553 
to Electricity and Magnetism, 
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 plus 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. 
lOG. 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, and 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 phsenomenon of the lateral discharge as this latter 
partakes of the nature of an ordinary electrical current. 
F/iil, Mag. S. 3. Vol. 16. No. 106, Suppl. July 1840. 2 P 
