The working substance of a clinical thermometer is mercury. Its bulb has a much larger volume than the capillary tube above it. Due to the influence of body temperature, the mercury in the bulb undergoes a slight change; the expansion of the mercury causes a significant change in the length of the mercury column within the tube.
Human body temperature varies between 35°C and 42°C, so the scale of a clinical thermometer is divided into 35°C to 42°C, with each degree further divided into 10 parts, thus allowing the thermometer to be accurate to 1/10 of a degree. When taking a reading, the thermometer must be removed from the armpit or mouth, at which point the temperature of the bulb will decrease. To ensure that the reading still represents body temperature, a special design is required: the very thin tube between the bulb and the straight glass tube-the constriction. The lower part of the thermometer, near the bulb, has a very narrow, curved neck. When measuring body temperature, the mercury in the bulb expands as the temperature rises, being forced through the constriction tube into the straight tube, rising from the neck to a certain position within the tube. When thermal equilibrium is reached with body temperature, the mercury column remains constant. When the outside temperature is low, the mercury contracts in volume upon contact with the cold air, breaking off at the narrow, curved neck. This prevents the mercury in the thin tube from flowing back into the bulb, maintaining the height the mercury column reached when in contact with the body. Therefore, when it leaves the body, it still indicates the body temperature. After use, a clinical thermometer should be "returned" by forcefully shaking the top of the thermometer downwards to allow the mercury that has risen into the tube to return to the bulb. Other thermometers should never be shaken; this is a key difference between a clinical thermometer and other liquid thermometers.
