Jean-Pierre Christin (31 May 1683 – 19 January 1755) was a French physicist, mathematician, astronomer, and musician. His proposal in 1743 to reverse the Celsius thermometer scale (from water boiling at 0 degrees and ice melting at 100 degrees, to where zero represented the freezing point of water and 100 represented the boiling point of water) was widely accepted and is still in use today.[1][2][3]
Christin was born in Lyon. He was a founding member of the Académie des sciences, belles-lettres et arts de Lyon and served as its Permanent Secretary from 1713 until 1755. His thermometer was known in France before the Revolution as the thermometer of Lyon. One of these thermometers was kept at the Science Museum in London.[4]
The first thermometers used in Lyon were made according to the indications of René Antoine Ferchault de Réaumur, who created the alcohol thermometer in 1730 and applied a graduation divided into 80 parts between the melting point of water (0°) and the boiling point (80°).
After experimenting, Christin considered that the mercury thermometer was preferable to the one with alcohol and pronounced in favor of its instrumentation in a conference on 13 June 1740. He noticed the idea of the centesimal division when, in his research, he observed that a quantity of mercury poured into a test tube, first condensed by the cold of crushed ice and then expanded by the heat of boiling water, moved its volume from 66 to 67 parts of the tube and represented them as 6600 and 6700 parts. By registering the mercury dilated in one hundred parts, he found it natural to divide the space traveled by one hundred hundredths, being that these new units, smaller than those of Réaumur, would be more in harmony with the sensations caused by the variations in temperature.
With this in mind, in 1743 he created the mercury-based "thermometer of Lyon" and with its new centigrade degree thermometer with 0 degrees to indicate the temperature at which the water reaches its point of fusion and 100 degrees where it reaches its boiling point.
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