ABSOLUTE ZERO — SIMON 



267 



;-T 



Before we consider the methods of 

 generating still lower temperatures, I 

 want to refer briefly to the measure- 

 ments of these temperatures. You know 

 that, in general, one measures tempera- 

 ture by the pressure of a diluted gas, 

 using it as a substitute for an ideal gas. 

 But at the lower limit of the liquid he- 

 lium range the only existing gas, the 

 helium, is stable only at a pressure so 

 small that it is of no use for a thermom- 

 eter. In this region one can use another 

 phenomenon for measuring the tem- 

 perature. 



The thermometer that I want to speak 

 of now is a magnetic one, and it depends 

 on the fact that paramagnetic suscepti- 

 bility is a function of temperature. In 

 a paramagnetic substance there exist 

 little elementary magnets, which we will 

 assume for the moment to be perfectly 

 free to point in every direction in space. 

 The thermal agitation has the effect of 

 making the directions of these elemen- 

 tary magnets have a random distribu- 

 tion. Applying a magnetic field, it will 

 try to turn them in its direction; on the 

 other hand, the thermal agitation tries 

 to establish a disorder with respect to 

 the direction of the dipoles. There will 

 be a compromise of these two effects, 

 and it is evident that the lower the tem- 

 perature, that means the lower the ther- 

 mal agitation, the more magnetized the 

 substance will become. Calculating this 

 numerically, one finds that in such a 

 substance the magnetic susceptibility figure 

 would be proportional to 1/T. That is 

 the famous Curie Law, which was de- 

 rived primarily for paramagnetic gases, because there the single 

 elementary magnets connected with the atoms or molecules are cer- 

 taiidy perfectly free. At first sight one might think that within 

 a solid body the condition of free elementary magnets could not be 

 realized. But experiments, made chiefly in the Leiden Laboratory, 

 have shown that there are some paramagnetic salts which follow 



4. — Simplified diagram 

 the apparatus. 



of 



