33Q SECTIONAL TRANSACTIONS.— A. 



Mr. J. G. Daunt. — Experiments on the thermal and magnetic behaviour 

 of supraconductors ( 1 1 .o). 



In continuation of the work of Mendelssohn and Moore on the magneto- 

 caloric cooling obtained with supraconductors, a method is described of 

 reaching very low temperatures by the adiabatic magnetisation of supra- 

 conductors. The advantages of the method are the comparatively low 

 magnetic fields required and the fact that the cooling substance is a metal, 

 to and from which heat can easily be transported by the free electrons. In 

 the experiments the working substance was a sphere of very pure poly- 

 crystalline tin, which was found to fulfil the necessary requirement of 

 reversibility in the transition between the normal and supraconductive 

 states. The suitability of various substances and alloys for the working 

 substance is discussed with regard to their thermal and magnetic properties. 



It was also determined that the threshold curve of tin is not a parabola. 

 Further experiments confirm the non-parabolic form of the threshold curve 

 of lead and mercury. A brief description of the method of measuring the 

 threshold curves is given. 



Dr. B. Rollin. — Properties of liquid helium (11.15). 



An account is given of the properties of helium in the condensed state. 

 Because of the large value of the zero point energy in relation to the lattice 

 energy, the behaviour of liquid helium is in many respects remarkable. 

 This seems to be the explanation of the impossibility of solidifying liquid 

 helium merely by reducing the temperature. 



The transformation which occurs in liquid helium at the X point (2 ■ 2° K.) 

 is especially interesting. At this point there is a large change in the entropy 

 of the liquid, so that below this point the liquid has only a very small entropy 

 and is therefore in a highly ordered state. As would be expected, the physical 

 behaviour of this ordered liquid phase is remarkable. It has been found 

 to have an extremely small viscosity and a very large thermal conductivity. 

 The high thermal conductivity is not only of theoretical interest but of 

 practical importance, especially in the attainment of thermal isolation of 

 vessels containing liquid helium. 



An outline is given of further researches to be made in connection with 

 condensed helium. 



Dr. A. H. Cooke. — The magnetic method of cooling (11.30). 



The principle of the magnetic method of cooling paramagnetic salts, by 

 which the lowest temperatures are at present attained, is explained, and an 

 account is given of the procedure developed in the Clarendon Laboratory, 

 Oxford, for experiments at temperatures down to o*oi° K. A survey is 

 made of the phenomena to be investigated in this region, and of the use of 

 the procedure for the measurement of very small energy changes. 



The possibility of reaching still lower temperatures by making use first 

 of the electronic magnetic moments and then of the nuclear moments of 

 substances is discussed. On considering the rates of attainment of thermal 

 equilibrium within the paramagnetic salts, and between the salts and other 

 substances, it is found that only in metals can the nuclear moments be 

 employed for such a process. 



Mr. G. L. Pickard. — The expansion method for liquefying helium ( 1 1 .45). 



The principle of the Simon expansion method of liquefying helium (or 

 hydrogen) is explained and a typical apparatus and working procedure are 



