987 



Prom the relation (9) ensues moreover (f=Q. In all I lie (P') occur 

 therefore six groups, of vviiich the three points Phave coincided into 

 one ; in the (jf) belonging to them sir groups with united lines p. 



Physics. Further Experiments with Liquid Helium, f. The Hall- 

 ejfect, and the magnetic cliange in resistance at low tem.pera- 

 tures. TX. The appearance of galvanic resistance in supra- 

 conductors, ivhich are hrouglit into a magnetic field, at a 

 threshold value of the field''. By H. Kamerlingh Onnes. Com- 

 munication No. 139/' from the Physical Laboratory at Leiden. 

 (Communicated in l!ie meetii.g i)f February 28, 1914). 



§ 1. Introfluction, first e.vperinienis. In my last paper upon the 

 properties of supra-conductors, and in the summary of my experiments 

 in that direction which I wrote for the .Tliird International Congress 

 of Refrigeration in (Chicago (Sept. 1913, Leiden Comm. Suppl. 

 N°. 34^), I frequently referred to the possibility of resistance being 

 generated in supra-conductors by the magnetic Held. There were, 

 however, reasons to suppose that its amount would be small. The 

 question as to whether the threshold value of the current might be 

 connected with the magnetic resistance by the field of the current 

 itself becoming perceptible could be answered in the negative, as 

 we had then no reason to think of a law of increase of the resist- 

 ance with tlie field other than proportional to it, or to the square 

 of it, and the law of increase of the potential differences at currents 

 above the threshold vahie could not l»e reconciled with either 

 supposition. A direct proof that in supra-conductors onl}^ an insigni- 

 ficant resistance was originated by the magnetic field was found in 

 the fact that a coil with 1000 turns of lead wire wound within a 

 section of a square centimetre at right angles to the turns round a 

 space of I cm. in diametei* remained supra-conducting, even when 

 a current of 0.8 ampère was sent through it. The field of the coil 

 itself amounted in that case to several hundred gauss, and a great 

 part of the turns were in a field of this order of magnitude, without 

 any resistance being observed. The inference was natural, that, even 

 if we should assume an increase with the squarn of the field, the 

 resistance would probably still remain of no importance even in fields 

 of 100 kilogauss. In my publication (see Report, Chicago, Suppl. 

 N". 34/^) I restricted my conclusion about the resistance in the 

 magnetic field to a limit of 1000 gauss, and I also remarked that 

 when it came to making use of the supra-conductors for the con- 

 struction of strong magnets without iron, it would be necessary in 

 the first place to investigate what resistance the magnetic field would 



64 



Pioceedings Royal Acad. Amsterdam. Vol. XVI. 



