222 Contributions to Electridty and Magnetism. 



53. I may also mention that the energetic action of the spiral 

 conductors enables us to imitate, in a very striking manner, the 

 inductive operation of the magneto-electrical machine, by means 

 of an uninterrupted galvanic current. For this purpose, it is only 

 necessary to arrange two coils to represent the two poles of a 

 horse-shoe 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 he- 

 lix No. 4, which is connected with a subsequent part of the in- 

 vestigation. 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 experi- 

 ments, 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 ob- 

 tained. 



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

 but this was not sufficient to prevent the magnetizing of a bar of 

 iron introduced into the opening at the centre. The effect ap- 

 peared to be confined to the inductive action. The same acci- 

 dent had before happened to another coil of nearly the same kind. 

 It was therefore noted as one of some importance. An explana- 

 tion was afterwards found in a peculiar accident of the secondary 

 current.* 



SECTION IV. 



On the Effects produced by interposing diff'erent Substances be- 

 tween the Conductors. 



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

 discharge, that the effect took place through interposed plates of 

 all substances, conductors and non-conductors.f The experi- 

 ment which I have given in paragraph 51 would appear to indi- 

 cate that the inductive action which produces the secondary cur- 

 rent might also follow the same law. 



* See paragraph 75. t Philosophical Transactions, 1821. 



