14 



able to test the magnetic condition of the coil while no othei* magnetic 

 objects are in the neighbourhood; it is also necessary to prevent the 

 induction currents which are generated when the field is produced 

 and when it disappears from partly or completely neutralising each 

 other (cf. § 4). 



It can therefore be easily seen that the follow- 

 ing procedure is advisable: the field is put on, 

 while the coil is in the cryostat at the centre of 

 the interferrum, everything being i)repared for 

 siphoning the liciuid helium into the cryostat. 

 The current generated at the production of the 

 field is then immediately dissipated by the ordinary 

 resistance of the coil before the heUuni is })Oured 

 over. Care is taken to keep the field below the 

 thresliold-value of the production of ordinary 

 resistance, which holds for the temperature at 

 which the experiment is going to be made. The 

 coil is tiien cooled l)y letting in liquid helium, 

 the held leuiaining luichanged. In this manner a 

 supra-conducting coil is obtained, closed in itself 

 -<!>- without a current placed in the magnetic field. 

 If the field is now put off and the apparatus 

 which have produced it are removed, a cui'rent 

 will remain in the coil which is smaller than or 

 in the limit equal to the threshold-value corre- 

 sponding to the tem[)erature of the coil. The presence of this current 

 can be established by its magnetic action outside the cryostat. 



In order to obtain a strong current it is advisable to cool the 

 coil as far down as possible, as thereby the threshold-value of the field 

 to be used for the induction and the threshold-value of the current 

 are both made as high as possible. For that reason the first experiment 

 was made at a temperature of I''. 8 K, the lowest temperature which 

 can be reached comparatively easily and maintained for a long time. 



§ 3. Calculation of the experummt. Assuming that the field 



diminishes proportionally to the time t from i/„ to and calling M 



dM 

 the magnetic i)otential of the coil in the field //,—- will be constant 



during the period of the disappearance of the field and the equation ') 



di . dM 



L h *'^^^— — 



dt dt 



1) Here is supposed thai r is independent of i below the threshold value iD of i. 



