Torsion is the strain set up in a rod or wire when one end is twisted with regard to the other. By considering the relative positions of two adjacent sections of a bar before and after twisting, It is clear that this strain is a shear strain. If a twisting movement or torque be applied to one end of a bar, whose other end is fixed, it is found that the twist produced is proportional to the torque, provided the limit of elasticity be not exceeded. In the case of bars of the same material and section but of different lengths, the twist for the same torque is proportional to the length; and in round bars of the same length, but of different diameters, it is inversely proportional to the fourth power of the diameter. In the case of bars or wires of the same length and diameter, but of different materials, the twist produced by a given torque is inversely proportional to the modulus of rigidity of the material - that is, to the ratio of the shear stress to the shear strain produced. The twist produced by a given torque applied to bars of different shapes depends upon the section. The fact that the angle through which the free end of a wire is twisted is proportional to the torque affords a convenient means of measuring twisting forces, and is used in a variety of instruments, such as the torsion balance, by means of which Cordoub investigated into the laws of electric and magnetic attraction. A little consideration will show that when an ordinary spiral spring is altered in length, the wire of which it is made is subjected to torsion, and such a spring obeys the same laws as a twisted wire, due allowance being, of course, made for the diameter of the mandrel on which it is wound.