732 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



by this arrangement, and it is also these that are of special interest to us. They will be only slightly 

 influenced by the plate currents, as the changes that might take place in the polarisation conditions must 

 be assumed, as a rule, to take place comparatively slowly. 



Possibly occurring thermo-electric forces will as a rule also undergo only slow, gradual changes. 



Finally, we have left the effect of the direct induction in the cable-system, produced by the mag- 

 netic variations. This is made as small as possible by placing the cables on the ground. 



In order to obtain an idea of its amount, we may make the following estimate. We will assume 

 that we have a surface of flow of 400 sq. metres. Further we will assume that the component of the 

 magnetic field at right angles to this surface varies with a velocity of 100 y per minute. In the system 

 there will than be induced an electromotive force with magnitude 



io 



o.ooi X 400 X 

 ~ 8 



60 



6.7 X io~ 7 volts. 



- volts 



Now the earth-resistance between the plates has been measured and found to vary between 150 i> 

 and 1500 i?. If we employ a mean value of 670 Q we find the strength of the current to be 



. _ 6.7 X io~ 7 

 670 



io~ 9 amp. 



This current is divided between the galvanometer and the shunt, generally in the proportion i 

 Thus through the galvanometer there will pass 



Tr, 9 



= 3 X io~ 12 amp., 



300 



and a current of this size will produce a deflection on the photographic paper of about 



3X 



3 X 



= o.ooi mm. 



Thus, even for so powerful a variation in the magnetic field, there will if our assumptions hold good 

 be only an imperceptible deflection, whereas in reality very considerable deflections are found with 

 variations of such magnitude. The surface of flow must therefore be of an altogether different order of 

 magnitude, if this kind of induction is to have any disturbing influence. 



It appears from this estimate that what we observe must be produced by actually existing earth- 

 currents. 



As regards the nature of the soil, the following may be said. 



It will be seen from the maps on p. 15 of Section I, that the observation-place in Kaafjord is situa- 

 ted in a region that is inclosed on all sides by high, steep mountains. 



Alien Fjord, moreover, sends a narrow branch, Kaa-Fjord, up into this mountain mass; and the 

 earth-current cables were laid upon the terraces above this branch-fjord, just at the foot of the excee- 

 dingly steep slope of Grytbotten Mountain. 



These mountains are probably very rich in well-conducting veins of copper ore. 



At Bossekop, the region surrounding the observation-place is flatter, and it would appear that th 

 the local conditions play a less important part. 



The reader is further referred to the description on p. 14, Part I. 



Records were kept at Kaafjord from the middle of November, 1902, to the end of February, 1903, 

 then the registerings were continued at Bossekop until April 2. 



