THE SEMI-CENTENNIAL ANNIVERSARY 47 



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, probably will not suffice. A marked preponder- 

 ance of electrons of one sign seems to be 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 

 experiments, are in harmony with those of Fowler in proving 

 that sun-spots are comparatively cool regions in the solar at- 

 mosphere. 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 whirl of ions in a tornado produces a magnetic field, it 

 is too feeble to be readily detected. Thus, while we have dem- 



* King has recently found that the current decreases very rapidly as the pressure increases, 

 but is still appreciable at a pressure of 20 atmospheres. 



