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 J lin. in diameter, carefully covered with silk 

 ribbon, were coiled together in a helix round a hollow cylinder of 

 wood, li 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 eifects 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, 1^- 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 ^ foot square. The helix 

 surrounding the bar teniiinated at a pile of a foot square, by means of 

 a commutator a 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 a^, B^ b^,), the needle advanced to 30°, 

 and on reversing the current so as to produce contrary magnetisms 

 {^g o„, B^ b^) 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^ b^, the needle advanced to 

 33^°; it receded on the contrary to 13° when the magnetism of the 

 bar and of the armature attracted one another, (A a ,B b ). After 



^ s n n s' 



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 



