to Electricity and Magjietism. 257 



Section IV. — On the Effects prodticed by interposing different 

 Substances between the Conductors. 



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

 trical discharge, that the effect took place through interposed 

 plates of all substances, conductors and nonconductors *. 

 The experiment which I have given in paragraph 5 1 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 



Fiff. 5. 



a represents coil No. 1, b helix No. 1, and c an 

 interposed plate of metal. 



Jyth of an inch thick, interposed. With this arrangemeut 

 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 withdrawn, they were very intense. 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- 

 culiar to the iron, on account of its temporary magnetism; 

 but this idea was shown to be erroneous by substituting a 

 plate of zinc of about the same size and thickness. With this 

 the screening influence was exhibited as before. 



59. After this a variety of substances was interposed in 

 succession, namely, copper, lead, mercury, acid, water, wood, 

 glass, &c. : and it was found that all the perfect conductors, 

 such as the metals, produced the screening influence; but 

 nonconductors, as glass, wood, &c. appeared to have no effect 

 whatever. 



60. When the helix was separated from the coil by a di- 

 stance only equal to the thickness of the plate, a slight sen- 

 sation could be perceived even when the zinc of y^th of an 

 inch in thickness was interposed. This effect was increased 

 by increasing the quantity of the battery current. If the 

 thickness of the plate was diminished, the induction through 



* Philosophical Transactions, 18,21. [or Phil. Mag., First Series, vol. 

 Iviiii.] 



Phil, Mag, S. 3. Vol. 16. No. 103. April 1840. S 



