rnoFESSOR Tho?^ison Oh Transient Electric Currents. 287 



cording as the capacity of the principal conductor exceeds or falls short of 

 a certain limit. If the principal conductor, and the length and substance 

 of the discharger, be given, the discharge will be continued or oscillatory 

 according as the electrodynamic capacity of the latter, depending as it 

 does on the form into which it is bent, falls short of, or exceeds a certain 

 limit. Lastly, if the principal conductor, and the length and form of the 

 discharger be given, the discharge will be continued or oscillatory, accord- 

 ing as the resistance of the discharger to conduction exceeds or falls short 

 of a certain limit. 



It ought to be remarked that, although the electrical equilibrium is not 

 rigorously attained, whatever kind of discharge it may be, in any finite 

 time ; yet practically, in all ordinary experimental cases the discharge is 

 finished almost instantaneously as regards all appreciable effects ; and 

 the great obstacle in the way of experimenting at all on the subject arises 

 from the difficulty of arranging the circumstances, so that the periods 

 of time indicated by the theory for the succession of various phenomena, 

 (as for instance, the alternations of the charges of the contrary electricity 

 on the principal conductor), may not be inappreciably small. 



It is not improbable that double, triple, and quadruple flashes of light- 

 ning which are frequently seen on the continent of Europe, and sometimes, 

 though not so frequently, in this country, lasting generally long enough to 

 allow an observer, after his attention is drawn by the first light of the 

 flash, to turn his head round and see distinctly the course of the light- 

 ning in the sky, result from the discharge possessing the oscillatory char- 

 acter. A corresponding jjhenomenon might probably be produced arti- 

 ficially on a small scale, by discharging a Leyden phial or other conduc- 

 tor across a very small space of air, and through a linear conductor of large 

 electro-dynamic capacity and small resistance. Should it be impossible, on 

 account of the too great rapidity of the successive fia.shes, for the unaided 

 eye to distinguish them, Wheatstone's method of a revolving mirror might 

 be employed, and might show the spark as several points or short lines of 

 light separated by dark intervals, instead of a single point of light, or of 

 an unbroken line of light, as it would be if the discharge were instantane- 

 ous, or were continuous and of appreciable duration. 



The experiments by Riess and others on the magnetization of fine steel 

 needles by the discharge of electrified conductors, illustrate in a very re- 

 markable manner the oscillatory character of the discharge in certain cir- 

 cumstances ; not only when, as in the case with which we are at present 

 occupied, the whole mechanical eflfect of the discharge is produced within 

 a single linear conductor, but when induced currents in secondary con- 

 ductors generate a portion of the final thermal equivalent. 



The decomposition of water by electricity from an ordinary electrical 

 machine, in which, as has been shown by Faraday, more than the electro- 

 chemical equivalent of the whole electricity that passes appears in oxygen 

 and hydrogen rising mixed from each pole, is probably due to electrical 



