There came the centigrade scale. The issue was that many solid/liquid thermometric substances didn't respond to temperature linearly so different thermometers produced different results. This was fixed with using gases in constant volume thermometers.

Is the centigrade scale temperature now defined as the reading on a constant volume thermometer? And what is the difference between the centigrade scale and the Celsius scale?

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    $\begingroup$ The unit name ‘degree centigrade’ was changed to ‘degree Celsius’ in 1948. $\endgroup$ – Loong Apr 7 at 9:41

Celsius and centigrade are synonymous. The boiling temperature of water has not been a fixed point for a very long time. Instead the two fixed points are absolute zero and the melting point (later triple point) of water.

But also this will change. From May 20th, temperature will be defined by fixing Boltzmann's constant. In practice, ITS-90 will remain in force and the triple point of water will remain the reference for the calibration of thermometers for ambient temperatures. https://www.nist.gov/si-redefinition/kelvin-its-90


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