Mercury-in-glass thermometer 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 and the boiling point at . He named it Centigrade (100 grades). Finally, Celsius proposed a method of calibrating a thermometer: These points are adequate for approximate calibration but both vary with atmospheric pressure.
Thermodynamic temperature as SPRTs, PRTs or Platinum RTDs) and monochromatic radiation thermometers. For some types of thermometer the relationship between the property observed (e.g., length of a mercury column) and temperature, is close to linear, so for most purposes a linear scale is sufficient, without point-by-point calibration. For others a calibration curve or equation is required. The mercury thermometer, invented before the thermodynamic temperature was understood, originally "defined" the temperature scale; its linearity made readings correlate well with true temperature, i.e. the "mercury" temperature scale was a close fit to the true scale. The thermodynamic temperature is a measure of the average