Question

why does the mercury level not fall on its own when the bulb of a clinical thermometer is removed from the mouth?

Answer 1

Mercury thermometers are designed with a narrow constriction at the bottom of the readout tube. As the temperature of the mercury rises, it is forced through the constriction by the increasing pressure in the bulb.

But as the temperature of the mercury falls again after being removed from the body, there is no corresponding pressure to pull the mercury down through the constriction. Consequently, the mercury level falls slowly enough to be imperceptible for several minutes, long enough for the temperature to be reliably read.

This is why you need to shake a mercury thermometer before you use it. The shaking forces the mercury back down through the constriction into the bulb.