ON ELECTRO-DYNAMIC INDUCTION. 317 



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 roth of an inch thick, 

 interposed. With this arrangement no 

 shocks could be obtained ; although, 

 when the plate was withdrawn, they 

 were very intense. 



58. It was at first thought that this 



a represents c^^^rrf^^ No. 1, and c ^ffect might bc pecuHar to the iron, on 



an interposed plate of metal. accouut of its temporary maguetism J 



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., ap- 

 peared 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 per- 

 ceived even when the zinc of Toth of an inch in thickness was inter- 

 posed. This effect was increased by increasing the quantity of the 

 battery current. If the thickness of the plate was diminished, the induc- 

 tion through it became more intense. Thus a sheet of tinfoil inter- 

 posed 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 by M. Arago. 

 The ingenious variation of this principle by Messrs Babbage and Her- 

 schell, furnished me with a simple method of determining this point. 



62. A circular plate of lead was interposed, which caused the induc- 

 tion in the helix almost entirely to disappear. A slip of the metal 



