the Electrical Discharge in vacuo, 19 



one over the other until the end of the former parts company 

 with the latter; when this is well done the strise are, in the 

 majority of cases, not observed. Of all the substances which 

 have been tried, the vapour of phosphorus succeeds best ; and 

 with this is seen a remarkable effect on the powder or smoke of 

 allotropic phosphorus (which is always formed when the strise 

 are observed) : this smoke traverses from pole to pole from the 

 negative to the positive side, showing, unless there be some latent 

 optical deception, a mechanical effect of the discharge under the 

 circumstances." 



Dr Robinson, who has a little misunderstood my meaning in 

 this passage, says (Proc. Royal Irish Acad., December 1856), 

 " Mr. Grove appears to think it arises from some vibration in 

 the metal of a contact breaker, which produces a fluctuation in 

 the inducing current." I did not mean to say that the effect 

 was due to any peculiar vibration of the metal of the contact 

 breaker, but to the interrupted and successive discharges of the 

 apparatus itself, — that the changes in the character of the dis- 

 charge attendant upon changes in the action of the contact 

 breaker, as well as the frequent absence of strise when only a 

 single disruption was effected, afforded evidence that the strise 

 were connected with the multiplied discharges. My notice was, 

 however, short and somewhat obscure, as, although I entered 

 fully on the matter in the Section-room, I had prepared no me- 

 moir on the subject. I still retain the opinion I then expressed, 

 though I do not assert it as a positive conviction ; the difficulty 

 of proving it arose from the circumstance that it was next to im- 

 possible to produce, by the ordinary modes, a single discharge 

 from the induced coil, for the following reasons. When one 

 extremity of the wire of the primary coil is drawn over the other, 

 an induced current is produced at the moment of disruption in 

 the secondary wire and a consequent discharge in the vacuum. 

 But at this same moment of disruption, the extra or induced cur- 

 rent in the ]n'imary M'ire itself finds a passage in the form of a 

 spark immediately the contact is broken, and this extra current 

 occasions necessarily a second induced current in the secondary 

 wire, which having a ready path opened to it by the discharge of 

 the previous current, would be discharged through that path, 

 though it might not have tension enough to overcome any great 

 resistance. Although, therefore, these discharges cannot be 

 separated to the eye, it by no means follows that there are not 

 two discharges when an apparently single disruption of the pri- 

 mary circuit takes place. 



I had repeatedly and in vain attempted to get rid of this diffi- 

 culty, and to produce what I could feel assured was a single 

 induced discharge, when a veiy simple plan occurred to me, 



C2 



