THE EARTH AND SUN AS MAGNETS 117 



rapidly — so rapidly, in fact, as to suggest the existence of an opposing 

 field. It is probable that the vortex which produces the observed field 

 is not the one that appears on our photograph, but lies at a lower level. 

 In fact, the vortex structure shown on spectroheliograph plates may 

 represent the effect, rather than the cause of the sun-spot field. We 

 may have, as Brestcr and Deslandres suggest, a condition analogous to 

 that illustrated in the aurora : electrons, falling in the solar atmosphere, 

 move along the lines of force of the magnetic field into spots. In this 

 way we may perhaps account for the structure surrounding pairs of 

 spots, of opposite polarity, which constitute the typical sun-spot group. 

 The resemblance of the structure near these two bipolar groups to the 

 lines of force about a bar magnet is very striking, especially when the 

 disturbed condition of the solar atmosphere, which tends to mask the 

 effect, is borne in mind. It is not unlikely that the bipolar group is 

 due to a single vortex, of the horse-shoe type, such as we may see in 

 water after every sweep of an oar. 



We thus have abundant evidence of the existence on the sun of 

 local magnetic fields of great intensity — fields so extensive that the 

 earth is small in comparison with many of them. But how may we 

 account for the copious supply of electrons needed to generate the 

 powerful currents required in such enormous electro-magnets ? Neutral 

 molecules, postulated in theories of the earth's field, will not suffice. 

 A marked preponderance of electrons of one sign is clearly indicated. 



An interesting experiment, due to Harker, will help us here. 

 Imagine a pair of carbon rods, insulated within a furnace heated to a 

 temperature of two or three thousand degrees. The outer ends of the 

 rods, projecting from the furnace, are connected to a galvanometer. 

 Harker found that when one of the carbon terminals within the furnace 

 was cooler than the other, a stream of negative electrons flowed toward 

 it from the hotter electrode. Even at atmospheric pressure, currents 

 of several amperes were produced in this way.^ 



Our spectroscopic investigations, interpreted by laboratory experi- 

 ments, are in harmony wuth those of Fowler in proving that sun-spots 

 are comparatively cool regions in the solar atmosphere. They are hot 

 enough, it is true, to volatilize such refractory elements as titanium, 

 but cool enough to permit the formation of certain compounds not 

 found elsewhere in the sun. Hence, from Harker's experiment, we may 

 expect a flow of negative electrons toward spots. These, caught and 

 whirled in the vortex, would easily account for the observed magnetic 

 fields. 



The conditions existing in sun-spots are thus without any close 

 parallel among the natural phenomena of the earth. The sun-spot 

 vortex is not unlike a terrestrial tornado, on a vast scale, but if the 



'Kmg has recently found that the current decreases very rapidly as the 

 pressure increases, but is still appreciable at a pressure of 20 atmospheres. 



