Oil Magnetic Storms. 669 



than this view supposes: the ions have, indeed, 

 considerable freedom to follow their own courses under 

 the electrostatic and electromagnetic forces affecting them. 

 Consequently, under the radial electrostatic force resulting 

 from the mutual repulsion of the injected charges^ the positive 

 ami negative ions present, will at first tend to move radially 

 in opposite 4 directions : the vertical electric current thus pro- 

 duced will be unidirectional, and so also will be the horizontal 

 electromagnetic force tending to deflect them along the circles 

 of latitude, owing to their motion across the earth's horizontal 

 magnetic field. This deflecting force will thus move both 

 positive and negative ions in the same latitudinal direction, 

 but the more mobile electrons will have much the greater 

 velocity in this direction, both because of their smaller mass 

 and of the greater deflecting force on them (proportional to 

 their greater radial velocity under the electrostatic force). 

 The horizontal current, not unidirectional for the two sets of 

 ions as is the vertical current, will thus be directed opposite 

 to the direction of motion of the electrons (opposite, because 

 of their negative charge). Now the direction of this 

 horizontal current is definitely known from the characteristic 

 diminution of the earth's horizontal magnetic force during a 

 magnetic storm : it is towards the west. Hence the motion 

 of the electrons must be easterly, which (considering the 

 direction of the horizontal magnetic field which deflects the 

 electrons to the east) indicates that the radial motion of the 

 negative electrons must be upwards. 



Consequently the electrostatic field impels electrons up- 

 wards, and positive ions downwards : it must therefore be 

 due to a negative charge on the earth. 



This simple argument is supported by another which, 

 though less weighty than the former, seems yet to have more 

 force than most of those on which attempts to decide the 

 sign of the injected charge have been based. The effects of 

 magnetic storms in the middle belt of the earth are definitely 

 greater over the afternoon than over the forenoon hemisphere 

 of the earth, and this suggests that the injection of corpuscles 

 is the more intense over the former hemisphere. Now the 

 simplest, paths of the particles are likely to be the more 

 numerous, and these (as calculated by Stormer) correspond 

 to entry of the particles on the afternoon side of the auroral 

 zones, if the particles are negative, and on the forenoon side 

 if positive. The circumstance mentioned is thus favourable 

 to the view that the particles are negative. 



Oct. 11, 1920. 



