PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. VI. 665 



These vortices round the spots on the magnetic globe, I have not succeeded in photographing with 

 he present arrangements. On account of the importance of all these phenomena, however, I have constructed 

 . vacuum-vessel of 320 litres' capacity, and can employ a magnetic globe with a diameter of 24 centimetres. 

 .Vith this new apparatus, I have succeeded in obtaining good photographs, which will be mentioned below. 



The discharges of the cathode-globe are partly continual discharges all over the surface, and partly 

 lisruptive at intervals; in the latter case they issue from the eruptive spots. 



Fig. 253 shows how a branch of discharge issuing from the spots sometimes follows the magnetic 

 ines of force in the neighbourhood of the equator, giving rise to a phenomenon which greatly resembles 

 he black filaments on the sun, studied by HALE, ELLERMAN, Fox, EVERSHED, DESLANDRES and D'AZAMBUJA. 



It will be of considerable interest to compare this experiment with some photographs of quiescent 

 trominences on the sun. Fig. 252 is a reproduction of one of Prof. MALE'S earliest prominence photo- 

 ; raphs taken at KENWOOD'S Observatory. I have unfortunately no data to enable me to decide whether 

 his prominence follows more or less the lines of magnetic force on the sun. 



I have sought by various methods to find 

 value for the very singular capacity of this globe 

 in-responding to disruptive discharges, a capacity 

 /liich seems to vary perceptibly according to the 

 onditions of the discharge. In the case of this 

 ;lube (8 cm. in diameter), this capacity varies 

 ,bout ,J of a microfarad, and if I assume that 

 he sun has a corresponding capacity C in the 

 elation of the square of the diameters, I find that 

 " = 3 X io 18 microfarads. 



In calculating the tension of the solar dis- 

 harges according to the value HQ = 3 X io"C.G.S. 

 u-c M. Abraham, Theorie der Klektrizitat, B. II, 

 i. 183, equation (120 bis)), I find that =6.4 X io s 



"Its. The energy J /2 2 C =-- 5.9X1 o 30 ergs, transformed into heat, will be sufficient to heat to 175 C. 

 globe of iron the size of the earth. 



Sun-spots may be considered as the eruptive centres of similar disruptive discharges, and the 

 uestion then immediately arises: Where shall we seek for the positive pole of these discharges, in 

 .hich the spots, or that which surrounds them, represent the cathode? 



There are several possible solutions to this question. 



In the first place, it might be imagined that the interior of the sun formed the positive pole for 

 normous electric currents, while perhaps the faculae, in particular, round the spots, formed the negative 

 oles. Or it might be imagined that the positive poles for the discharges were to be found outside the 

 hotosphere, for instance in the sun's corona, the primary cause of the discharge being the driving away 

 f negative ions from the outermost layers of the sun's atmosphere in some way or other for instance, 

 s ARRHENIUS has assumed, by light-pressure after condensation of matter round them. Finally, it might 

 >e assumed and this, according to the experimental analogies, seems the most probable assumption 

 iat the sun, in relation to space, has an enormous negative electric tension of about 600 million volts. 



The first assumption has the advantage of appearing to give a natural explanation of the move- 

 lent of the sun-spots in various latitudes, provided that the sun's magnetisation is the opposite to that 

 'f the earth. 



