Thermometer

A thermometer is a device which measures temperature or temperature gradient, using a variety of different principles. There are various types of thermometers, with thermometers used for medical purposes being amongst the most well known type.

Medical thermometers

A medical/clinical thermometer showing the temperature of 38.7 °C

Mercury-in-glass thermometers were the traditional method of 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 expansion causes the mercury to expand and flow 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.

Close up of a maximum thermometer. The break in the column of mercury is visible.

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. Rectal temperature is considered the method of choice for infants.

Electronic clinical thermometer

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.

Other kinds of medical thermometers exist: tympanic thermometers measure the temperature of the tympanum by infrared measurement; band thermometers are applied to the patient's brow.

Other types of thermometer

• Bi-metal mechanical thermometer
• Electrical resistance thermometer
• Galileo thermometer
• Infrared thermometer
• Laboratory thermometer
• Mercury-in-glass thermometer
• Reversing thermometer
• Room thermometer
• Silicon bandgap temperature sensor
• Six's thermometer
• Thermistor
• Thermocouple

See also

• Timeline of temperature and pressure measurement technology