Question

what is the purpose of mercury in thermometer..?

Answer 1

Mercury can detect very low heat and therefore gets heated fast. Mercury is helping to detect fever in thermometer.

Answer 2

Usually, the thermometer is used to find out the heat of a person's body, by bringing the tip of the thermometer in contact with the person's armpit, or placing it under the tongue. What the mercury does is, it expands rapidly due to the heat increase that happens, hence showing the temperature, which is read by marked readings.