THE EARTH AND SUN AS MAGNETS 



III 



Fig. 5. The Auuor^v. 



more penetrating rays sometimes reach an altitude of 25 miles, the 

 lowest hitherto found for the aurora. The passage through the at- 

 mosphere of the electrons which cause the aurora also gives rise to the 

 irregular disturbances of the magnetic needle observed during magnetic 

 storms. 



The outflow of electrons from the sun never ceases, if we may reason 

 from the fact that the night sky is at all times feebly illuminated by 

 the characteristic light of the aurora. But when sun-spots are numer- 

 ous, the discharge of electrons is most violent, thus explaining the fre- 

 quency of brilliant auroras and intense magnetic storms during sun- 

 spot maxima. It should be remarked that the discharge of electrons 

 does not necessarily occur from the spots themselves, but rather from 

 the eruptive regions surrounding them. 



Our acquaintance with vacuum tube discharges dates from an early 

 period, but accurate knowledge of these phenomena may be said to 

 begin with the work of Sir William Crookes in 1876. A glass tube, 

 fitted with electrodes, and filled with any gas, is exhausted with a suit- 

 able pump until the pressure within it is very low. When a high voltage 

 discharge is passed through the tube, a stream of negatively-charged 

 particles is shot out from the cathode, or negative pole, with great veloc- 

 ity. These electrons, bombarding the molecules of the gas within the 

 tube, produce a brilliant illumination, the character of which depends 

 upon the nature of the gas. The rare hydrogen gas in the upper at- 

 mosphere of the earth, when bombarded by electrons from the sun, 

 glows like the hydrogen in this tube. Nitrogen, which is characteristic 

 of a lower level, shines with the light which can be duplicated here. 



But it may be remarked that this explanation of the aurora is only 

 hypothetical, in the absence of direct evidence of the emission of elec- 

 trons by the sun. However, we do know that hot bodies emit electrons. 



