Clinical thermometer is a sensitive Fahrenheit scale thermometer. For measuring temperature of human body this thermometer is extensively used. For this reason this thermometer is called clinical thermometer or doctor’s thermometer.
Construction: It consists of a cylindrical bulb B which is filled with mercury. A capillary tube T of uniform and fine bore is attached to the bulb. Just above the bulb at point C the tube is made narrow and bent. Since the temperature of human body varies from 95°F to 110°F, the thermometer is calibrated in this temperature range. Each degree is divided into five small equal divisions. Besides, as the temperature of a healthy man is 98.4°F, so special bold or red mark is made on the surface of the thermometer at 98.4°F.
Working Principle: The thermometer is thoroughly shaken before measuring the temperature of the body. Then the mercury level drops and remains inside the bulb. In this condition if the thermometer is placed under the tongue or under the arms, the temperature and consequently the volume of the thermometer increases. As a result, some mercury flows through the capillary tube C from the bulb B. When the thermometer is taken out of the body, the mercury level contracts. Consequently, the mercury above the level C goes back to the bulb. But since the mercury level above C cannot come back to the bulb through the fine capillary tube, it remains above C. So the upper level of the mercury surface indicates the temperature of the body. Before using the thermometer again it should be shaken thoroughly so that mercury level goes back to the bulb.
Even after the thermometer removed from the body. The temperature can be read. Since it is measured the highest temperature of a body. So it is called maximum thermometer.