THE EAETH^S MAGNETISM BAUER. 211 



than that of the positive, or, in other words, the earth's total negative 

 charge must be distributed through the larger sphere, and, if that be 

 the whole earth itself, then for the chief term involved in the mag- 

 netic potential, the surface of the sphere' containing the positive 

 charge need be, on the average, only 0.4X10~^ cms., i. e. four-tenths 

 of the radius of an ordinary molecule, below that of the earth's 

 surface to give a magnetic field of the required strength. Taking the 

 average atomic weight of the earth's substance in round numbers as 

 50, the mean volume density of either charge would be about 3.3X10^^ 

 electrostatic units. 



At present there is little hope that a magnetic field, caused just as 

 supposed, can be detected in the laboratory. For a sphere of 15 centi- 

 meters radius, rotating 100 times a second, the magnetic intensity at 

 the poles would be but one hundred-millionth part (10-^) of that of 

 the earth. We thus see that the quantities involved in the solution 

 of one of the great problems confronting the student of the earth's 

 physics — the origin of the earth's magnetic field — may be of such a 

 minute order as to be beyond the ken at present of the laboratory 

 experimentalist. Perhaps the effects become appreciable in the case 

 of the earth because of the fortunate fact that it is a body of suffi- 

 cient size and angular velocity. 



On the other hand, the geophysicist is at a great disadvantage in 

 that he is unable to bring his earth-magnet into the laboratory and 

 to experiment upon it — to reverse the direction of rotation, for 

 example, and see what would happen! Fortunately for him, how- 

 ever, nature comes to his relief somewhat and performs experiments 

 for him on his great magnet on a world-wide scale, by producing 

 in an incredibly short time manifold and at times startling varia- 

 tions and fluctuations in the apparently fixed magnetization of the 

 earth. Thus, on September 25, 1909, there occurred the most re- 

 markable magnetic storm on record, during which, within a few 

 minutes, the earth's magnetic movement, or intensity of magnetiza- 

 tion, was altered by about one-twentieth to one-thirtieth part. The 

 earth's magnetic condition was below par for fully three months 

 thereafter. As this severe storm was accompanied by a brilliant dis- 

 play of polar lights, this is the most appropriate place to recall that 

 Halley made the first suggestion of a connection between the aurora 

 borealis and the earth's magnetism. (PI. 8, fig. 1.) 



It is firmly believed that a long step forward will have been taken 

 toward the discovery of the origin of the earth's magnetism when 

 once we have found out what causes it to vary in the surprising 

 manner shown by the secular or long-period changes, by the magnetic 

 storms, and the numerous other fluctuations, such as the diurnal 

 variation, for example. The keynote of modern investigation in 



