Medical thermometers are used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth (oral temperature), under the armpit (axillary temperature), or into the rectum via the anus (rectal temperature).
The traditional mercury-filled medical thermometer works in the same way as a meteorological maximum thermometer. The thermometer consists of a mercury-filled bulb attached to a small tube. There is a constriction in the neck close to the bulb. As the temperature rises, the force of the expansion pushes the mercury up the tube through the constriction. When the temperature falls, the column of mercury breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. To reset the thermometer, it must be swung sharply.
When it is designed for use in humans, the typical range of this kind of thermometer is from about 35°C to 42°C or 89.6°F to 109.4°F. The temperature is obtained by reading the scale inscribed on the side of the thermometer. In the 1990s, mercury-based thermometers were found too risky to handle; the vigorous swinging needed to "reset" a mercury maximum thermometer makes it easy to accidentally break it, and spill the poisonous mercury. Mercury thermometers have largely been replaced with electronic digital thermometers, or, more rarely, thermometers based on liquids other than mercury (such as heat-sensitive liquid crystals). Other modern options include digital Infrared contact or non-contact thermometers, which are also called scanner thermometers. Most medical thermometers may be used to take oral, axillary, vaginal, or rectal temperatures.
To eliminate the risk of patient cross-infection, disposable single-use clinical thermometers and probe covers are employed in clinics and hospitals.