In the described bimetal strip, which metals form the bonded pair?

• A. Copper
• B. Nickel
• C. Copper or brass with Invar
• D. Aluminum

Answer: (C)

A bimetal strip works because two bonded metals have different coefficients of thermal expansion. When temperature changes, the metal that expands more causes the strip to bend, while the metal that expands less resists, creating curvature.

Invar is famous for its very low coefficient of thermal expansion, so pairing it with a metal that expands more—like copper or brass—produces a strong bending effect as temperature rises. Therefore, the bonded pair described is copper or brass with Invar.