Syam Sundar Neti Puducherry 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 prope