ABSOLUTE ZERO — SIMON 261 



increases, but as the whole state of order must be constant the tem- 

 perature must fall (fig. 5, D). (Corresponding exactly to the adia- 

 batic expansion of the gas.) 



Within the last few yeai-s this method has been used experimen- 

 tally, and at nearly the same time Giauque and MacDougall in Cali- 

 fornia, de Haas and Wiersma in Leiden, and Dr. Kiirti and I began 

 to work with it. We developed the technique so that it is now fairly 

 easy to work with this procedure, and we can show you an experiment 

 with it here in this room. 



We will look once again at figure 4. There you see within the 

 lower vessel two different paramagnetic salts A and B, so that we 

 can do two different experiments. To carry out the cooling by this 

 method one has to have the substance in thermal contact with the 

 surroundings when the field is vswitched on and isolated when the 

 field is switched off. We do this automatically by suspending the 

 substance in a little glass tube closed at both ends and filled with 

 about 1 cubic centimeter of helium at room temperature. This gas 

 makes a heat contact with the surrounding helium bath during mag- 

 netization. Upon switching the field off the substance cools quickly, 

 and the helium gas has to condense on it, as the vapor pressure falls 

 rapidly with falling temperature. Choosing the right dimensions 

 and vacumn conditions, one can in this way use the cooling substance 

 itself as a pump. 



Now we will begin with an experiment,* taking first the upper 

 substance, A, manganese ammonium sulphate. We bring the magnet 

 into position and switch on a field of about 10,000 gauss. We have 

 to wait now for about a minute until the heat of magnetization is 

 carried away. Next we remove the magnet and bring the coils for 

 the temperature measurement into position. The substance has 

 cooled down, the thermometer points to about 0.25°. At the same 

 time you notice that the temperature keeps quite steady. 



We will now make another experiment with the lower substance, 

 B, iron ammonium alum, and the same field. We have now reached 

 0.1°, and you see that again there is hardly any change of tem- 

 perature. 



Now we will look at table 1, which givas the vapor pressures of 

 helium at different temperatures. Although they have not been 

 measured experimentally, these figures are very accurate, since we 

 have all the necessary data at our disposal for calculating the vapor 

 pressures according to the second law of thermodynamics. You see 

 that at 0.1° the vapor pressure is 10"^^, and at 0.25 it would be about 

 10-^2, so that there is practically no gas which could transmit heat 



* During the lecture a wire of the thermometer circuit broke, so that the experiment 

 could not be performed. However, it was shown to a large number of the audience three- 

 quarters of an hour later when the trouble had been repaired. 



