In many branches of engineering there is an increasing demand for light materials which will operate in conditions of elevated temperature and /or under severe temperature gradients. Magnesium alloys have found extensive use in the field of aviation, due to their lightness and retention of structural strength at elevated temperatures. When this type of structure is subjected to service conditions, it is likely that the general temperature level will change, in addition to the establishment of temperature gradients. A detailed analysis of the structural displacement, loading and stress under such conditions would undoubtedly be aided by a knowledge of the mathematical “law” describing the coefficient of linear thermal expansion as a function of temperature. A single integration of this equation over the limits of temperature will give the displacement per unit length. Integration of the product of this equation and Young's modulus of elasticity, ie integration of the thermal modulus, provides an accurate determination of the thermal stresses promoted by internal restraint, resulting from temperature gradients in the structure.