Question

When a thermometer is inserted into hot water, the mercury or alcohol is sometimes observed to fall slightly before it rises. Why?

Answer 1

Solution:

There are two physical properties that affect the thermometer as it comes into thermal equilibrium with the body in which the temperature is to be measured. They are the thermal expansion and the thermal conductivity of the materials that compose the thermometer. Glass has a relatively low thermal conductivity, approximately 1 W/(m-°C), and a low volume expansion coefficient, approximately 10^-5 (°/C) while mercury has a volume expansion coefficient of about 10^-4 (°/C), a factor of 10 greater than glass. As energy flows into the thermometer the glass warms first and starts to expand and its volume increases. Because of the low thermal conductivity of the glass the mercury does not expand initially, so its volume doesn't change initially and the height of the mercury column actually decreases due to the larger capillary volume of the glass. Eventually the glass comes into thermal equilibrium and ceases to expand while the mercury continues to expand until it too comes into thermal equilibrium with both the glass and the body. The rate of the expansion of the mercury is determined by in part by the thermal conductivity of the glass.