2*?8 



Physics. — "Further experiments /rit/i liquid helium. ./. The imi- 

 tation of an Ampère molecular current or a permanent magnet 

 by means of a supra-conductor. (Cant.). By Prof. H. Kamerli^gh 

 Onnes. Coramunication N". 140c from the Physical Labora- 

 tory at Leiden. 



(Communicated in the meeting of May 30, 1914). 



§ 5. The main experiment repeated. Although the original experi- 

 ments on the persistence of a current which is started in a closed 

 supra-conductor have established the fact that the diminution of the 

 current with the time is very small (at least if it is assumed that 

 in the phenomena hitherto unknown magnetic properties do not plaj' 

 an important part), still for the magnitude of the change only an 

 upper limit could be tixed. As no change of the current was obser- 

 ved, this upj)er limit was determined by the uncertainty in the 

 measurement of the current. The only fact established so far was 

 that, if the change had been greater than 10 "/o» it would have 

 been observed. 



In repeating the experiment it was attempted to determine the 

 change itself or otherwise to reduce its upper limit, in so far 

 as the conditions under which the experiment had to be made 

 allowed this. The same supra-conducting lead-coil was used. As before 

 the current was produced in the closed conductor by induction in 

 order that the circuit might be kept free from connections other 

 than of lead fused together ^). The current was again measured by 

 compensating the action of the coil on a compass-needle by means 

 of a current in a subsidiary coil. The arrangement was however 

 improved by this coil (of insulated copper wire) being placed in a 

 small vessel with liquid air in a fixed position with respect to the 

 needle. When the compensation was obtained, the experimental coil 

 was turned 180° about a vertical axis and again compensated with 

 the current in the second coil reversed, the magnetic moment of 

 the experimental coil being deduced from the mean of the two 

 observations. 



If the diminution of the current can actually be calculated 

 from the residual micro-resistance given in Comm. No. 133, viz. 



— -^ = 0.5X10-1^ or with r.>9ooK.= 734-<2^v no.sK. =37, the time 



1) An attempt is at present being made to manufacture a supra-conducting 

 current-key. 



2) This value given already in Comm. N^ \4-0b is a more accurate value than 

 the one given in Comm. N'\ 133. 



