PART III. KARTH CURRENTS AM) KAKTH MACNKTISM. CHAP. I. 



757 



There will hereafter be more frequent opportunities of studying these phenomena, as the Norwegian 

 ate, at my request, has conceded means to keep the Haldde Observatory in continual activity. Figure 276 

 lows the observatory as it looked in 1912; but at the present time large new buildings are being added, 

 id it is very well equipped with up-to-date instruments. 



IB May 



Fig. 275. 



THEORETICAL INVESTIGATION OF THE CURRENTS 

 HAT ARE INDUCED IN A SPHERE BY VARIATION OF EXTERNAL CURRENT-SYSTEMS. 



153. In the foregoing Article, we have had occasion to draw attention to conditions which indicate 

 ie existence of earth-currents that are induced by variations in the outer polar current-system, which 

 p have assumed as the cause of the polar magnetic storms. 



In the next place these currents exerted a considerable influence upon the magnetic apparatus, so 

 <at especially the smaller details in the phenomena had mainly to be regarded as the effect of the 

 i rth-currents. 



In order to arrive at greater clearness, it may be interesting to make some calculations as to how 

 sch currents on the whole will flow in the earth, and what magnetic effects they will produce. 



A comparison of the results that can be obtained by the aid of the theory and the actual obser- 

 tions, will of course only hold good of the main features of the phenomena, as in the calculations we 

 1 ve to make a number of simplifying assumptions, which in reality are by no means exact. 



Fig. 276. Haldde Observatory. 

 Hirkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 



96 



