400 Prof. Thomson on Transient Electric Currents, 

 •essed when the condition 



UA>A-C<^^ .... . (15) 



is fulfilled. Thus if the interval of time -j^ ro-ri> at which 



I _ ^ \ 

 VCA 4A^/ 



the successive instants when the strength of the current is a 

 maximum follow one another, be sufficiently great, and if the 

 evolution of heat in any part of the circuit by the current during 

 several of its alternations in direction be sufficiently intense to 

 produce visible light, a succession of flashes diminishing in 

 intensity and following one another rapidly at equal intervals 

 will be seen. It appears to me not improbable that double, 

 triple, and quadruple flashes of lightning which I have frequently 

 seen on the continent of Europe, and sometimes, though not sd 

 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 lightning in the sky, result from the discharge possessing 

 this oscillatory character. A corresponding phsenomenon might 

 probably be produced artificially on a small scale by discharging 

 a Ley den phial or other conductor across a very small space of 

 air, and through a linear Conductor of large electrodynamic 

 capacity and small resistance. Should it be impossible on 

 account of the too great rapidity of the successive flashes 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 inter- 

 vals, instead of a single point of light, or of an unbroken line of 

 light, as it would be if the discharge were instantaneous, 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, illus- 

 strate in a very remarkable manner the oscillatory character of 

 the discharge in certain circumstances ; not only when, as in the 

 case with which we are at present occupied, the whole mecha- 

 nical effect of the discharge is produced within a single linear 

 conductor, but when induced currents in secondary conductors 

 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 oscillations in the dis- 



