In a bimetal thermometer, what causes the needle to move?

In a bimetal thermometer, the movement of the needle is caused by the difference in temperature between the two metals that make up the bimetallic strip. A bimetal thermometer consists of two different types of metals that have different coefficients of thermal expansion, meaning they expand and contract at different rates when subjected to temperature changes.

When the temperature changes, one metal will expand more than the other, causing the bimetallic strip to bend. This bending translates into movement of a pointer or needle that indicates the temperature on a dial. The mechanism relies on the differential expansion of the two metals in response to temperature fluctuations, which provides a readable output on the scale of the thermometer.

The other options do not accurately reflect the principle by which a bimetal thermometer operates. For instance, the expansion of both metals evenly would not result in any bending since they would increase their length proportionally, negating the differential effect needed for movement. The contraction of metal wires is unrelated to the operation of the thermometer, and the welding process of the metals does not influence the thermal response but is rather a fabrication detail. Thus, the correct understanding centers around the differential expansion of the two metals in response to varying temperatures.