I9I0.] TERRESTRIAL MAGNETIC DISTURBANCES. 137 



currents in the atmosphere. If then the regions of these upper 

 electric currents are at any time made by some cause more conduct- 

 ing, electricity is immediately set in motion, which in turn induces 

 a subsidiary magnetization in the earth. The effect then which we 

 actually observe on the compass needle is the joint result of the 

 newly generated electric currents in the atmosphere and the induced 

 magnetization of the earth. 



The direction followed by the new current depends upon its 

 origin, upon the direction of the electromotive force of the primary 

 electric field already existing at that point, and upon the deflecting 

 effect of the earth's magnetic field and of the earth's rotation on the 

 flowing current. In other zvords, zvhile zve must doubtless look 

 chiefly to extra-terrestrial agencies for the ionimng of the air and 

 thus splitting it up into carriers of positive and of negative charges, 

 ive are compelled to look to the atmospheric electric field and to the 

 earth's rotation for furnishing the energy necessary to drive the ions 

 over the earthy arnd by their motion produce the effects observed 

 during a magnetic storm. 



We shall tentatively designate the theory which thus aims to 

 account for terrestrial magnetic disturbance as " the ionic theory of 

 magnetic disturbances." It is of interest to quote here from 

 Schuster's extremely suggestive paper^ on the " Diurnal Variation 

 of Terrestrial Magnetism " : 



Outbreaks of magnetic disturbances, affecting sometimes the whole of 

 the earth simultaneously, may be explained by sudden local changes of con- 

 ductivity which may extend through restricted or extensive portions of the 

 atmosphere. I have shown in another place that the energy involved in a 

 great magnetic storm is so considerable that we can only think of the earth's 

 rotational energy as the source from which it ultimately is drawn. The earth 

 can only act through its magnetization in combination with the circulation 

 of the atmosphere, so that magnetic storms may be considered to be only 

 highly magnified and sudden changes in the intensity of electric currents 

 circulating under the action of electric forces which are always present. 



How can we account, on the basis of the " ionic theory," for the 

 velocity of motion of magnetic disturbances and, hence, for the time 

 required to make a complete circuit of the earth? Starting from the 

 well-established law as to motions of ions in gases, it is found that in 



' Phil. Trans. R. Soc, A, Vol. 208, 1908, 184-1S5. 



