685 



it was not certain (conip. the remarks about niej'curj in glass in 

 § 7 and § 1 1 Conim. VII of' this series) that the picëin remained 

 adherent lo the tin wire everywhere. Tlirongh this coil a current of 

 0.8 amp. (see § 15) could be sent, without the threshold value being 

 reached. The number of ampere windings per cm^ was then about 

 800. If tiie disturbing potential phenomena had not been greater 

 than with the shorter wire of the same section which was washed 

 by liquid helium over its entire surface, and if the difficuKy mentioned 

 in note 2 §4 does not come into play, it would have been possible 

 to supply this coil with up to 9000 ampere windings per cm'. If, 

 therefore, the potential phenomena which frustrated this iji the 

 experiment reported, in accordance with the opinion expressed in 

 Comm. N". VII of this series, particularly in § 11, may be ascribed 

 to ''bad places" in the wire, and if we may therefore be confident 

 that they can be remoxed (for instance by fractionising the wire) 

 and if moreover the magnetic field of the coil itself does not produce 

 any disturbance (note 2 § 4) then this miniature coil may be the 

 prototype of magnetic coils without iron, by which in future much 

 stronger magnetic fields may be realised than are at present reached 

 in the interferrum of the strongest electromagnets '). 



^) J. PfiRRiN (Soc d. phys. 19 Avril 1907) made the suggestion of a field of 

 100000 gauss being produced over a fairly large space, by coils without iron, 

 cooled in liquid air. Ch. Fabry (Journ. d. Phyy H'évr. 1910) woi'ked out this idea. 

 He finds that the energy absorbed in such a coil, in watts is represented by the 

 formula 



where a is a length in centrimetres, vdiich determines the size of the coil, for a 

 cylindrical one the radius of the internal space, y the latio of the metallic area in 

 a section through the coil at right angles to the windings to the area of this 

 section, K a purely numerical coefficient, which depends upon the form of the 

 coil, and which in cylindrical coils with wire of equal section does not differ mucli 

 from 0,18, : the specific resistance of the metal of the windings in ohms, centi- 

 metre, H the magnetic field in gauss. 



In order to get the desired field of 100000 gauss in a coil with an internal 

 space of 1 cm. radius, with copper as metal, and cooling by liquid air 100 kilo- 

 watt would be necessary, pulling K at 0.20 and '^ at 1,5 (which last number 

 might well be 6 times as large). The electric energy supply, as Fabry remarks, 

 would give no real difficulty, but it would arise from the development of Joulb 

 heat in the small volume of the coil to the amount of 25 kilogramme calories per 

 sec. which in order to be carried off by evaporation of liquid air would require 

 about 0,4 litre per second, let uo say about 1500 litres per hour. 



We may add to Fabry's objection that the preparation of 1 litre of liquid air 

 per hour is at present to be reckoned as requiring nol much less than '/o K VV. 

 According to this standard, 7 times as much work would be necessary for the 



44 



Proceedings Royal Acad, Amsterdam. Vol. XVI. 



