Expansion

Expansion means increase of dimension in one, two, or three directions. We thus have linear, areal, or voluminal expansion. Heat is a general cause of expansion for most materials, a definite increase in temperature of the substance being associated with a corresponding increase in its dimensions. The term coefficient of expansion is hence applied to the proportional increase in dimension of a body per degree rise in temperature. Such expansion by heating must be allowed for in practice, and inasmuch as most solids expand with great force the effect may be rendered useful in many applications. The expansion of an iron girder bridge on a hot day may amount to three or four inches or more, and provision must therefore be made to render such expansion free. The walls of a building that are in danger of falling outwards may be drawn together by iron bars that are expanded by heating and then allowed to cool and contract; the iron tires of wheels are fixed on when hot, so as to bind the parts of the wheel more tightly together when cold; the expansion by undue heating of metals in close neighbourhood of each other may bring them into actual contact, and by so doing set up an electric circuit and ring an alarm bell [Electric Bells]; and many other practical applications of extension by heating might be cited. But indiarubber, and in certain states sundry other materials, do not expand on heating but suffer contraction. [Heat.]