On Electro- 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 

 y\th of an inch thick, interposed. With this arrangement no 

 shocks could be obtained ; although, when the plate was with- 

 drawn, they were very intense. 



Fig. 5. 



a coil No. 1, h helix No. 1, and c 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 zinc 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, 

 &c. ; and it was found that all the perfect conductors, such as the 

 metals, produced the screening influence ; but non-conductors, 

 as glass, wood, &c., appeared to have no eflect 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 y\i\\ of an inch in thickness 

 was interposed. This eflject was increased by increasing the 

 quantity of the battery current. If the thickness of the plate v/as 

 diminished, the induction through it became more intense. Thus 

 a sheet of tinfoil interposed produced no perceptible influence ; 

 also 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 



