PART I. ON MAGNETIC STORMS. CHAP. III. 



297 



FURTHER COMPARISON WITH THE TERRELLA-EXPERIMENTS. 



74. In order to obtain a clear idea of the way in which the various light-phenomena around our 

 terrella appear under conditions answering to the earth's positions at the various seasons, I have made 

 three series of experiments representing an equinox, and the summer and winter solstices. 



For each of these seasons, 12 photographs have been taken in four groups of three. The position 

 of the magnetic north pole in the four groups answers respectively to noon, 6 p. m., midnight and 6 a. m. 



In order to obtain a position answering to the summer or winter solstice, the discharge-tube was 

 inclined so that its axis was at an angle of 23^2 below or above the horizontal position answering to 

 the equinox. The terrella was suspended by a universal joint in such a manner that it always main- 

 tained the desired position in relation to the cathode rays during a rotation of the terrella answering 

 to the diurnal revolution of the earth. 



Thirty-six photographs have thus been taken, with the highest possible magnetisation of the ter- 

 rella with a magnetising current of 33 amperes, corresponding to a magnetic moment of about 10 ooo 

 cm. 5 /a gr.Va sec. i (see fig. 70, p. 155). 



I have also taken 36 photographs of the terrella in exactly the same positions as the above, but 

 with a magnetising current of only 15 amperes, corresponding to a magnetic moment of about 6200. 

 These 72 photographs, with descriptions, will be found farther on in this work. 



It will be interesting, however, to describe here some few examples of these with their photo- 

 graphs, because of the great significance of the light-phenomena observed, in the explanations of magnetic 

 storms given in the preceding pages. 



In the eight photographs following, the terrella has a position answering to the winter solstice and 

 6 a. m. at the earth's magnetic north pole. 



The experiment represented is almost the same, but the photographs are taken from eight different 

 points of view. 



r>g- 134- 

 Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 



38 



