A clinical thermometer is also a mercury-in-glass type thermometer, designed for measuring the temperature of the human body. Since the temperature of the human body varies over a short range, the thermometer scale is marked from 95F to 110F or 35oC to 43oC. The normal temperature of a healthy person is 98.4 F or 36.9oC. Another special feature of this thermometer is the constriction in the stem just above the mercury bulb. When the thermometer is placed beneath the tongue of the patient, the mercury expands and pushes through the constriction but when the thermometer is removed from the mouth, the constriction prevents the expanded mercury in the stem from falling back into the bulb. Thus, the current temperature can be read even after some time. The thermometer has to be shaken to bring the expanded mercury back to the bulb. A clinical thermometer should not be sterilized in hot water otherwise the mercury will expand too much and break the glass.
This is used mainly by doctors and nurses to take body temperature readings. It is called a clinical thermometer because it is a diagnostic devise used by clinicians, (doctors) to help reach a clinical diagnosis.