EARTH AND SUN AS MAGNETS HALE. 157 



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 ex- 

 planation of the secular variation of the earth's magnetism, and 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 mag- 

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



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

 ness, though they may disappear as the result of more extended in- 

 vestigation. Its chief advantage lies in the possibility that it may 

 explain the secular variation of the earth's magnetism by a preces- 

 sional 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 observa- 

 tional results already obtained suggest that the phenomena of ioniza- 

 tion in the solar atmosphere may turn out to be the predominant in- 

 fluence. 



The lines which show the Zeeman effect originate at a compara- 

 tively low level in the solar atmosphere. Preliminary measures indi- 

 cate that certain lines of titanium, which are widely separated by a 

 magnetic field in the laboratory, are not appreciably affected in the 

 sun. As these lines represent a somewhat higher level, it is probable 

 that the strength of the sun's field decreases very rapidly in passing 

 upward from the surface of the photosphere — a conclusion in har- 

 mony with results obtained from the study of the corona and prom- 

 inences. Thus it may be found that the distribution of the electrons 

 is such as to give rise to the observed field or to produce a field oppos- 

 ing that caused by the rotation of the body of the sun. It is evident 

 that speculation along these lines may advantageously await the ac- 

 cumulation of observations covering a wide range of level. Beneath 

 the photosphere, where the pressure is high, we may conclude from 

 recent electric furnace experiments by King that free electrons, 

 though relatively few, may nevertheless play some part in the pro- 

 duction of the general magnetic field. 



