PART I. ON MAGNETIC STORMS. CHAP. II. 



8r 



(1) Under certain circumstances there is formed round the terrella a very steady, luminous ring. 

 As the system itself is confined within the form of a flat torus, the trajectories of the corpuscles in 

 consequence form approximately entire circles (see fig. 37). Owing to terrestrial magnetism, such nega- 

 tive corpuscles in space, coming from the sun, must then move from west to east round the earth. 



(2) At some height above the terrella, and on the side turned towards the cathode, we shall be 

 able to get very well characterised systems. The existence of these systems may be shown by a 

 phosphorescent screen, as illustrated in fig. 38 a, b, c, where the terrella is placed in three different 

 positions in relation to the screen, as indicated by the diagram below the images. 



The precipitations appear only on one side of this screen, and their inner border is sharply 

 defined. The system is of considerable breadth. It does not remain in the neighbourhood of the 

 equator, but extends on both sides, and fades away towards the poles, or unites with the polar system. 



a. b. c. 



Fig. 38. 



The three figures, 38 a, b and c, show how cathode rays are drawn in towards a highly magnetic 

 terrella. Both terrella and fixed screen are covered with phosphorescent substances. In position a, the 

 screen points straight towards the cathode, that is to say, the plane of the screen is perpendicular to 

 that of the cathode. In position b, the planes make an angle of 45 with one another; and in position 

 c, the screen is parallel with the cathode-surface. 



We can see how the rays are drawn in in rings or zones round the magnetic poles on the terrella 

 itself; but the phenomenon to which we shall here pay special attention, is the strong light that is found 

 only on the east side of the screen, and which is due to cathode rays that turned back before reaching 

 the terrella. They are caught by the screen, however, and rendered visible. It will be seen that the 

 mass of the rays turn back and come into contact with the screen in position b, answering to the after- 

 noon side of the terrella. Professor STORMER has calculated the trajectories of electrically charged cor- 

 puscles sent by the sun towards the earth, and has, amongst other things, studied the course of the 

 trajectories at the earth's magnetic equator. 



Birkeland, The Norwegian Aurora Polaris Expedition, 1902 1903. 11 



