590 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



a retrograde motion. In the course of this they have an opportunity of positive precipitation on our star- 

 screen and of the precipitation which we have called B on the equatorial screen in our earlier attempts. 

 When we after this look back on the photographs from earlier experiments take for instance fig. 204 

 we shall be able to see and understand them with much greater clearness than before. Look at the 

 admirable pictures in the first column (Nos. i, 4, 7, 10 and 13). We see directly how much of the 

 spherical triangular light pictures are wanting, it is rays that have turned before they have struck out 

 for the terrella, and we find them again in the precipitation on the eastern side of the vertical screen. 

 The more highly the terrella is magnetised, the greater will be the number of the rays of what we call 

 group A that will be converted into rays of group B. We have also seen that the end of the first line 

 of precipitation on the equatorial screen has moved away from the terrella, when this is magnetised to 

 an exceptionally high degree, the bulk of the rays nearest the terrella in the line of precipitation, have 

 been obliged to turn completely back. 



It is interesting to observe that in Nos. 7 and 10 we have a section of the ray-masses over 

 the poles at right angles to that shown in fig. 209, Nos. 3 and 7. 



We may now conclude by analogy that it is not only rays belonging to the first triangular figure 

 of precipitation that can be made to turn round by stronger magnetisation. 



We have mentioned that such precipitation appeared three times on the eastern side of the vertical 

 screen when the screen was turned in a positive direction through 360. The first precipitation was 

 strong and well defined, the second less strong, and the third slighter still. It is in this way that the 

 bulk of the rays in the middle of the three triangular figures of light disappear from the terrella, tin 

 rasy being thrown back before they reach the terrella, when the magnetism is sufficiently strong. 



Applying this fact to the earth, we should expect that a 

 station of medium latitude, for instance 65 , would not only have 

 powerful positive magnetic storms attaining a maximum at 6 

 p. m., but would also have slighter ones about i a. m., and a very 

 slight one about 8 a. m. (see p. 566). I hope to have an opport- 

 unity later on to investigate this matter. 



Nos. ii 16, fig. 215, are of experiments with a small 

 eight-armed screen, placed above the south pole of the terrella. 

 In the first three of these, the magnetising current was 10 am- 

 peres, the discharge-current 23 milliamperes, and the tension 

 2400 volts. The positions have the same hour-angles as before. 

 In the last three photographs the magnetising current was 

 20 amperes. Discharge-current 22 milliamperes with a tension 

 of 2700 volts. In the record of these experiments, the follow- 

 ing account is given: "Experiments were also made with a current 

 of 12 amperes to the small terrella. With this arrangement of 

 an equatorial screen there was no trace of negative precipitation 

 on the night-side. Great positive precipitation, on the other hand, 

 was found on branch 2, but on none of the other branches" 



Subsequent experiments have also proved that if the equatorial screen is large enough, the nega- 

 tive precipitation on the night-side in the polar light-ring disappears. 



In Nos. 14 & 16, precipitation A and B are exceedingly distinct upon the screen, and exactly as 

 with the large terrella (figs. 201 and 202). As we have already remarked, the circumscribing surfaces 

 of the vacuum-box have therefore nothing to do with the shape of this precipitation. 



Fig. 2 1 6. 



