664 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



If the metallic globe surrounding the electro-magnet is not smooth, but has sharp points on its 

 surface, for instance near the poles, the disruptive discharges would issue at these points, and it will )>,. 

 necessary to use a stronger magnetisation to make the patches arrange themselves in zones round the equator 

 From the results obtained by SWABE, WOLF, CARRINGTON and SPOERER, we know that the sun-spots 

 arrange themselves just in two zones between 5 and 40 N and S latitude, in such a manner that in 

 the minimum-period of the spots, they begin to show themselves in high latitudes, and then descend until 

 at their maximum-period they have reached a latitude of about 16 north and south. If we remember 

 especially that the spots are the centres of emission of very stiff' cathode-rays (Ho =3 X io"C. G. S.), which 



give rise to auroras and magnetic perturbations on our earth, it would appear 

 as if the sun-spots were the foot-points of disruptive electric discharges from tlv 

 sun. The possible depressions in the enveloping photosphere by the sun 

 which many astronomers believe to exist, can be easily explained by rel- 

 to an experiment with discharges from a quicksilver cathode in a vacuum-tube 

 (see fig. 201. Winkelmann's Handbuch der Physik, 4, p. 530). The pressure- 

 that the discharge here exerts upon the surface is probably proportional to the 

 energy of the discharge, which, as we shall see, must be enormous in tin 

 of the sun. 



If the pressure of the gas increases, the pencils of rays no lon^t T 

 radially from the globe, as in fig. 249, but the disruptive discharges an 



seen to manifest themselves in the shape of a star with four or five arms (see fig. 250), coming from an 

 eruptive spot, and almost following the surface of the non-magnetic globe, to meet often at a point on 

 the globe diametrically opposite. 



Fig. 250. 



Fig. 251. 



These discharges from opposite points (this is not clearly seen in fig. 250, however) brought to my 

 mind a very strange picture of some enormous eruptions on the Sun (see fig. 251), reproduced from 

 "Marvels of the Universe". On June 26th, 1885, M. TROUVELOT saw two huge prominences, each more 

 than three hundred and fifty thousand miles in height, rising from the sun. Flames of such dimensions 

 are exceedingly rare; it is therefore all the more significant that they rose exactly opposite to each othti 

 from the ends of the same diameter. 



It almost always happens too, in the experiment in which the cathode-globe is magnetised, that 

 there are two or three luminous branches turning in a spiral about the eruptive spot and near the stir- 

 face of the globe. These vortices move in the opposite direction to that of the hands of a watch on 

 the hemisphere containing the magnetic north pole, and in the same direction on the opposite hemisphere. 



This corresponds exactly with the results recently obtained by HALE, ELLERMAN, and Fox relative 

 to vortices in the hydrogen filaments and calcium vapour round a sun-spot, provided it is admitted, as 1 

 have found, that the sun and the earth are inversely magnetised (Comptes Rendus, Jan. 22, 1910). 



