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 doesnï¿½t 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.
Answered by: Steve Beecher, Ph.D., Physics Professor, Nazareth College of Rochester
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