122 WRITINGS OF JOSEPH HENRY. [1838 



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 in- 

 stead 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 introduced into the opening at the centre. 

 The effect appeared to be confined to the inductive action. 

 The same accident had before happened 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 be- 

 tween the Conductors. 



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

 electrical discharge, that the effect took place through in- 

 terposed plates of all substances, conductors and non-con- 

 ductors.* The experiment which I have given in para- 

 graph 51 would appear to indicate that the inductive action 

 which produces the secondary current might also follow the 

 same law. 



57. To test this the compound helix was placed about five 

 inches above coil No. 1, Fig. 5, and a plate of sheet iron, 

 about ^th of an inch thick, interposed. With this arrange- 

 ment no shocks could be obtained ; although, when the 

 plate was withdrawn, they were very intense. 



58. It was at first thought that this effect might be pe- 



* Philosophical Transactions, 1821. 



