PART I. ON MAGNETIC STORMS. CHAP. II. 



97 



<i. b. c. 



Fig. 46. 



a horizontal current for a long distance between Dyrafjord and Axeleen. This would satisfactorily ex- 

 plain the constant direction that the perturbation in this and other similar cases shows. 



In order to obtain a clear conception of the conditions, we will once more have recourse to my 

 experiments with the terrella. The experiments shown in fig. 46, a, b and c, follow directly on to 

 those in fig. 38, a, b and c. In fig. 46 a, the terrella is so turned that the screen forms an angle of 

 135 with its first position (fig. 38 a}. In 

 the next experiment (fig. 46 b), the angle 

 is 1 80. The angles are here measured from 

 west to east. Fig. 46 c shows how the ca- 

 thode rays strike the terrella; when the lat- 

 ter is not magnetic, but is in the same 

 position as in the experiment given in 

 fig. 46 b, only the half that is turned towards 

 the cathode becomes luminous with phos- 

 phorescence. 



It will be seen from figs. 46 a & b how 

 the cathode rays behave when the terrella is 

 very powerfully magnetised. 



We will here especially direct our at- 

 tention to the luminous wedge that is thrown 

 upon the screen at about the 7oth parallel 

 of latitude north. 



In figs. 47 a & b, we have a confirma- 



tion of the way in which the rays whirl round 



Birkeland. The Norwegian Aurora Polaris Expedition, 19021903. 



fig. 47- 



13 



