316 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



ductors enables us to imitate, in a very striking manner, the inductive 

 operation of the magneto-electrical machine, by means of an uninter- 

 rupted galvanic current. For this purpose it is only necessary to ar- 

 range two coils to represent the two poles of a horseshoe magnet, and to 

 cause two helices to revolve past them in a parallel plane. While a 

 constant current is passing through each coil, in opposite directions, the 

 effect of the rotation of the helices is precisely the same as that of the 

 revolving armature in the machine. 



54. A remarkable fact should here be noted in reference to helix 

 No. 4, which is connected with a subsequent part of the investigation. 

 This helix is formed of copper wire, the spires of which are insulated 

 by a coating of cement instead of thread, as in the case of the others. 

 After being used in the above experiments, a small discharge from a 

 Leyden jar was passed through it, and on applying it again to the coil, 

 I was much surprised to find that scarcely any signs of a secondary 

 current could be obtained. 



55. The discharge had destroyed the insulation in some part, but 

 this was not sufficient to prevent the magnetizing of a bar of iron in- 

 troduced into the opening at the centre. The effect appeared to be 

 confined to the inductive action. The same accident had before hap- 

 pened to another coil of nearly the same kind. It was therefore noted 

 as one of some importance. An explanation was afterwards found in 

 a peculiar action of the secondary current. 



SECTION IV. 



On the Effects produced by interposing different Substances between 



the Conductors. 



56. Sir H. Davy found, in magnetizing needles by an electrical dis- 

 charge, that the effect took place through interposed plates of all sub- 

 stances, conductors and nonconductors.* The experiment which I 

 have given in paragraph 51 would appear to indicate that the induc- 

 tive action which produces the secondary current might also follow 

 the same law. 



• Philosophical Transactions, 1821. 



