A bimetal thermometer consists of?

A bimetal thermometer operates based on the principle of thermal expansion of two different metals that are bonded together. This construction allows the thermometer to accurately measure temperature changes because each metal expands at a different rate in response to temperature variations. When the temperature changes, the differential expansion between the two metals causes the bimetal strip to bend or twist. This movement is then translated into a readable measurement on the thermometer scale.

The choice involving two dissimilar metals welded together specifically describes the fundamental component of a bimetal thermometer, making it the correct answer. The use of distinct metals is crucial since their differing thermal expansion properties enable the thermometer to function effectively.

In contrast, other options do not accurately reflect the design or working principle of a bimetal thermometer. For example, a filament or a setup involving two wires connected to a battery does not utilize the thermal expansion principle essential for measuring temperature in this context. Thus, the construction using two dissimilar metals is what characterizes and enables the bimetal thermometer's practical application in temperature measurement.