Thermometer

The traditional method of measuring human body temperatures was with a Mercury-in-glass thermometer whose tip was 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.

Oral temperatures may only be used on patients that are capable of holding the thermometer correctly in their mouth securely, which excludes small children or people who cough too much or vomit (this is now less of a problem with fast-reacting digital thermometers, but was certainly an issue with mercury thermometers taking 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.