I'AUT III. KARTII CfRRENTS AND EARTH MAGNETISM. CHAP. I. 



745 



During the time of the most rapid increase of the perturbation the earth-currents obtain a maximum. 

 When the disturbance is at its maximum the galvanometer has nearly its normal position, and when 

 the disturbance diminishes at the greatest rate we get another maximum of galvanometer deflection, but 

 now to the opposite side of the normal line. The storms which show this type of variation are 

 .specially those which we called polar elementary storms. 



A number of elementary storms showing a correspondence of this type are graphically represented 

 n vector diagrams (fig. 270). 



Fig. 271. 



If we look at these vector diagrams we notice that the current vector when passing from one 

 lirection to the opposite is not turned round quite gradually, but the vector is kept in the same line of 

 lirection. This peculiarity with regard to direction will be seen from the plates (PI. XXX XXXIII), 

 ml is even better illustrated by some of the rapid records, e.g. PI. XXXIV and XXXV. 



Even the rapid oscillations seem to pass along the same direction which is seen from Table CXII, 

 ,'hich shows that the ratio between corresponding amplitudes in the two directions is about constant. 



If we try to deduce the direction of the earth-currents from the variation of the magnetic force at 



laafjord by applying Lenz's law, we find a current-direction nearly opposite to that actually observed. 



liis circumstance may seem remarkable. In order to prove that there was not some error in the deter- 



lination of sensitiveness, I again, in May, 1910, made earth-current measurements with earth connec- 



ons in exactly the same places as before, and found the condition confirmed. 



It is to some extent doubtful where the cause of this peculiar circumstance is to be sought; but it 

 reasonable to assume that the local conditions in the ground have a very essential part to play. 



