204 CLASSICS OF MODERN SCIENCE 

 from the magnet was not formed by soldering, but by two cups of 

 mercury, which permitted the galvanic action on the magnet to be in- 

 stantaneously suspended and the polarity to be changed and rechanged 

 without removing the battery from the acid ; a succession of vivid 

 sparks was obtained by rapidly interrupting and forming the communi- 

 cation by means of one of these cups ; but the greatest effect was pro- 

 duced when the magnetism was entirely destroyed and instantaneously 

 reproduced by a change of polarity. 



It appears from the May number 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 Edinburgh, who obtained a spark 

 on the 30th of March ; my experiment being made during the last two 

 weeks of June. A simple notification of his result is given, without 

 any account of the experiment, which is reserved for a communica- 

 tion to the Royal Society of Edinburgh ; my result is therefore en- 

 tirely 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 sub- 

 ject, but which more important duties will not permit 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 phenomena as those 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 connected 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 instead of the short wire, though no spark will be 

 perceptible when the connection is made, yet when it is broken by 

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

 is produced. If the action of the battery be very intense, 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 sub- 

 stituted a spark will again be obtained. The effect appears somewhat 

 increased by coiling the wire into a helix; it seems also to depend in 



