to 'Electricity and Magnetism, 257 
Section IV . — On the Effects produced by interposing different 
Substance 's 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 
Fig. 5. 
y'yth 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withdrawn, they were very intense. 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, 1821. [or Phil. Mag., First Series, vol. 
Iviiii.] 
Fhil, Mag. S. 3. Vol. 16. No. 103. April 1840. S 
