Mercury-in-glass thermometer

The mercury-in-glass thermometer is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the height of the mercury within the tube, which varies according to the temperature.

The thermometer was used by the originators of the Fahrenheit and Celsius temperature scales.

Anders Celsius devised the Celsius scale, which was described in his publication the origin of the Celsius temperature scale in 1742.

Celsius used two fixed points in his scale: the temperature of melting ice and the temperature of boiling water. This wasn't a new idea, since Isaac Newton was already working on something similar. The distinction of Celsius was to use the melting temperature and not the freezing temperature. The experiments for reaching a good calibration of his thermometer lasted for 2 winters. By performing the same experiment over and over again, he discovered that ice always melted at the same temperature. He found a similar fixed point in the temperature of boiling water vapour. At the moment that he removed the thermometer from the vapour, the mercury level climbed slightly. This was related to the rapid cooling (and contraction) of the glass.

The air pressure influences the boiling point of water. Celsius claimed that the level of the mercury in boiling water is proportional to the height of the barometer.

When Celsius decided to use his own temperature scale, he chose to set the boiling point of pure water at 0º C and the freezing point at 100º C. Later it was decided by a great Swedish instrument maker that it was better the other way around.

Finally, Celsius proposed a method of calibrating a thermometer. This follows the following three steps and is the same everywhere in the world, a method which can also be used to construct a thermometer:

1. Place the cylinder of the thermometer in melting pure water and mark the point where the fluid in the thermometer stabilises. This point is the freeze/thaw point of water.
2. In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour.
3. Divide the length between the two marks into 100 equal pieces.