STRIDE IN THE ELECTRIC DISCHARGE. 379 



ON THE STRI^ SEEN IN THE ELECTRICAL 

 DISCHARGE IN VACUO. 



Phil. Mag., July 1858. 



IN a paper communicated to the Royal Society in January 

 1852 I published the first notice of transverse bands or striae 

 which are seen in the discharge from a Ruhmkorff coil in the 

 vacuum of an air-pump, wherein is placed a piece of phos- 

 phorus ; and in a postscript to that paper (' Phil. Mag./ vol. 

 iv.) I stated that the ^striae could be seen in several attenuated 

 gases, probably in all. Some time after the publication of my 

 paper, but without having seen it, M. Ruhmkorff observed the 

 same phenomenon in the vapour of alcohol ; and it has subse- 

 quently been experimented on by Messrs. Masson, Quet, Du 

 Moncel, Robinson, and Gassiot, the last of whom has elabo- 

 rately examined it in the Torricellian vacuum. 



No satisfactory rationale of the phenomenon has been given. 

 In my first paper I gave no opinion as to its cause ; but subse- 

 quently, in a communication to the British Association at Hull, 

 I stated my belief that it was closely connected with the inter- 

 ruptions of the contact-breaker. The following is the notice of 

 my communication in the ' Athenaeum,' August 30, 1856 : ' Mr. 

 Grove has observed that the mode of breaking contact has a 

 marked influence on the phenomenon. If, for instance, the 

 arm of the contact-breaker be made to rest on a slight spring 

 placed underneath it, the bands become narrower. If a 

 single breach of contact be effected, most observers have 

 remarked that the effect is still perceptible ; but it is very 

 difficult to effect a single breach of contact ; the fusion of the 

 metals at the point of contact, with the vibration accompany- 

 ing the movement, occasions a double or triple disruption. The 

 best mode is to place two stout copper wires across each other, 

 and with a firm hand draw one over the other until the end of 

 the former parts company with the latter ; when this is well 

 done the striae are, in the majority of cases, not observed. Of 

 all the substances which have been tried, the vapour of phos- 



