572 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



The next two photographs were taken during another experiment, from positions with hour-angles 

 of 90 and 180. 



The equatorial ring has not come out particularly well here, but on the other hand the polar rings 

 are quite distinct. We shall give better illustrations of the polar phenomena, however, later on, and will 

 therefore not dwell upon them now. These earlier photographs were taken during experiments in which 

 the discharge-tube was cylindrical, and not with the prismatic discharge-receptacle, which we used sub- 

 sequently. In No. 3, there are indications of the equatorial ring having been brighter out from the 

 terrella than close to it. 



As will be seen later on, in the chapter on zodiacal light, I easily succeeded in producing these 

 equatorial rings round a magnetic globe, which itself served as cathode. It was sufficient to employ a 

 difference of a few hundred volts in the tension between the electrodes, in order to produce the discharge 

 under these conditions. 



It is therefore not impossible that these rings in every case occur owing to the magnetic globe 

 having become negative in relation to its nearest surroundings in the discharge-tube. 



In the meantime it is a fact that a considerable number of rays move round the terrella, from west 

 to east, close to the equator; this has been demonstrated by nearly all the numerous experiments which 

 have just been described. 



STUDY OF RAYS OF GROUP B. 



112. We now pass on to experiments made with terrella No. 5 provided with a vertical screen over 

 its north pole, the plane of this screen passing through the axis of rotation, which still coincided with 

 the magnetic axis of the terrella. The screen was placed thus in order that the course of the rays in 

 the polar regions over the terrella could be studied. At the same time, the former horizontal screen 

 was retained (see fig. 205), now, however, entire, without the three slits, in order to prevent the formation 

 of polar precipitation by rays of group A. 



The two radial wires, about 4 cm. in length, standing out from the terrella, were also retained, in 

 order that their shadows thrown upon the screens might give information as to the course of the rays. 

 The photographs that are reproduced here distinctly show the position of the screens and wires. At first 

 a round hole was made in the vertical screen, and later on a slit was added. 



It now appeared that when the plane of the vertical screen formed an angle of about 30 with the 

 line of direction from the centre of the terrella to the cathode, characteristic precipitation became visible 

 upon the screen, extending far over the screen towards the axis, when the terrella was magnetised with 

 8 amperes. When, on the other hand, 25 amperes were employed upon the terrella, the precipitation 

 had moved right out to the right margin of the screen, seen from the cathode. With the employment 

 of 14 amperes, the precipitation was so situated that its innermost edge lay farther in than the above- 

 mentioned hole in the screen. 



The nine photographs in fig 209 represent various results of the experiments made. 



Nos. i and 2 represent experiments in which the hour-angle of the vertical screen was 30, this 

 angle being reckoned to the wing of the screen in which was the hole. The photographs were taken 

 from positions with hour-angles of 300 and 120-, and looking from above at an angle of from 15 to 

 20 with the horizon. The pressure was 0.0014 m m -> the discharge-current 24 milliamperes, and the 

 magnetising current 14 amperes. 



Nos. 3 and 4 are of a similar experiment, in which the vertical screen, with the terrella, is turned 

 160, in order to obtain clearer precipitation. We say then that the hour-angle of the screen is 190 

 and the photographs were taken from places with hour-angles of 310 and 90. The pressure and 

 magnetising current were as before, but the discharge-current and the tension were respectively 20 milli- 

 amperes and 2700 volts. 



