686 



y. Certainty that the potential phenomena observed are due to 

 such inipeifections in the vviie would be of no less value for ajiotlier 

 tempting group of experiments. As soon as the super-conductivity of 

 mei-cury was established, the question forced itself upon me, in 

 connection with the great value which according to the electron 

 theoiy of metals is ascribed to the free path of the electrons^) (comp. 

 § '12 /?), whether electrons moving at speeds by which they cannot 

 penetrate a thin plate, e.g. a Lp:nard's window of solid mercury, at 

 temperatures near the ordinary temperature^), or at least not without a 

 change of direction, would be able to do this better if the foil were 



cooling than for the current. By a judicious use of the cold of the vapours this 

 number can be reduced, but the proportion will remain unfavourable. 



Moreover, as Fabry shows, the dimensions determined by a, to make it pos- 

 sible for the heat to be carried off, would need to be much larger, by which at 

 the same lime the amount of liquid gas used becomes greater. The cost of 

 carrying out Perrin'c plan even with liquid air might be about comparable to 

 that of building a cruiser! 



If we calculate in the same way the cooling witli liquid hydrogen in the case 

 of silver and if we assume that the resistance of silver (according to Kamerlingh 

 Onnes and Clay) at the boiling point of hydrogen is 0,009 of that at the ordinary 

 temperature, we arrive at a more favourable figure, namely, that at a = 1 cm., 

 700 Hters of liquid hydrogen would be needed per hour, but the ratio of cooling 

 work and electric work becomes more unfavourable yet, putting the preparation 

 of a litre of liquid hydrogen in the same way as above at l^/g K. W. But the 

 figure for liquid hydrogen would also on the ground mentioned above have to be 

 considerably increased. Although an installation which will give as much liquid 

 hydrogen as is necessary for the cooUng could be made after the pattern of the 

 present Leiden plant, it would be of such an extraordinary size that with liquid 

 hydrogen also, the method described perhaps involves more difficulties than a 

 further increase of the size of the coil, in order to be able to cool with running 

 Wiiter (as introduced by Weiss) while this method also has its advantages with 

 a view to Ihe use of the field. 



The possibility of using the super-conductors tin an I lead, gives a new depar- 

 ture to the idea of Perrin of procuring a stronger magnetic field by the use 

 of coils without iron. With super-conductors no Joule heat needs to be carried 

 off (or at any rate only lO'-* times less than with ordinary conductors) and thus 

 with currents below the threshold value the difficulties mentioned above disappear. 

 If the conditions mentioned in the text can be fulfilled, then even a coil of 25 cm. 

 dian:eter of lead wire, constructed as the one in § 15, immersed in helium, could 

 give a field of 100000 gauss, without perceptible het being developed in the 

 coll. Some such apparatus could be made at Leiden if a relatively modeS(_ 

 financial support were obtained. In the mean time this remark may serve to put 

 the problem of very strong magnetic fields which are becoming indispensable for 

 various investigations in new form. 



1) Comp. note 3 p. 1113. Leiden. Comm. N'>. 119. Febr. 1911. 



~) Whether the same electron passes through, or whether the movement is 

 carried from the one to the other, does not affect the experimental question. 



