Lightning Discharges and of the Aurora Borealis. 347 



the hand which holds the vacuum-tube through the air and 

 the walls or floor of the room to the other terminal of the 

 transformer. We can change this brush-discharge or lumi- 

 nosity at either terminal of a transformer into a disruptive 

 discharge by lessening the distance between the terminals or 

 by increasing the electromotive force. 



I am fully aware that the oscillatory discharge of lightning 

 with its disruptive effects, which I have noted, its permanence 

 of path, and the fading of the disruptive discharge into the 

 brush- discharge or mere luminosity at either of the spark- 

 terminals, is a far simpler phenomenon than the luminosity 

 produced in rarefied tubes; for in the latter phenomenon we 

 have the dissociation and impact of molecules, and we must 

 consider all the problems of atomic motion in addition to 

 those of the oscillatory nature of electrical waves. It is not 

 my purpose to enter into a consideration of the molecular 

 movements involved in oscillatory discharges in vacuum-tubes; 

 but having discussed some of the general features of dis- 

 charges of electricity in air at the ordinary pressure, I shall 

 endeavour to trace the connexion between such discharges 

 and the phenomenon of the Aurora Borealis. To my mind the 

 luminosity in a vacuum-tube held in one hand while the other 

 hand grasps the terminal of a Ruhmkorff coil closely repre- 

 sents the phenomenon of the Northern Lights ; for we have in 

 this case a discharge of electricity from a higher level to a 

 lower through a rarefied medium. Although, in this paper, 

 I restrict myself to a discussion of the general relations 

 between discharges of lightning and the phenomenon of 

 the Aurora Borealis, and do not enter into a study of the mole- 

 cular movements excited by electrical discharges, I am impelled 

 to devote a few words to the subject of the stratified discharge, 

 and to show that it has no connexion with the oscillatory 

 discharge of electricity such as we are considering. The 

 distances between the stratifications do not seem to be 

 changed by modifying the period of forced oscillation given 

 to the transformer over a wide range. ] have produced them 

 by employing an interrupter of a Ruhmkorff coil giving from 

 sixty to one hundred vibrations per second, and by the use 

 of two alternating machines — one giving 300 to 400 alterna- 

 tions per second, and the other 900 to 1000 alternations per 

 second. The distances between the stratifications do not seem 

 commensurate with the rate of alternation of the exciter of 

 the transformer. On the other hand, the distance between the 

 stratifications is not dependent upon the amount of self- 

 induction in the circuit. In one case quadrupling the self- 

 induction reduced the distance between the stratifications 



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