Mercury-in-glass thermometer

A mercury thermometer is a liquid thermometer that uses mercury as the thermometer fluid. Mercury is first mentioned in a paper by Guillaume Amontons, which described the change in the reading of mercury barometers with temperature. The importance of this was recognized in 1718 by the physicist Daniel Gabriel Fahrenheit, who replaced the alcohol used until then with mercury. Others had done this before, but since he also added scales and his thermometers worked reliably and reproducibly thanks to calibration instructions and special manufacturing processes, he is considered the inventor of the mercury thermometer in particular, but also of the thermometer as a scientific measuring instrument in general.

Since mercury has a thermal expansion coefficient that is almost independent of temperature and does not wet the glass of the thermometer tube, mercury thermometers were the most widely used thermometers until the 1970s, especially for precise measurements. Mercury thermometers can be used in the temperature range of -38 °C (freezing point of mercury) and 350 °C (boiling point of mercury).

If thallium is added to the mercury, the measuring range can be extended to -58 °C.

The mercury thermometer can also be used up to above its boiling point if the mercury contains nitrogen under high pressure. In this way, the measuring range can be extended to 750 °C.

Since mercury as a liquid metal is electrically conductive, it is possible to build mercury thermometers that can carry out switching operations when a certain temperature value is reached by means of a contact inserted in the tube, which can be used in thermostats, for example.

Since the 1970s, mercury thermometers have been increasingly replaced by other types of thermometers, especially alcohol thermometers, because of the toxicity of mercury, in particular the mercury vapours that occur in the event of damage (contained in small globules and released into the air as soon as the thermometer breaks). A clinical thermometer with mercury as an indicator contains up to 1 g of mercury. At 20 °C room temperature, this corresponds to a sphere with a diameter of approximately 5.2 mm. Since 2009, the sale of mercury thermometers has been banned within the EU, with the exception of the scientific and medical sectors. Since 10 April 2014, the sale of mercury thermometers for commercial and industrial uses has also been banned.

See also

• Expansion thermometer