

PART. II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 565 



It will be seen that almost everywhere the uppermost, polar part of the light-figures on the 

 terrella, consists of the point of intersection of two strips of light which intersect one another at often 

 a considerable angle, sometimes, indeed, more than 90. It was interesting to see that the corre- 

 sponding angle between the strips of light when they fell upon a vertical screen, was always much 

 smaller than on the terrella, being quite acute. It was enlarged on the terrella by the oblique projection 

 of the strips of light from west to east. The apex, when it touched the vertical screen, looked like a 

 section of the wedge of light or the horn, often seen in the air about the auroral zone during 

 experiments. 



Figures 7 12, when considered as a connected group, give an indication of the reason for the 

 appearance of all these secondary precipitations when the rays are soft and the magnetic force great. 

 We receive the impression that some of the rays in the great bundle of rays that is working its way 

 round the terrella from west to east, turn back once or oftener near the polar zones, describing some- 

 thing that resembles an epicycloidal curve. The stronger the magnetism, the more loops do the rays 

 make, and the steeper the incline at which they intersect the magnetic horizontal plane. We shall return 

 to these cases of secondary precipitation in the next section of the experiments. 



We shall now in passing mention some experiments that are closely connected with the preceding 

 ones, but which nevertheless originally formed the transition to the study of rays of group B. 



When the screen had an hour-angle of about 90, there might sometimes be noticed on its east 

 side a remarkable shadow of the wire that conveyed the current to the terrella, this being caused by 

 rays that have come over the polar regions of the terrella, and have then turned right round so that 

 they come near the earth in the auroral zone with a tangential motion from east to west. 



Nos. 13, 14 and 15 of fig. 204, are from experiments such as these. In both experiments the pres- 

 sure was o.oi mm., 21 milliamperes passed through the discharge-tube with a tension of 3200 volts, and 

 25 amperes were employed for magnetising. The positions of the screen were with hour-angles of 

 82 and 87; and in both cases the photographs were taken from places with hour-angles of 130 and 310. 



Nos. 13 and 15 show distinctly how the shadow of the metal wire at a distance of 3 or 4 centi- 

 metres to the east, is thrown upon the screen in the form of two lines meeting in a point, which runs 

 farther in towards the terrella in No. 13 than in No. 15. No. 14 gives the corresponding view of the 

 phenomenon from the opposite side. What is particularly interesting about this last-mentioned photograph 

 is that part of the conducting wire coated with phosphorescent matter is distinctly seen above the 

 screen, illuminated by the rays, and thus casting a shadow back upon the east side of the screen. The 

 rays which cause the formation of the shadow of the conducting wire come from above and strike a 

 part of the wire that is more than 2 cm. above the north pole of the terrella. They then shoot down 

 and bend westwards, coming in contact with the screen as the photographs show. It is a striking fact 

 that in spite of the bending and twisting of the separate rays, the pencil of rays succeeds in throwing 

 relatively clear shadows. 



Several experiments of this nature were made without photographing them, and the particularly 

 sharp shadow of the conducting wire, with the characteristic point directed towards the auroral zone 

 was always noticeable. The experiments were, as we have said, an introduction to the study of what 

 we called rays of group B, which give us the foundation for the explanation of the positive polar storms. 



In photographs 13 and 15, the characteristic light-figures on the east side of the screen will have 

 been noticed. These are of another kind than the precipitation upon the east side of the vertical screen, 

 which we saw when the hour-angle of the screen was small. Upon closer investigation it appeared that 

 a slight precipitation of returning rays also took place upon the east side of the screen when the hour- 

 angle of the latter was about 250. It would thus seem as if this phenomenon could be obtained in 

 three positions of the screen, although the last, with 250, was certainly very inconspicuous. 



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



