Mercury thermometers operate on the principle of thermal expansion. They consist of a glass tube with a bulb at one end that contains mercury, a liquid metal. When the temperature rises, the mercury inside the bulb heats up, causing it to expand. As the volume of mercury increases due to this expansion, it rises in the narrow tube.
The glass tube is very thin and has a small diameter, so even small changes in temperature cause noticeable movements of the mercury column. This movement is calibrated against a scale marked on the glass tube, typically in degrees Celsius or Fahrenheit, allowing users to read the temperature.
The properties of mercury make it ideal for this purpose because it has a high coefficient of thermal expansion, meaning it expands significantly with temperature changes. Additionally, it remains a liquid at a wide range of temperatures, only solidifying at extremely low temperatures (around -39 degrees Celsius or -38 degrees Fahrenheit). This characteristic allows mercury thermometers to measure temperatures within a broad range accurately.
However, despite their accuracy, mercury thermometers have fallen out of favor due to the toxic nature of mercury. Accidental breakage can release mercury vapors, which are hazardous to health if inhaled. As a result, alternative thermometers, such as digital ones or those using safer substances like alcohol or gallium, have become more popular for temperature measurement.
