PART III. EARTH CURRENTS AND EARTH MAGNETISM. CHAP. I. 



TABLE CXXV (continued). 



775 



As regards form, the current-charts exhibit, as might be expected, a great resemblance to the 

 harts for the equipotential curves on the earth's surface and the curves for constant values of P ? . It 

 vill further be seen that the current-lines in the second extreme case draw closer together about the 

 .torm-centre than in the first extreme case. The form, however, in its main features, is very similar, 

 during a polar storm, therefore, we should suppose, if the conduction-conditions in the earth were as 

 deal as we have assumed, that a current-system would be formed, of which the form at the surface 

 vould be something between these two extreme cases. 



For large values of Ju, we see that the current-lines from the neighbourhood of the stormcentre 

 ollow more closely the parallel-circles than for small values of J /t . In a latitude of about 40 in 

 >articular, we notice that it is often in a N S direction that the comparatively powerful earth- 

 urrents occur. 



This may possibly have some significance in explaining the peculiar fact that in Germany, for 

 nstance, the direction of the earth-currents is so markedly N S. It may even be remarked that the 

 nain direction for the earth-currents in Germany is approximately perpendicular to the auroral zone. 



Another peculiarity in the occurrence of the active systems of precipitation, which also certainly 

 >lays a part in this respect, is the ease with which the systems of precipitation appear to form at the 

 Norwegian stations at about midnight, Greenwich time, a circumstance which we have frequently pointed 

 >ut before. 



If the current-strength in the outer system varies sinusoidally, there is in the first extreme case a 

 >hase-difference of 90 between the strength of the current in the outer system and that in the inner 

 ystem. 



In the second extreme case there is a phase-displacement of 45 at the surface, and changing 

 ery rapidly inwards. The whole current-system might approximately be imagined replaced by a system 

 hat was concentrated in an infinitely thin globular cup, and the current-strength in this imaginary 

 urrent-system must be assumed to oscillate in time with the current-strength in the outer system. 

 The direction of this current will be the reverse of that in an outer current-sheet, which we may 

 magine replacing the outer system. 



If phase-displacement can be observed, there should be a means of forming a conception of the 

 .arth's conductivity. The observations in the north seem to show that the conditions follow the first 

 ather than the second extreme case; but I think that here one ought to be very careful in drawing 

 my conclusions whatever concerning this circumstance, especially as x must be supposed to vary within 

 /ery wide limits. In the next place, as regards the magnetic effect of the induction currents, we can 



Pe 



especially point out how the relation 7,- varies when one retires from the current-system. In the first 



-* i 



:ase, it decreases greatly as one retires from the storm-centre ; in the second extreme case, this is not so. 



