SECTIONAL TRANSACTIONS.— A. 329 



pronounced enhancement of b and/or d relative to a, and probably 

 also a slight increase. The increase of c/a responsible for red 

 aurora; of type A was explained by the writer as due to presence of 

 ozone, and this is supported by the observed effect of sunlight on 

 this same ratio, because we may expect that the ozone concentration 

 is greatest on the day-side of the earth. 



Discussion on Low temperature physics (10.20). 



Dr. H. Grayson Smith. — Saturation currents in supraconductors (10. 20). 



In a continuation of the experiments which have been performed at 

 Toronto on the supraconductivity of thin films of tin, it has been definitely 

 shown that there is an upper limit to the current in the supraconducting 

 state, apart from the effect of the magnetic field caused by the current. 



It has recently been found that : (1) For films of sufficient thickness their 

 normal transition points were unaffected. The current strength at which 

 resistance reappeared was considerably less than that required to cause the 

 critical magnetic field at the surface. (2) Penetration of an external 

 magnetic field through a film commenced with a field strength somewhat 

 less than half the threshold value. 



The experiments seem to be in qualitative agreement with the theory 

 of F. and H. London concerning the electromagnetic behaviour of supra- 

 conductors. They indicate that the surface layer in which the supra-current 

 flows is considerably deeper than hitherto supposed, namely ~ io -4 cm. 



Dr. K. Mendelssohn. — Normal and anomalous supraconductors 

 (10.35). 



Two kinds of supraconductors can be distinguished in respect to their 

 thermal and magnetic behaviour : 



(1) ' Normal ' behaviour is shown by some very pure metals (e.g. Sn, Pb, 

 Hg, Tl) with undistorted crystal lattice. These substances show a rever- 

 sible change to zero induction at the threshold curve, and the difference of 

 free energy between the supraconductive and the non-supraconductive state 

 can be deduced from the threshold values by simple thermodynamic 

 equations. The transition between the two states and the conditions under 

 which equilibrium can be reached has been investigated in detail on samples 

 of varying purity, crystal size and geometrical shape. 



(2) Supraconductive alloys do not show a reversible change to zero 

 induction, and the penetration of a magnetic field does not coincide with the 

 destruction of supraconductivity. Experiments on the thermal behaviour 

 (specific heat, heat of transition, magneto-caloric effect) of these ' anomalous ' 

 supraconductors show that the simple thermodynamical treatment cannot 

 be applied in their case. The question arises whether this ' anomalous ' 

 behaviour is solely due to secondary effects like inhomogeneity of the 

 sample or whether it must be ascribed to an essential difference in the 

 constitution of the supraconductive state. 



In order to explain the behaviour of the ' anomalous ' supraconductors 

 phenomenologically we have compared them with a magnetic ' sponge ' 

 the skeleton of which consists of regions of very high threshold value. 

 Experiments are described by which this hypothesis has been tested and 

 which allow us to trace the intermediate stages between ' normal ' and 

 ' anomalous ' supraconductors. 



