530 PROF. JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 
contrary increase that of liquids. Is this the state of bodies which 
Mr. Faraday calls electro-tonic ? 
22. 
In the supplement of No. 105. of the Institute for May 13, 1835, 
there is a notice of a memoir by Mr. Faraday the publication of which 
we are looking for. The experiment cited at the end of this notice ap- 
peared to me so striking and important in connection with the subject 
of the present memoir that I did not delay repeating it. Two copper 
wires, 400 feet long and 2 lin. in diameter, carefully covered with silk 
ribbon, were coiled together in a helix round a hollow cylinder of 
wood, 14 inch in diameter. The ends of these two wires were united 
in a single one. The effect of this combination was beyond all my 
expectations ; for by employing a voltaic pair of silver and zinc plates, 
which had only a surface of half a square inch, I obtained at the mo- 
ment of disjunction a brilliant spark, and a violent shock which could 
scarcely be borne. The same effects took place when the pair of 
plates were reduced to a wire of platina and zinc. After having 
placed a cylinder of soft iron in the hollow of the wooden cylinder, the 
action was still more considerable. These effects were not much in- 
creased by the enlargement of the surface of the pair. A conducting 
wire of 400 feet having been employed alone, the spark and the shock 
were much more feeble; but on uniting in a circuit the two ends of the 
second wire of 400 feet, there was neither spark nor shock. This is 
perfectly conformable with Mr. Faraday’s experiment. 
Upon this I made the following experiment: In the hollow of the 
wooden cylinder I placed a cylinder of soft iron, 13 inch in diameter, 
forming the armature of the bar of soft iron. We will call the corre- 
sponding extremities of the bar and the armature A,a; B,b. The two 
wires of 400 feet of the helix coiled round the armature were united in 
one of 800 feet, the ends of which were conducted by a multiplier to 
the poles of a voltaic pair of plates about 1 foot square. The helix 
surrounding the bar terminated at a pile of a foot square, by means of 
a commutator @ bascule. The deviation was 16°. The current which 
magnetized the horse-shoe bar being directed so as to produce in A the 
same magnetism as in @ (A, a, B, by), the needle advanced to 30°, 
and on reversing the current so as to produce contrary magnetisms 
(A, a, B,, 6,) the needle receded from 16° to 10°, returning after a few 
oscillations to its first position at 16°. By employing a single wire of 
400 feet, the other wire not forming a circuit, the deviation of the needle 
was 21°. By the arrangement A, a,,-B, 6, the needle advanced to 
334°; it receded on the contrary to 13° when the magnetism of the 
bar and of the armature attracted one another, (A_a,B 6). After 
having united in a circuit the second wire of 400 feet, the deviation 
of the needle having been the same as before, that is 21°, the needle 
