends on its inherent elasticity. A fine wire of a few feet in length may be twisted through several revolutions without impairing its elasticity, but if carried beyond a certain point, the fibers or particles will be torn asunder and assume a new position, as, for example, in a lead wire, before finally breaking.
This article was originally published with the title "Torsion" in Scientific American 13, 21, 163 (January 1858)