Why does a mercury-in glass thermometer first drop slightly when placed in boiling water?
When placed in boiling water, the materials comprising the thermometer respond in the order in which the heat penetrates them. As the heat first permeates the glass cylinder, the cylinder expands, enlarging the bore. For the moment until the mercury is also heated, the constant volume of mercury fills the expanded bore to a slightly lower depth. When the heat reaches the mercury and it also expands, its volume to increase, and the liquid depth rises in the bore.
