An instrument for measuring a person's body TEMPERATURE. Clinical thermometers previously comprised a glass capillary tube sealed at one end with a mercury-filled bulb at the other. The mercury expands (rises) and contracts (falls) according to the temperature of the bulb, which may be placed under the tongue or arm or in the rectum. Current thermometers use an electric probe linked to a digital read-out display, providing an instant reading. Hospitals have electronic devices that maintain constant monitoring of patients’ temperatures, pulse rates and blood pressure.