258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



Curie's Law down to the helium temperatures with great accuracy, 

 and this means that the elementary magnets within them have a very 

 high degree of mobility. The salts concerned are especially those of 

 the rare earth and iron groups. I cannot go into a theoretical ex- 

 planation of this behavior, but I want to mention that it is in accord- 

 ance with our theoretical ideas. 



So it is easy to construct a thermometer with such a paramag- 

 netic substance, using its susceptibility as a measure for the temper- 

 ature. And it is evident that a thermometer of this kind becomes 

 very sensitive at low temperatures. For example, between 4° and 1° 

 the susceptibility changes in the proportion 1 : 4. We have a ther- 

 mometer of this kind in our apparatus. We measure the suscepti- 

 bility by bringing a system of two coils around the place where, in- 

 side the apparatus, the salt is situated, and sending an alternating 

 current through the primary coil ; the induced e. m. f . in the second- 

 ary depends on the susceptibility of the salt which is fixed within 

 these coils. This induced e. m. f. is amplified in a little set (which 

 the Cambridge Instrument Co. have very kindly lent me for the 

 demonstration) ; then it is rectified and sent to a galvanometer that 

 shows the deflections on this scale. Of course, the system is com- 

 pensated in such a way that the deflection zero corresponds to a sus- 

 ceptibility zero within the coils. Thus the deflections on the scale 

 are a direct measure of the susceptibility, and as this changes with 

 1/T, the deflections are proportional to 1/T. At the moment the 

 thermometer is showing about 2°. 



Now we can continue considering how to approach nearer to the 

 absolute zero. As I have mentioned already, the lowest temperature 

 reached by reducing the vapor pressure of liquid helium is 0.7°. At 

 this point the vapor pressure is so small that it is in practice impos- 

 sible to proceed further. A gas with a still lower boiling point does 

 not exist. Very probably the new helium isotope, helium 3, dis- 

 covered in the Cavendish Laboratory, will be more suitable for 

 reaching low temperatures, if it can ever be obtained in sufficient 

 quantities, but there will be no difference in the order of magnitude. 



You may be wondering why we should bother to get still nearer to 

 absolute zero, as it seems so difficult now to get down any further. 

 What can still happen in this small region? To answer this ques- 

 tion we have to ask another. When can one predict that some- 

 thing will happen in a certain temperature region? 



Let us assume that the phenomenon we are interested in is con- 

 nected with an energy change of a certain quantity. Then the 

 thermal agitation will have an influence on it when it itself reaches 

 this order of magnitude. So we see that from this point of vievv^ 

 there is no sense in speaking of an absolutely high or an absolutely 



