The mercury thermometer is the most well-known device used to measure temperature.
The mercury thermometer is the most well-known device used to measure temperature.

We use a thermometer to measure temperature throughout science and engineering. Thermometers come in several types, but the liquid-in-glass mercury thermometer is the most well-known. The operation of the mercury thermometer is simple to understand once all of the different parts are identified. The main parts of a mercury thermometer are the capillary, the bulb, the scale, and the expansion chamber.


A mercury thermometer takes advantage of liquids' expansion when the temperature increases. The bulb is the lowest part of the thermometer, which has a spherical shape. This section of the thermometer acts as a reservoir to hold the mercury. If the temperature is sufficiently high, the mercury travels up the capillary.


The capillary of a mercury thermometer is the long cylindrical tube that is connected to the bulb. As temperature increases, mercury flows up the capillary. The further the mercury moves up the capillary, the higher the measured temperature. The capillary ends in a section known as the expansion chamber.

Expansion Chamber

The expansion chamber of a mercury thermometer can be found at the top of the capillary. The function of the expansion chamber is to form a larger volume through which the mercury can fill if the maximum temperature scale is exceeded. It is undesirable for mercury to reach the expansion chamber since it means the thermometer is no longer sensitive to increases in temperature.


The scale is the series of lines that are etched into an area to the side of the capillary. The scale allows the temperature to be read off in units of degrees. The type of degree unit depends upon the specific thermometer. Two commonly used temperature scales, are degrees Celsius and degrees Fahrenheit, which are found on everyday thermometers. An alternative scale that measures temperature in degrees Kelvin is often used by scientists and engineers.