On liapitl Poriodic Variations of Terrestrial Magnetism. 79 



of the magnetic field on tlie cluuige of the current,^^ assun)iiig 

 the cm-rent already caused l)y other agents. In § 11) we have 

 suggested that the zonal part of the atmospheric current seems 

 to be more liable to the fluctuations than tlie part running in 

 the meridional direction. This is in some measure favourable 

 to the hypothesis. 



If the mechanical vibration of the atmosphere ])e once 

 started, the periodic fluctuation of current naturally results on 

 account of the induction due to the general terrestrial field. 

 The intensity of the induced current will be greatest at tlie 

 equator for a given amplitude of the oscillation. 



The cause of the initial impulsive increase of the current 

 may on one hand be considered as the result of the motion of the 

 atmosphere brought al:>out in a purely mechanical way, for 

 example, an abrupt advent of the vertical flow of the upper 

 atmosphere caused by the release of some instability of equi- 

 librium; or we may conceive a limited portion of the atmosphere 

 excited» to a vigorous vertical current, as in tlie centre of a 

 cyclone, caused as a secondary disturbance accompanying the 

 daily exchange of air between the day and night hemispheres. 



On the other hand, the abrupt increase of current may 

 also be attributed to an external agent, for example, a sudden 

 increase of the conductivity of air caused by the inflow of the 

 corpuscular radiation from the sun.'^ 



Among the above two possibilities, the latter is more 

 plausible, being in accordance with the commonl}^ accepted view 

 as regards the origin of magnetic disturbances, which is strongly 

 supported by the intimate relation existing between the solar 

 activity and the disturbances in general. Fig. 10 is also 

 favourable to tliis hypothesis. The former is rather doubtful, 



1) In view of this consideration, an interesting physical problem presents itself : 

 Consider u current passing through highly rarefied gases, acted upon by an external 

 magnetic field ; will the gases really flow «.s- a whole as Fleming's rule specifies for a solid 

 conductor? As far as we are aware, the answer is not yet given either experimentally or 

 theoretically, especially for such a case as is analogous to the atmosperic currents. 



2) A. Schuster concluded on the basis of energy consideration that the E.M.F. of the 

 current causing magnetic disturbances is to be attributed to the motion of tlie atmosphere, 

 the sun's radiation serving only as an agent increasing the conductivity. 



