PART. II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 597 



At present we are acquainted with (1 rays, which pass through about i millimetre of mercury; 

 md they are accompanied by y rays, much more penetrating still. 



LENARD(') has made investigations for the purpose of finding a relation between the velocity of an 

 electron and the coefficients of absorption for corresponding rays in different substances. 



He arrived at the result that the absorption increases more than a million times when we pass 

 rom /? rays of radium to cathode rays with a velocity equal to a hundredth part of that of light. 



It seems probable, however, that the penetrability of our rays should be much greater than that 

 )f the p rays of radium; but no simple law has yet been found that can be employed for calculating 

 he absorption when the velocity is known. 



Several physicists have found that the p rays are absorbed according to an exponential law, and 

 hat the velocity does not change when the rays pass through matter; but it would appear that these 

 esults are not certain. 



We can point to yet another circumstance that indicates that the corpuscular rays coming from 

 he sun must be extremely inflexible. After HALE'S discovery of the comparatively powerful magnetic 

 ield that is found round the sun-spots, it is an obvious conclusion that the sun on the whole is magnetic. 

 This conclusion is also obvious for other reasons. The corona's rays in the polar regions of the sun 

 lave led several investigators to believe that the sun is magnetic, with poles near those of the axis of 

 otation. 



It now appears that no rays can emanate from the equatorial regions of the sun out into space, if 

 he sun is assumed to have a magnetisation that can be compared with that of the earth, and the rays 

 ire supposed to be no more inflexible than the hitherto known corpuscular rays, i. e. if Rontgen 

 ays and y rays are not corpuscular rays. 



It is another matter altogether when we assume that the rays actually have the inflexibility that 

 ,ve have above inferred that they must have, from aurora and terrestrial magnetic phenomena on the 

 ;arth. We are then even able to give a plausible explanation of a phenomenon that has been studied 

 :>y Ricc6(-), and which has to do with magnetic storms. Ricco has observed that there is a difference 

 )f time of from 40 to 50 hours between the passage of a large spot to the central meridian and the 

 naximum of a magnetic perturbation that it produces on the earth. He concludes from this that the 

 velocity with which the corresponding rays are propagated ought to be between 900 and 1000 kilo- 

 netres per second. 



It is easy, by quite simple calculations, to determine the path that a corpuscular ray going straight 

 )ut, with the velocity of light, from the sun's magnetic equator will describe when the stiffness of the 

 ays is that assumed above, and the sun is supposed to act upon the rays like an elementary magnet 

 with a definite moment M. 



I have calculated from ST6RMER*s( 3 ) formulae that the sun should have a magnetic moment of order 

 to 28 , or about 150 times greater than that of the earth and inversely magnetic, in order to deflect our 

 ays by an angle corresponding to this retardation of from 40 to 50 hours. 



The probable existence of such corpuscle-rays from the sun as those here treated of, is 

 ;ven now admitted by several men of science, and it will certainly be soon acknowledged that these 

 lew solar rays, which I have thus discovered, enter deeply into many terrestrial conditions, even if they 

 -annot compare in importance with the wondrous rays we have hitherto been acquainted with. Owing 

 o the magnetic condition of the earth, the new solar rays, as we have seen, principally enter the polar 

 egions. 



(') Annalen der Physik, t. XII, 1903, p. 714. 



(-) Nature, November 4, 1909. 



( 3 ) Archives des Sciences physiques et naturelles, Vol. XXIV, Chap. IV, 1907, p. lai. 



Birkeland. The Norwegian Aurora Polaris Expedition 1902 1903. 76 



