Why mercury is used in thermometers Mercury is used in thermometers because It has a high coefficient of expansion so that even a small rise in temperature brings about sufficient expansion which can be detected in the capillary of the calibrated part of the thermometer. Mercury is a liquid from −39∘C to 357∘C. Mercury is a good heat conductor. As a result, the mercury level indicates the temperature as soon and accurately as feasible. Mercury does not adhere to the surface of the glass. As a result, it can freely move inside the capillary glass tube. Mercury expands uniformly across the entire temperature range. As a result, the temperature is accurate.