1832] WRITINGS OF JOSEPH HENRY. 79 



It appears from the May No. of the Annals of Philosophy 

 that I have been anticipated in this experiment of drawing 

 sparks from the magnet by Mr. James D. Forbes of Edin- 

 burgh, who obtained a spark on the 30th of March; my 

 experiments being made during the last two weeks of June. 

 A simple notification of his result is given, without any ac- 

 count of the experiment, which is reserved for a communi- 

 cation to the Royal Society of Edinburgh ; m}^ result is 

 therefore entirely independent of his and was undoubtedly 

 obtained by a different process. 



Electrical self-induction in a long helical wire. 



I have made several other experiments in relation to the 

 same subject, but which more important duties will not per- 

 init me to verify in time for this paper. I may however 

 mention one fact which I have not seen noticed in any work, 

 and which appears to me to belong to the same class of phe- 

 nomena as tliose before described; it is this: when a small 

 battery is moderately excited by diluted acid, and its poles 

 which should be terminated by cups of mercury, are con- 

 nected by a copper wire not more than a foot in length, no 

 spark is perceived when the connection is either formed or 

 broken ; but if a wire thirty or forty feet long be used in- 

 stead of the short wire, though no spark will be perceptible 

 when the connection is made, yet when it is broken by draw- 

 ing one end of the wire from its cup of mercury, a vivid 

 spark is produced. If the action of the battery be very in- 

 tense, a spark will be given by the short wire; in this case 

 it is only necessary to wait a few minutes until the action 

 partially subsides, and until no more sparks are given from 

 the short wire; if the long wire be now substituted a spark 

 will again be obtained. The effect appears somewhat in- 

 creased by coiling the wire into a helix; it seems also to de- 

 pend in some measure on the length and thickness of the 

 wire. I can account for these phenomena only by suppos- 

 ing the long wire to become charged with electricity, which 

 by its re-action on itself projects a spark when the connec- 

 tion is broken. 



