How does the internal energy of a medical thermometer change when measuring temperature?

To change the internal energy of any body, this body needs to do work (or work on it) or take advantage of the phenomenon of heat transfer. The thermometer uses heat transfer to measure temperature. As a result of the presence of a temperature difference between the thermometer and the person, heat energy will be transferred, as a result of which the internal energy of the thermometer will increase, the mercury in the thermometer will begin to heat up and expand, showing the person’s temperature on the thermometer scale.