Heat causes metals to expand. The expansion of unit of length for one degree is called the linear coefficient of expansion. The increase per degree for unit of surface is called surface expansion; for unit of volume it is called cubic expansion. A steel joist 3 ft. long is, for example, about 1/8 in. longer in summer than in winter; hence long steel structures must not be rigidly fixed at both ends. Steel car-rails are laid about 1/2 in.apart to allow for expansion. The amount of expansion of various substances in length, area, and cubic contents or capacity is given in the following table. For each degree of heat the metal expands the fraction of an inch indicated.