580 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



EXPERIMENTS FOR DETERMINING THE TANGENTIAL COMPONENT OF THE POLAR 

 PRECIPITATION IN RELATION TO THE SURFACE OF THE TERRELLA. 



113. In the preceding pages, it has frequently been stated that the polar precipitation in the 

 neighbourhood of the auroral zone was produced by rays that came in to the terrella fairly perpendicularly. 

 By the previous investigations, therefore, it was not made clearly apparent how the tangential component 

 in the precipitation was directed at the various places on the surface of the terrella. 



By the experiments illustrated in fig. 212, however, the matter has been give n, by special ami 

 ments, all possible clearness, and it will be seen what a remarkably striking analogy comes out betv 

 the situation and direction of the various instances of precipitation upon the terrella, on the one side, 

 and the situation of the positive and negative districts of precipitation during the polar magnetic storms 

 on the earth, described in Chapter I of the present part, on the other. 



The photographs of which these illustrations are reproductions, were unusually successful. As tln-v 

 were to make clear one of the most important points in the theory, they were chosen with care from 

 a great number of more or less good ones. Any one with experience of similar experiments, will easily 

 understand the labour that this entailed. 



The experiments were made with terrella No. 4, with a diameter of 8.2 cm., which was sns/>i'ii</i'il 

 by the magnetic equator, so as to give the best possible opportunity of photographing the polar precipi- 

 tation from the side of the discharge-box. Upon one magnetic pole in this case the south pole 

 a star-shaped screen was placed, consisting of 8 branches of a height of about 15 millimetres, standing 

 on their edge. 



Nos. i, 2 and 3 were taken from an experiment in which the discharge-current was 24 milliamp 

 the magnetising current 20 amperes, and the tension 2500 2300 volts. The pressure was 0.006 mm. 



The first two photographs were taken, looking towards the centre of the terrella, in a plane with 

 an easterly hour-angle of 270, the first with a declination of + 24, the second with 24. The 

 third photograph was taken in the plane of the horizon from a place with an hour-angle of 240. 



Nos. 4, 5 and 6 were taken during a similar experiment, in which the discharge-current was 23 

 milliamperes, the magnetising current 20 amperes, and the tension 2500 volts. The pressure was 0.009 mm< 



The terrella was turned 15, so that the line from the centre to the magnetic south pole had an 

 hour-angle of 285. The photographic apparatuses were in the same position as before. It was intended 

 that the conditions should answer more or less to the position of the earth in summer. 



Nos. 7, 8 and 9 are of a similar experiment with a discharge-current of 24 milliamperes, a magnet- 

 ising current of 20 amperes, and a tension of 2400 volts. The pressure was 0.009 mm - 1 h' s ume ' 

 however, the terrella was turned 15 in the opposite direction, so that the hour-angle of the line to the 

 magnetic south pole was now 255. The purpose of this was similarly to make the conditions answer 

 to some extent to the position of the earth in winter. 



At the top of all the photographs, there is a hook, which has nothing to do with the suspension 

 of the terrella, and ought not to have been there at all, as it has nothing to do with the present experi- 

 ments. The cathode in the discharge-tube is, as will be understood, on the right of the terrella. The 

 left side will therefore answer to the night-side. For the purpose of easy reference, we will number 

 the eight branches that form the star-shaped screen, beginning with the middle branch on the right of 

 the picture the branch which, as we have said, points towards the cathode -- and continuing in the 

 reverse direction to the hands of a clock. 



It will at once be noticed that the principal precipitation on the three branches, 4, 5 and 6, on the 

 night-side, is found on the west side of each branch. There is no precipitation on the east side, but 

 a dark, narrow shadow is to be seen in the polar band of light on the terrella itself. In No. 2 there i: 



