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SCIENCE PROGRESS 



these first measurements have a particular interest. When the 

 super-conductive state was first discovered it seemed as if many 

 hitherto unattainable physical possibilities would be achieved 

 at last. One was the production of a large magnetic field, 

 say io 5 gauss, without the use of iron. Clearly, if the field 

 itself had no contrary effect, and fairly large currents could be 

 employed, a large field could be obtained in a limited space 

 with the expenditure of a comparatively small amount of energy. 

 Unfortunately for this vision it was found that the currents 

 large enough to give the desired fields created such disturbances 

 in the super-conductor itself as to bring it out of this condition 

 into one where Joule-heat was developed to an extent which 

 appears to render the attainment of the desired field impossible. 

 The exact course of the threshold value with various con- 

 ductors and temperatures is shown in Table II. It will be 

 noted that the current densities vary very much for the various 

 conductors, and as a comparison the relative specific con- 

 ductivities are given. The results for lead and tin are not 

 nearly as complete as for mercury, but they are sufficient to 

 show the similarity of the phenomena. Table III. shows the 

 increase of potential difference with current density at a 

 temperature where the mercury is super-conductive. It is clear 

 that there is a very sharp increase at current densities just 

 above 1024, so that this is properly termed a threshold value. 



TABLE II 



Current densities in amperes per mm? at which potential difference appears 

 at the extremities of the conductor. 



