# Lord Kelvin's wonderful prediction

My question is straight forward. How did Lord Kelvin accurately determine that -273.16°C is absolute zero. I guess this was not determined by experiments cause the equipment was not so developed. ( Please note that I am not sure that it was Lord Kelvin only who gave the concept of absolute zero.)

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• You do understand the physics principle involved, don't you? – Cosmas Zachos Dec 14 '17 at 17:16
• I recommend that you familiarize yourself with some elementary termodynamics, before asking this question. It is explained there. – Alexandre Eremenko Dec 14 '17 at 17:50

Long ago I had to make a project about this topic. I disagree with other answers: Lord Kelvin, with the help of Joule, did actually find the absolute scale, because there's much more you are missing.

It's true that it was well known that $T=-273 ºC$ was acually the temperature at which wolume predicted for ideal gases gets zero. You can see it in many books like Callen thermodynamics.

However, setting $T=ºC+273$ is NOT enough to consider it an "absolute scale", because it still depends on the instrument you use. You must DEFINE temperature in such a way it doesn't depend on the substance used. You always measure a physical property related to T, but not directly T. What Kelvin did was finding a relationship between T and energies, heats and works, which are universal physical quantities that do not depend on the sustance. For this, he used the results of the ideal Carnot's engine.

Liek this, it doesn't depend on the substance but on measurable energies/heats. This is now indeed an absouet scale.

For the full development, check this book:

Gray, Andrew (1908). Lord Kelvin: an account of his scientific life and work

which you can find here: https://archive.org/details/lordkelvinanacc01graygoog

It was observed that if you change the temperature of a gas at constant pressure, its volume changes linearly with the temperature (Charles' law), at least if the temperature is low enough. The slope of the line turns out to be proportional to the volume of the gas at some reference temperature, say $T_0 = 0^\circ$C, with the proportionality constant $\alpha$: $\Delta V / \Delta T = \alpha V_0$. That means if you extrapolate the $V$ vs. $T$ line back to zero volume, you get a minimum possible temperature $T_\text{min}$, which is given by $T_\text{min} = T_0-1/\alpha$. Furthermore, that minimum temperature turns out to be the same for different gases, or for the same gas at different pressures, or for different initial volumes $V_0$. That is, it's a universal constant and not the property of a particular gas. Gay-Lussac first measured it in 1802 to be $-267^\circ C$, and more recent measurements have put it at $-273.15^\circ C$.

He didn't accurately determine the value - just the principle that there was an absolute zero temperature scale.

Although probably fairer to say he popularised it, or put it on a firmer theoretical foundation, since Guillaume Amontons seems to have made a reasonable experimental estimate of absolute zero 150 years earlier.