137 
were of the same metal and kept at a difference of potentials 
equal to about three-quarters of that produced by a single 
cell of Daniell’s. Hence, and from my measurement of the 
electrostatic effects of a Baniell’s battery, published in the 
Proceedings of the Royal Society, for February and April, 
1860, I find that plates of zinc and copper held parallel to 
one another at any distance, D, apart which is a small 
fraction of the linear dimensions of their opposed surfaces, 
and kept in metallic communication with one another, 
exercise a mutual attraction equal to 
A 
2 x 10 -10 x jy 2 grammes weight. 
Hence if they were allowed to approach from any greater 
distance, I)', to the distance D, the work done by their 
mutual attraction is 
2 x 10- 10 x 
A(D' — D) 
D'D 
centimetre grammes ; 
which, if D is very small in comparison with D', is very 
approximately equal to 
2 x 10- 10 x ~ 
Now suppose a pile to be made of a great number (N + l) 
of very thin plates alternately of zinc and copper, kept in 
metallic connection while they are brought towards one an- 
other. Let their positions in the pile be parallel, with 
narrow spaces intervening. For simplicity let the thick- 
ness of each metal plate and intervening space be D. The 
whole work done will be 
2 x 10- 10 x Ng- 
The whole mass of the pile (if we neglect that of one of the 
end plates) is 
NAD P , 
where p denotes the mean of the densities of zinc and cop- 
per. Hence, if h be the height to which the whole mass 
must be raised against a constant force equal to its weight 
