746 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903. 



A consideration of the country in which the earth-current measurements were made, confirms thi* 

 assumption. 



We have previously described this 'country (p. 732), and from the description it is evident that 

 local conditions would probably exert a great influence on the earth-current conditions, taking into 

 account that the earth-current lines are only 400 metres long. 



If we compare the current-directions found in Kaafjord with the sketch-map on p. 15 and with fig. 271 

 we see that the direction of the earth-current is parallel with that in which the mountain-ridge and tlu 

 branch-fjord run. Now in inductions of this kind, the main direction of the earth-current should be 

 E W; but if we look at the shape of the fjord and of the mountain mass on the maps, we see that if 

 on the whole the earth-currents are influenced by local conditions of this kind, it would be by no means 

 unlikely that in the regions surrounding Kaafjord, a peculiar deflection of the current-lines such 

 have here observed might take place. In order, therefore, to come to a clear understanding of this 

 question, it would be better to observe the earth-currents with considerably longer earth-conn 

 and in more level country. 



It was chiefly for this reason indeed that at the beginning of March we moved our station to 

 Bossekop, where the ground is less rough. 



There proved to be a considerable difference in the earth-current conditions We no longer find 

 such a marked constancy in the current-directions. As the vector diagram for March 31 shows, the 

 currents may here flow under various azimuths. From the same diagram it appears moreover that when 

 the magnetic force varies in strength, the directions are throughout in accordance with those we 

 expect to find according to Lenz's law, especially as regards the currents with direction NW SE. 



Unfortunately, however, we have only very few successful records of typical perturbations from 

 Bossekop. 



EARTH-CURRENTS AND POSITIVE EQUATORIAL PERTURBATIONS 



148. The characteristic properties of the positive equatorial perturbations are given in the first pan 

 of this work : 



Discussing the various systems which might produce these perturbations, we found it very diffinil: 

 to explain their properties by supposing that earth-currents were the primary cause of these distur- 

 bances. 



In lower latitudes the perturbing force is directed towards the north nearly along the magnetic 

 meridian, and it can maintain a considerable value for a great many hours. 



At Kaafjord successful earth-current records have been obtained for the E W circuit during tin 

 most typical equatorial perturbation observed by us, namely, that of January 26. The galvanonn i< i 

 the N S line being in some way out of order no oscillations were recorded in this line. 



Looking at the curves in No. 10, PI. XXXII, we notice that the H and D curves show small, hut 

 still quite noticeable deflections lasting for several hours In the earth-current curve there is absolute!) 

 no deflection of long duration to be noticed, but merely sudden oscillations about the normal line. 



ON THE SIMULTANEITY OF EARTH-CURRENTS AND MAGNETIC DISTURBANCES 



149. The question regarding the simultaneity of the occurrence of earth-currents and magnetk 

 storms was first discussed by Airy and since then it has been subject to considerable attention ti 

 most authorities. 



