12 



The spi no-occipital roots have, however, followed the shifting of their 

 nucleus and have come verj' near the vagus roots. 



The posterior viscero-motor column is considerably shortened at 

 its frontal extremity, which most probablj' results from tiie 

 absence or extreme reduction of the motor IX, and perhaps even 

 of the frontal motor X root (Johnston) in connection with the absence 

 or reduction of the two posthyomandibular branchial sacks (Stockard). 



Physics. — "Farther experiments with liquid helium. J. 1 lie imitation 

 of an Ampère molecular current or of a permanent magnet by 

 means of a supra-conductor." Communication N". 1046 from the 

 Physical Laboratory at Leiden. By Prof. H. Kamerlingh Onnes. 



(Communicated in the meeting of April 24, 1914). 



§ 1. Introduction. If a current is generated in a closed supracon- 

 ductor, from which no other work is required than what is necessary 

 to overcome the possible remaining micro-residual resistance of the 

 conductor, it follows, from the small value that the micro-residual 

 resistance can have at the most, that the current will continue 

 for a considerable time after the electromotive force that set it in 

 motion has ceased to work. The time of relaxation r in which 

 the current decreases to e-^^^ of its value is given by the ratio 



— of the self-induction L and the resistance r of the circuit. When 

 r 



r approaches zero, this period may rise to very high values. Whereas 

 the time of relaxation is extremely small in ordinary cases (for the 

 coil with which we are about to deal for instance, of the order of 

 a hundredthousandth of a second) when the resistance in the supra- 

 conducting condition becomes say 1,000,000 or even 1,000,000,000 

 times smaller it may increase so much, that the disappearance of 

 the current can be observed ; it may e\'en take place extremely slowly. 

 From the moment that I had found in mercury a supra-conductor 

 at the lower temperatures which can be obtained with liquid helium, 

 I was desirous to demonstrate the persistence of a current iji a con- 

 ductor of this kind, and amongst other things to take advantage of 

 it in the further investigation of the microresidual resistance of the 

 supra-conductor^). But it was only after the previous study of various 



1) For the salce of brevity we use the word resistance here in the sense of 

 quotient of potential difference and current strength. In supra-conductors (see 

 Gomm. No. 133) we can at present only speak of current and potential difference ; 

 whether the relation between these two can be expressed by means of the concep- 

 tion of specific resistance, has still to be investigated. (Comp. note 1 § 3). 



