September 12, 1912] 



NATURE 



39 



negligible, which is far from the case in some of 

 the hypothetical cases dealt with by Stormer. 

 Hp may for mathematical purposes be regarded 

 as the characteristic constant for a definite species 

 of ray. 



In his earliest work, Stormer treated the 

 ■earth as a uniformly magnetised sphere, of 

 moment 852 x lo-^ C.G.S. Assigning to Hp 

 ^■alues ordinarily conceded to kathode rays and 

 ;3-rays of radium, he found that no electrified 

 particle coming from the sun could reach the 

 •earth's atmosphere except in the immediate 

 vicinity of a magnetic pole. If, for instance, Hp 

 lis 1000, the greatest angular distance from the 



abatement. Stormer 's next idea was suggested 

 apparently by experiments of Birkeland's, which 

 showed that kathode rays in a vacuum tube con- 

 taining a magnetised sphere can form a luminous 

 ring in the magnetic equator. Stormer supposes a 

 gigantic ring of kathode rays to form and persist 

 for some time in the plane of the earth's magnetic 

 equator, and the auxiliary magnetic field thus 

 produced is believed to render access to low lati- 

 tudes possible to other kathode rays coming from 

 the sun. 



In the absence of a ring', as already remarked, 

 rays require Hp to equal 10'' to permit of their 

 getting as far as 22° from the magnetic pole, but 



:.— Enlargement of a photograph of an aurora on February 2S, 191. 



pole is only 38°, and to increase the angle to 

 22° — which answers roughly to the position of 

 maximum aurora — one must assign to Hp the 

 enormous value 10". To account for the observed 

 occurrence of aurora in low latitudes. Prof. Birke- 

 land has had to assume no less a value than 

 7 X 10*, which implies a very close approach in v 

 to the velocity of light. 



Prof. Stormer sets out in quest of another 

 fxplanation. His first idea was to substitute for 

 the field of a uniformly magnetised sphere that 

 derived from the earth's Gaussian potential, based 

 ultimately on the work of Carlheim Gyllenskold. 

 The mathematical difficulties were thereby much 

 increased, while the physical difficulties showed no 

 NO. 2237, VOL. go] 



this becomes possible to rays the Hp of which ;:; 

 only 1000, provided there is an equatorial ring of 

 140 times the earth's radius, carrying a current 

 of about 60 million amperes. The rays forming 

 the ring are supposed to be quite distinct. Their 

 value of Hp increases from 100 when the ring's 

 radius is 1400 times the earth's to 10'' when the 

 former radius is 14 times the latter. For a given 

 size of ring, the distance from the mag- 

 netic pole which rays coming from the sun 

 can attain increases with the current in 

 the ring. In the case of the largest angular 

 distance considered in detail, 4i'6°, the rays 

 forming the ring and those coming from the 

 sun have both 10'' for their value of Hp, and the 



