408 Electro-Magnetic Experiments. 
In these experiments the connection of the battery with the wires 
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 rernoving the battery from the acid ; a succession of vivid sparks 
was obtained by rapidly interrupting and forming the communication 
by means of one of these cups; but the greatest effect was produced 
when the magnetism was entirely destroyed and instantaneously re- 
produced by a change of polarity. 
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 Edinburgh, 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 
account of the experiment, which is reserved for a communication to © 
the Royal Society of Edinburgh; my result is therefore entirely inde- 
pendent of his and was undoubtedly obtained by a different process. 
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. [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 must 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 ifa wire thirty or for- 
ty feet long be used, instead of the short wire, though no spark will 
be perceptible when the connection is made, yet when it 1s broken 
by drawing one end of the wire from its cup of mercury a vivid spark 
is produced. If the action ef 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 substi- 
tuted a spark will again be obtained. ‘The effect appears somewhat 
increased by coiling the wire into a helix ; it seems also to depend in 
some measure on the length and thickness of the wire ; I can account 
for these phenomena only by supposing the long wire to become 
charged with electricity which by its reaction on itself projects a 
spark when the connection is broken. 
ETN NS TSE NEN TT 1s PME IT 
* From a natural magnet. 
