Torsion
in mechanics, the force with which a body, as a thread, wire, or slender rod, resists a twist, or the force with which it tends to return to its original state on being twisted. Such machines as capstans and windlasses, also axles, which revolve with their wheels, are, when in action, subjected to be twisted, or undergo the strain or torsion.
If a slender rod of metal be suspended vertically, so as to be rigidly fixed at the point of suspension, and then twisted, through a certain angle, it will, when the twisting force ceases to act, untwist itself or return in the opposite direction with a greater or less force or velocity till it comes to rest in its original position. The limits of torsion within which the body will return to its original state depend on its elasticity, and the force with which it tends to recover its natural state is termed elasticity of torsion. This force is always proportional to the angle through which the body has been twisted.
If a body is twisted so as to exceed the limit of its elasticity, its particles will either be wrenched asunder or it will take a set, and will not return to its original position on the withdrawal of the twisting force.
In surgery, the twisting of the cut end of a small artery in a wound or after an operation, for the purpose of checking hemorrhage. The bleeding vessel is seized by an instrument called a torsion forceps, drawn out for about a quarter of an inch, and then twisted round several times, till it cannot untwist itself. a