Compound bar is used to demonstrate unequal expansion in different material on heating. Highlights the role of temperature and materials in practical applications such as in thermostats. 

Other compound bars use a laminated metal known as bimetal. Although those are correct for usage, but are wrong to demonstrate the principle of unequal expansion. This bar consists of two metals, brass and steel (iron), joined together by rivets end to end, fitted to a wooden handle. On heating, the joined strips bend in a particular direction, thereby showing unequal expansion.