What happens when a bimetallic temperature gauge is heated?

When a bimetallic temperature gauge is heated, one metal expands more than the other due to the differing thermal expansion coefficients of the two metals used in the gauge. This characteristic results in the metals bending as they are heated, which translates this mechanical movement into a reading on the gauge. The design leverages the property that different metals will expand at different rates, creating a bending moment that can be measured and converted into a temperature reading.

This principle is fundamental to the operation of bimetallic temperature gauges, making option A the correct choice. Other options fail to capture the essential action of thermal expansion and its role in temperature measurement.