A compound bar is a device that demonstrates the principle of thermal expansion and contraction, a fundamental concept in thermodynamics.

This device was invented by the English clockmaker, John Harrison, as part of his marine chronometer, a navigation device that allowed sailors to determine longitude at sea.

The bar consists of two bonded metal plates, one of pure copper and the other of steel. When at the same temperature, they remain straight due to the equal expansion or contraction of both metals. However, when heated, the two metals expand at different rates due to their distinct coefficients of thermal expansion, causing them both to bend.

On a microscopic level, as the metals are heated, the kinetic energy of the molecules increases. This results in the molecules vibrating more rapidly and occupying more space. Both plates grow, but since copper molecules expand up to 40% more than steel, the copper bar becomes longer than the steel bar, causing both to curl slightly in the direction of the steel.

The demonstration can be inverted by cooling the plates, causing the molecules to lose kinetic energy and plates to contract. As the metals contract at different rates, they bend slightly in the direction of the copper, thereby providing a tangible illustration of thermal contraction.