A large mercury in glass thermometer.
The thermometer was used by the originators of the
Anders Celsius, a Swedish scientist, 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 condition of melting and not that of freezing. 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 calibration mark on the thermometer. He found a similar fixed point in the calibration of boiling water to
water vapour (when this is done to high precision, a variation will be seen with atmospheric pressure; Celsius noted this). 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.
When Celsius decided to use his own temperature scale, he originally defined his scale "upside-down", i.e. he chose to set the boiling point of pure water at 0 °C (212 °F) and the freezing point at -100 °C (-32 °F).
 One year later, Frenchman
Jean-Pierre Christin proposed to invert the scale with the freezing point at 0 °C (32 °F) and the boiling point at 100 °C (212 °F).
 He named it Centigrade (100 grades).
Finally, Celsius proposed a method of calibrating a thermometer:
- Place the cylinder of the thermometer in melting ice made of pure water and mark the point where the fluid in the thermometer stabilises. This point is the freeze/thaw point of water.
- In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour.
- Divide the length between the two marks into 100 equal parts.
These points are adequate for approximate calibration but both vary with atmospheric pressure. Nowadays, the
triple point of water is used instead of the freezing point (the triple point occurs at 273.16 kelvins (K), 0.01 °C).
Before the discovery of the true thermodynamic temperature, the thermometer defined the temperature; thermometers made with different materials would define different temperature scales (a coloured alcohol thermometer would give a slightly different reading than a mercury thermometer at, say half-scale). In practice, several materials gave very similar temperatures to each other and, when discovered, to the thermodynamic temperature.