C 187 3 
XVI. An account of an apparatus on a peculiar construction for 
performing electro-magnetic experiments. By W. H. Pepys, 
Esq. F. R. S. 
Read April 10, 1823. 
This apparatus £P1. XXII.] was made under my directions 
for the London Institution. It consists of two plates, each 
fifty feet in length, and two feet in width ; the one copper, 
and the other zinc, making a superficial surface of four hun- 
dred feet. They are rolled or wrapped round a cylinder of 
wood with three strands or ropes of horse hair between each 
plate, to prevent contact of the metals ; and to maintain 
these in their situation, notched sticks are occasionally intro- 
duced in the rolling. Two conductors of copper near three 
fourths of an inch in thickness are secured to the end of each 
plate, from which the power is dispensed upon immersion in 
the acid. 
To allow of the free use of so bulky an instrument, it is 
suspended by ropes and pullies, with a counterpoise weight, 
to allow its immersion in a tub of dilute acid, or when not in 
use, in one of water ; it requires about fifty-five gallons of 
fluid, and the strength of the solution used has been about 
one-fortieth of strong nitrous acid. 
Upon immersing the instrument in the dilute acid, and 
uniting the two conductors, magnetic needles on their stands 
were very sensibly affected for five feet from the conductors. 
Cylindrical bars of steel placed in the interior of a glass 
tube, surrounded by a spiral of wire, and forming part of 
