tliat these experiments luul proved that a current generated in a 

 circuit which is supra-conducting- over its whole iengtii, continues 

 without electro-motive force, we could investigate in how I'ar an 

 electric contact interposed in an otherwise supra-conducting circuit, 

 measured by the amount of conductivity of supra-conductors, might 

 be considered as having no resistance. The immediate cause of this 

 investigation was a suggestion made by my colleague Kuenkn, whether 

 the current the relaxation period of whicli was to be stndied, 

 might not be obtained in the coil by short-circuiting. 



1 thonght then, that the transitional resistance in a contact to 

 be manipulated nnder liquid helium could hardly be made small 

 enough for this pnrpose. The transitional resistance of a stop- 

 contact treated with all due care at ordinary temperature is not 

 likely to be less than 0,0001 -Q, which is still 100,000 C.G.S. while 

 the micro-resistance of the coil itself is only 37. It has now^ been 

 found, however, that transitional resistances such as we are con- 

 sidering can become very small at low temperatures. A qnite moderate 

 pressnre, between two pieces of lead appeared to be suflicient for 

 the purpose. The arrangement is shown in Fig. 2. The small lead 

 plate p, provided with three small cones directed upwards and 

 coimected with the coil through a spirally-wound part of the lead 

 wire whicli acted as a spring, is attached to a thin rod (partly formed 

 of wood) and was |)ressed against the block soldered to the glass tube 

 by screwing up the rod, the force being accui'ately regulated by means 

 of a spring (see top of tig. 1). The tube is pi-oxided with a number 

 of side-openings to prevent the very much intensified heat convec- 

 tion (caused by resonance phenomena) which occurs in tubes closed 

 at the toj) when the bottom is at a \ery low temperature, and 

 which would lead to excessive evaporation of the helium. 



By means of this simple kej' we were enabled to arrange the 

 following experiment. To each extremity of the windings of the coil 

 two wires were attached (tig. 1 and 2). By means of the one paiiw/c a 

 current can be sent through the coil. The other pair //b can be con- 

 nected to a ballistic galvanometer. Moreover the two ends are connected 

 to the two parts of the supra-conducting key. With the key and the 

 galvanometer open, a current is sent through the cooled coil, in 

 the neighbourhood of which the compass-needle has been mounted. 

 The coil is then closed in itself, which gives no change in the 

 deviation of the needle. One can then convince oneself as long as 

 one likes, that the side-current, which in ordinary cases is imme- 

 diately extinguished, remains unaltered in the supra-conductor ; the 

 galvanometer connection is then closed, which also brings no change 



34* 



