234 ELECTRICITY OF THE ATMOSPHERE AND AURORA BOREALIS. 



Polar ligbt, considered as an electric discharge, produces the follow- 

 ing results :* 



1. An electric current, produced by the discharge itself, which takes 

 i:)lace slowly. 



2. Kays of light consisting of an infinite number of sparks, each spark 

 giving rise to two currents of induction, proceeding in opposite di- 

 rections. 



3. An electric current, proceeding in an opposite direction from that 

 of the charge, and originating in the electro-motive force, discovered by 

 M. Edluud in the electric spark. 



In order to develop these currents, a closed circuit is necessary. It 

 is true that in the i)henomenon of polar light this does not properly 

 exist, but in that case the earth on one hand, and the ran^fied air of the 

 upper regions of the atmosphere on the other, are immense reservoirs 

 of electricity, which produce the same effect as if the circuit was closed. 



According to the theory of M. de la Kive, the i)Ositive electricity of the 

 air, discharging itself into the ground, produces a current I will call 

 principal ; this current is re-enforced by one of the currents of induction, 

 which M, Edlund has shown accompany the production of the electric 

 spark ; that is to say, the one which, going in the opposite direction from 

 the charge, can alone acquire a certain degree of intensity. Bat this 

 principal current thus re-enforced is counterbalanced in part by the one 

 which has the contrary direction, and which produces the electro-motive 

 force of the spark. We see by the observations made with telegraph 

 wires during the appearance of polar light that it is sometimes the one, 

 sometimes the other of these two currents which gains possession of the 

 wire, the first being generally predominant, since it has been observed 

 that the current given by telegraphic wires is more frequently" directed 

 from the north to the south than from the south to the north. 



Under the circumstances which accompany the production of polar 

 light, the latter contains in itself all the conditions necessary in order 

 that magnetism may act upon it ; for a ray of this light constitutes a 

 current, flexible throughout, and consequently obedient to the law dis- 

 covered by Plucker, according to which such a current necessarily takes 

 the form of a magnetic curve. It is here we should look for the cause 

 of the formation of the crown, taking at the same time into considera- 

 tion "the variations in conductibility of the rarefied air of the upper 

 regions of the atmosphere. 



When an arch of i)olar light appears, we consider that it forms part 

 of a radiant ring, whose center coincides vei-y nearly with the mag- 

 netic pole. The rays of this ring are parallel to the direction of the 



* The author precedes this part of his work with the description of an experiment, in which 

 he tried, but unsuccessfully, the action of a magnet upon a series of discharges produced 

 by Holtz's machine under certain conditions. We omit this description, as it would be un- 

 intelligible without a figure to illustrate it, and it is not of ^reat importance, since the results 

 of the experiment were negative. ' 



