A clinical thermometer consists of a narrow glass rod filled with a liquid like mercury or alcohol which easily expands on heating. Only a part of the tube is filled with the liquid. Another important part is a bulb. It is the portion which came into contact with the human body to measure temperature. At the time of contact, the temperature of the bulb will be the same as that of the human body and by this temperature, liquid level rise. The markings on tube help to identify the temperature.

The table shown below gives the differences between clinical and laboratory thermometers.