The difference in lengths of two rods, made up of different materials, remains constant at all temperatures. Show that their lengths at `0^(@)C` are inversely proportional to their coefficients of linear expansion.

Explanation

Coefficients of thermal expansion of concrete and iron are almost the same. Thus on increase or decrease of temperature, iron and concrete expand or contract almost equally and concrete joints do not get separated from the rods. No thermal stress develops between them. If rods of other metals were used, the two exapansions or contractions would not match and cracks would develop.