Question

At the time of Onnes and Dewar, it was already known that the lowest temperature they could ever reach was -273 degrees celsius. So:

- How did they know this since the lowest temperature actually reached before the liquefaction of hydrogen by Dewar was around 60k?

- How was absolute zero determined by them?

Answer 1

Before Dewar, Lord Kelvin predicted that the absolute zero temperature was -273.150 C, i.e. on kelvin scale 0K. It was according to ideal gas equation.

the liquefaction of hydrogen by Dewar was around 60k. After achieving this he tried to liquify Helium. But he couldn't. But Onnes liquified helium. It was known at the time that helium liquify just 40 above absolute zero. In this way onnes found thay temperature of liquifivation of helium was 4.2K. In this way they determined absolute zero temperature.