THE APPROACH TO THE ABSOLUTE ZERO OF 

 TEMPERATURE^ 



By F. Simon, D. Phil. 

 Clarendon Latoratory, Oxford University ; Formerly Professor of Physical 



Chemistry at Breslau 



[With 1 plate] 



Speaking in this building, with all its associations with low tem- 

 peratures, a long introduction would be superfluous, but I would like 

 to remind you of some data. In figure 1 a temperature scale is given 

 in degrees Centigrade, and you see some important fixed points 

 marked on it. 



Now is this scale infinite at both ends or not? We know that heat 

 consists in the unordered motion of the smallest particles, the atoms 

 or the molecules, and the intensity of this irregular motion rises 

 with increasing temperature (in an ideal gas it is directly propor- 

 tional to it). So it is evident that there will be no limit to high 

 temperatures, as there is none to the intensity of the motion, but, 

 of course, there will be a lower limit to the temperature scale, at 

 the point where the thermal motion stops altogether. This point 

 is therefore with justification called the absolute zero. Though it 

 has not been reached in experiment, and as we will see later on, it is 

 by principle impossible to reach it absolutely, its position can be 

 given with great accuracy, and it is found to lie at —273.1° C. A 

 rational temperature scale has, therefore, to begin at this point. In 

 this scale, the Kelvin scale, the absolute zero of temperature is given 

 by the number zero, and any other temperature by adding 273.1° 

 to the number of degrees Centigrade. On the right-hand side of 

 figure 1 the temperatures are indicated in degrees Kelvin, and you see 

 that the boiling point of the most volatile gas, helium, lies about 4° 

 from the absolute zero. In figure 2 you see the liquid helium range 

 given in more detail. By reducing the pressure over the liquid 

 helium, one can easily get down to about 1°. By improving the 

 isolation, and using a huge pump, Keesom succeeded in reaching 0.7°, 

 which was the lowest temperature obtained until a few years ago. 



^ Reprinted by permission from Royal Instilution of Great Britain Weekly Evening 

 Meeting, Friday, Feb. 1, 1935. 



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