564 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



Nos. i, 2 and 3 of fig. 204 were obtained under the same conditions as the three preceding photo- 

 graphs, except that the hour-angle of the screen was 265. No. 2 shows that it is the second principal 

 line of the rays' intersection that falls upon the screen. Two secondary precipitations are also seen upon 

 the screen ; but we shall return to these later. 



The last two experiments show that the angles 155 and 265 correspond to the first and second 

 lines of the rays' intersection with the horizontal plane (the magnetic equator), when the magnetising 

 current is 28 amperes. By other experiments, the angles corresponding to the first three lines of inter- 

 section were found to be 155, 265 and 365, for the same magnetisation, and the angular distance 

 between the second and third lines of intersection is thus only 100. 



By experiments with a magnetising current of 8 amperes on the terrella, the angles were found 

 to be 168, 272 and 370 



In experiments with the terrella highly magnetised, it was very interesting to watch the changes 

 in the phenomena as the terrella became warm and gave off" gas. To begin with, 8 distinct secondary 

 precipitation-figures were once observed upon the night-side of the terrella, partly overlapping one 

 another, and coming closer together towards the morning-side. The number of the patches of precipita- 

 tion increased as the terrella grew warmer and gave off" more gas, and finally there appeared continuous 

 polar bands, answering to the north and south auroral zones. 



Nos. 4, 5 and 6 were obtained under the same conditions as the preceding photographs, except 

 that the hour-angle of the screen is now 250, and therefore somewhat less than what would answer 

 to the second line of intersection of the rays. It is also clearly seen in photograph 5 that the rays 

 have not yet drawn together so that all pass through the slit. The third patch of precipitation on the 

 terrella in No. 6 also bears evident signs of this. 



The next six photographs are from a series of experiments that were made with the screen in the 

 same position, but with pressures of 0.0009 mm., 0.0019 mm., 0.0052 mm., 0.012 mm., 0.02 mm. and 

 0.05 mm. The only pressures represented here are 0.0009 mm. and 0.012 mm. In the cases of the 

 lowest pressures, vapour has certainly, as already mentioned, played an important part. 



Nos. 7, 8 and 9 are from experiments with a pressure of 0.0009 mm - ar) d the screen at an hour- 

 angle of 40. The photographs were taken from the same three positions as before, with respectively 

 90", 180 and 270 east hour-angle. 



The strength of the current through the discharge-tube varied from 18 to 22 milliamperes, and the 

 tension between the electrodes from about 4500 to 3500 volts. The current magnetising the terrella 

 was of 28 amperes. 



No. 7 shows precipitation of returning rays upon the screen. In No. 8 the second precipitation is 

 seen solitary, but with the third precipitation there are several secondary patches. 



Nos. 10, ii and 12 were taken under a pressure of 0.012 mm., with a current of 24 milliamperes 

 through the discharge-tube, a tension of about 2500 volts, and a magnetising current of 28 amperes. 

 The figures give a hint of the transition to the continuous band of light round the poles of the 

 terrella, which appears with softer cathode rays; and it will be seen that the parts about the magnetic 

 equator become more and more free from precipitation. 



Experiments were made for the purpose of determining what tangential motion in relation to the 

 terrella those rays had which formed the precipitation in the polar regions on the night and morning 

 side of the terrella. The experiments were made with various pressures, and both the primary and the 

 secondary precipitation was examined by means of the screen. It appeared in every case that the rays 

 had a motion parallel with the auroral zone in a direction from west to east. Corresponding precipitation 

 upon the earth would thus give rise to negative polar storms, as the various cases of secondary preci- 

 pitation summed themselves up in their magnetic effects very much as shown in the case of tin rays in 

 the diagrammatic figure 50 a, in Section I. 



