The earliest clinical thermometers were mercury based, consisting of a small bulb filled with mercury and connected to a thin glass neck. As the temperature rises, the mercury expands, causing it to be forced up the neck. However, due to the toxicity of mercury and the danger of these thermometers breaking, many clinical thermometers use other methods to determine temperature, such as infrared energy. These clinical thermometers use an instrument called a thermopile to detect infrared energy, which is directly related to temperature. The thermometer then converts the measurements from the thermopile to temperature.
The most common usage of a clinical thermometer is to take "daytime" temperatures during the course of a normal day. This temperature can be used to look for a fever, which is often an indication of infection. Another use of a clinical thermometer is in the reading of a basal temperature, which is the temperature of a person immediately upon awakening. This reading is more sensitive to small changes in body temperature, and allows for testing for ovulation or for changes in the function of the thyroid.
The most common method of taking someone's temperature is by getting an oral reading. This involves sticking the thermometer in the patient's mouth. If this isn't possible, or if a more accurate reading is necessary, a rectal measurement may be necessary. This is done by inserting the thermometer in the patient's rectal cavity with the aid of lubrication. In some cases, thermometers may take readings from other cavities such as the ear.