THE EARTH AND BUN AS MAGNETS 123 



Schuster, assuming the magnetic fields of the earth and sun to be 

 due to their rotation, found that the strength of the sun's field should 

 be 440 times that of the earth, or 264 gausses. This was on the sup- 

 position that the field-strength of a rotating body is proportional to the 

 product of the radius and the maximum linear velocity of rotation, but 

 neglected the density. Before inquiring why the observed and theoret- 

 ical values differ, we may glance at the two most promising hypotheses 

 that have been advanced in support of the view that every large rotating 

 body is a magnet. 



On account of their greater mass, the positive electrons of the neutral 

 molecules within the earth may perhaps be more powerfully attracted by 

 gravitation than the negative electrons. In this case the negative 

 charge of each molecule should be a little farther from the center of 

 the earth than the positive charge. The average linear velocity of the 

 negative charge would thus be a little greater, and the magnetizing 

 effect due to its motion would slightly exceed that due to the motion of 

 the positive charge. By assuming a separation of the charges equal to 

 about four tenths the radius of a molecule (Bauer), the symmetrical 

 part of the earth's magnetic field could be accounted for as the result 

 of the axial rotation. 



This theory, first suggested by Thomson, has been developed by 

 Sutherland, Schuster and Bauer. But as yet it has yielded no explana- 

 tion of the secular variation of the earth's magnetism, and other impor- 

 tant objections have been urged against it. While it should not be 

 rejected, the merits of other theories must not be overlooked. 



Chief among these is the theory that rests on the very probable 

 assumption that every molecule is a magnet. If the magnetism is 

 accounted for as the effect of the rapid revolution of electrons within 

 the molecule, a gyrostatic action might be anticipated. That is, each 

 molecule would tend to set itself with its axis parallel to the axis of the 

 earth, just as the gyrostatic compass, now coming into use at sea, tends 

 to point to the geographical pole. The host of molecular magnets, all 

 acting together, might account for the earth's magnetic field. 



This theory, in its turn, is not free from obvious points of weakness, 

 though they may disappear as the result of more extended investiga- 

 tion. Its chief advantage lies in the possibility that it may explain the 

 secular variation of the earth's magnetism by a precessional motion of 

 the magnetic molecules. 



On either hypothesis, it is assumed, in the absence of knowledge to 

 the contrary, that every molecule contributes to the production of the 

 magnetic field. Thus the density of the rotating body may prove to be 

 a factor. Perhaps the change of density from the surface to the center 

 of the sun must also be taken into account. But the observational 

 results already obtained suggest that the phenomena of ionization in 

 the solar atmosphere may turn out to be the predominant influence. 



