Why is a mercury thermometer used to measure body temperature?

Mercury thermometer is a very convenient device for measuring the body temperature because of the many advantages it has. The mercury thermometer works on the principle of thermal expansion of mercury filled in a small bulb connected to a capillary tube. In principle, any other liquid can be used equally well; but a few considerations work in favour of mercury. Mercury has highly linear expansion behaviour in the temperature range of interest for clinical usages; which means that the amount of expansion is same for the same rise in temperature. And the extent of expansion is sufficient for observation of temperature changes of 0.1 degree. This helps in graduating the thermometer easily and accurately. Further, mercury, unlike water or alcohol, does not wet the walls of the glass capillary along which the liquid climbs up and down. If it wetted, it would leave a small amount of it stuck to the walls and would render the reading erroneous. Another unique property of mercury is its surface tension in contact with the glass walls. It has an upward curvature enabling reading the temperature easily. Also, the metallic lustre of mercury is convenient for taking the reading. It may be added that there do exist thermometers using other liquids than mercury, but mercury becomes the material of choice because of the above advantages, which result in convenient usage, handy shape and size, and the affordable cost of the thermometer.

Source: thehindu.com