Page:Scientific Memoirs, Vol. 1 (1837).djvu/542

530 contrary increase that of liquids. Is this the state of bodies which Mr. Faraday calls electro-tonic?

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 appeared 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 ¾ lin. in diameter, carefully covered with silk ribbon, were coiled together in a helix round a hollow cylinder of wood, 1½ 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 moment 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 increased 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, 1½ inch in diameter, forming the armature of the bar of soft iron. We will call the corresponding 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 ¼ foot square. The helix surrounding the bar terminated at a pile of a foot square, by means of a commutator à 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 $$a(A_n a_n,\, B_s b_s,)$$, the needle advanced to 30°, and on reversing the current so as to produce contrary magnetisms $$(A_sa_n,\, B_n b_s)$$ 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 $$A_n a_n,\, B_s b_s$$, the needle advanced to 33½°; it receded on the contrary to 13° when the magnetism of the bar and of the armature attracted one another, $$(A_sa_n,\, B_n b_s)$$. 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