Medical thermometers are traditionally mercury-in-glass thermometer used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth (oral temperature), under the armpit (axilliary 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 bulb containing mercury attached to a small tube. As the temperature rises, the mercury expands and flows up the tube. The temperature is obtained by reading the scale inscribed on the side of the thermometer. There is a constriction in the neck close to the bulb. As the temperature rises the mercury is forced up through the constriction by the force of the expansion. 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.

In the 1990s, mercury thermometers were found too risky to handle and have largely been replaced with electronic thermometers, or, more rarely, thermometers based on liquids other than mercury. In some places it may be illegal to sell products which can contain mercury, such as thermometers. Both kinds may be used orally, axilliarily, or rectally.

Oral temperature may only be taken from a patient who is capable of holding the thermometer in their mouth correctly and securely, which generally excludes small children or people who are overcome by coughing or vomiting (this is now less of a problem with fast-reacting digital thermometers, but was certainly an issue with mercury thermometers, which took several minutes to register a temperature). Another counter-indication is if the patient has drunk a hot or cold liquid beforehand, in which case one has to wait or use another method.

Rectal temperature, especially if taken by a person other than the patient, should be facilitated with the use of lubricant (such as petroleum jelly (now discouraged) or a water-based personal lubricant). Although rectal temperature is the most accurate, this method may be considered embarrassing in some countries or cultures, especially if used on patients older than young children and if not taken the correct way a rectal temperature can be uncomfortable and in some cases painful for the patient. Rectal temperature is considered the method of choice for infants for the general public; however, the rectal route is least desirable in infants from a nursing point of view.

Other kinds of medical thermometers exist such as the tympanic thermometer that measures the temperature of the tympanum by infrared measurement, and the band thermometer that are applied to the patient's brow.

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