On Ellectro-Dynamic Induction. 223 
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 
sth of an inch thick, ster poagek With this arrangement no 
shocks could be obtained; although, when the plate was with- 
drawn, they were very ‘ptenee, 
Fig. 5. 
a coil No. 1, d helixiNo. 1 , and ¢ an interposed plate of metal. 
58. It was at first thought that this effect might be peculiar to 
the iron, on account of its temporary magnetism; but this idea 
was shown to be erroneous by substituting a plate of zine of about 
the same size and thickness. With this the screening influence 
was exhibited as before. 
59. After this various other substances were interposed in suc- 
cession, namely, copper, lead, mercury, acid, water, wood, glass, 
&e. ; and it was found that all the perfect cmickcat, such as the 
metals, produced the screening influence ; but non-condutctors, 
as glass, wood, &c., appeared to have no effect whatever. 
60. When the helix was separated from the coil by a distance. 
Only equal to the thickness of the plate, a slight sensation could 
be perceived even when the zinc of ,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 it became more intense. Thus 
@ sheet of infoil interposed produced no perceptible influence ; 
four sheets of the same were attended with the same result. 
A certain thickness of metal is therefore required to produce the 
Screening effect, and this thickness depends on the quantity of 
the current from the battery. 
61. The idea occurred to me that the screening might, in some 
way, be connected with an instantaneous current in the plate, 
Similar to that in the induction by magnetic rotation, discovered 
