1838] WRITINGS OF JOSEPH HENRY. 123 



€uliar to the iron, on account of its temporary magnetism ; 

 but this idea was shown to be erroneous by substituting a 



Fig 5. — a represents coil No. 1, b helix No. 1, and c an interposed plate of 



metal. 



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 ef- 

 fect whatever. 



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

 tance only equal to the thickness of the plate, a slight sensa- 

 tion could be perceived even when the zinc of -^ of an inch 

 in thickness was interposed. This effect was increased by 

 increasing the quantity of the battery current. If the thick- 

 ness of the plate was diminished, the induction through it be- 

 came more intense. Thus a sheet of tinfoil interposed pro- 

 duced 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 Herschel., furnished me 

 with a simple method of determining this point. 



