to Electricity and Magnetism. 553 



necessary to recall the well-known fact, that when the knob 

 of a jar is electrified positively, and the outer coating in con- 

 nexion with the earth, then the jar contains a small excess of 

 positive electricity beyond what is necessary to perfectly neu- 

 tralize the negative surface. If the knob be put in communi- 

 cation with the earth, the extra quantity, or the free electri- 

 city, as it is sometimes called, will be on the negative side. 

 When the discharge took place in the above experiments, the 

 inner riband became for an instant charged with this free 

 electricity, and consequently threw off from the outer riband, 

 by ordinary induction, the sparks described. It therefore be- 

 came a question of importance to determine, whether the in- 

 duced current described in paragraph 100 was not also a re- 

 sult of the lateral discharge, instead of being a true case of 

 a secondary current analogous to those produced from gal- 

 vanism. For this purpose the jar was charged, first with the 

 outer coating in connexion with the earth, and again with the 

 knob in connexion with the same, so that the extra quantity 

 might be in the one case iplus and in the other minus ; but 

 the direction of the induced current was not affected by these 

 changes; it was always the same, namely, from the positive 

 to the negative side of the jar. 



105. When, however, the quantity of free electricity was 

 increased, by connecting the knob of the jar with a globe 

 about a foot in diameter, the intensity of magnetism appeared 

 to be somewhat diminished, if the extra quantity was on the 

 negative side; and this might be expected, since the free 

 electricity, in its escape to the earth through the riband, in this 

 case would tend to induce a feeble current in the opposite 

 direction to that of the jar. 



106. The spark from an insulated conductor may be con- 

 sidered as consisting almost entirely of this free or extra elec- 

 tricity, and it was found that this was also capable of pro- 

 ducing an induced current, precisely the same as that from 

 the jar. In the experiment which gave this result, one end 

 of the outer riband of the cylinder (100) was connected with 

 the earth, and the other caused to receive a spark from a con- 

 ductor fourteen feet long, ind nearly a foot in diameter. The 

 direction of the induced current was the same as that of the 

 spark from the conductor. 



107. From these experiments it appears evident that the 

 discharge from the Leyden jar possesses the property of in- 

 ducing a secondary current precisely the same as the galvanic 

 apparatus, and also that this induction is only so far connected 

 with the phaenomenon of the lateral discharge as this latter 

 partakes of the nature of an ordinary electrical current. 



Fhil, Ma^. S. 3. Vol. 16. No. 106. Siippl. My 1840. 2 P 



