142 BAUER— SOLAR ACTIVITY AND [April 23, 



Hence, as a first approximation, we may confine ourselves 

 chiefly to the polar atmospheric electric currents. Owing to the 

 direction in which they go — approximately southwest to northeast, 

 if negative ones, and reverse for positive currents — it follows that 

 their efl:'ect on a compass needle placed on the earth's surface is 

 similar to that of the earth's own magnetism. Or, in other words, 

 the magnetic system equivalent to the atmospheric electric system is 

 precisely similar to that of the earth and the magnetic axis of the 

 atmosphere is hence in the same general direction as the earth's. 



There is this difference, however, between the two magnetic 

 fields of the earth and of the atmosphere, viz., that while their 

 north magnetic poles are both in the northern hemisphere, that of 

 the earth is below the surface, whereas that of the atmosphere 

 is in the regions above ; hence, while the effects on the compass 

 needle are the same for both, the efifects on the dip needle are 

 opposite. The earth's field makes the north-seeking end of the 

 needle dip below the horizon in the northern magnetic hemisphere, 

 whereas the magnetic field of the atmosphere, were it alone acting 

 (the earth's field being eliminated) would make the north-seeking 

 end of the needle point above the horizon. Hence the efifects of 

 the two fields on the vertical magnetic component are opposite in 

 the same hemisphere. 



In accordance with the well-known laws of induced magnetism 

 or electricity, the atmospheric magnetic field would have to be 

 related to the earth's own field in the following manner: First, 

 the strength of the induced electric currents must be directly 

 proportional to the earth's intensity of magnetization, the electrical 

 conductivity of the atmospheric layers in which the currents flow, 

 and the velocity of the air-currents ; secondly, the magnetic axis 

 of the atmospheric electric field must suffer a displacement with 

 reference to the earth's magnetic axis in a direction opposite to 

 that of the earth's rotation (since the earth is moving more slowly 

 than the air-currents) ; hence, the atmosphere's north magnetic 

 pole would have to lie west of that of the earth's. The angle of 

 displacement depends upon the same quantities as did the strength 

 of the currents, different functions, however, being involved ; it 

 cannot exceed 90°. 



