The change in any linear dimension of a solid, such as its length, width or thickness, is called linear expansion. The change in length of bar is directly proportional to product of the change in temperature and its original length. Suppose the original length
of a bar is L₀ when the temperature is T₀ , as shown in Figure 7. If the length of the bar is changed to L when the temperature is changed by ∆T, then the change in its length ∆L, is given by

∆L = α L₀ ∆T

where ∆T = T - T₀ and the proportionality constant a, is called the coefficient of linear expansion.