IN VOLTAIC ELECTRICITY AND ELECTRO-MAGNETISM. 
291 
to them, were immersed in the extreme cells of a battery of thirty, four-inch 
plates ; metallic contact between the copper plates and the ends of the battery 
being carefully avoided. The connexion being made with the cups of the gal¬ 
vanometer, the deflecting force was found to be equal to 90 degrees of torsion 
of the glass thread. The copper plates were then immersed in the extreme 
cells of the same battery, containing one hundred and twenty pair of plates, 
when it was found that the degrees of torsion were somewhat less than 180. 
Hence the electro-magnetic effects of the two batteries were nearly as the 
square root of the number of plates. Hence the electro-magnetic effects of two 
batteries are within certain limits proportional to the quantities of water de¬ 
composed. 
18. These principles will enable us to account for a fact in electro-magnetism 
which has never been explained in a satisfactory manner. Since the discoveries 
in electro-magnetism, it was observed that no increase of electro-magnetic 
power is gained by increasing the number of plates in a battery, when the 
extreme plates were connected by metallic contact with the ends of the battery. 
This unexpected result has been accounted for by supposing that voltaic electri¬ 
city, having tension like common electricity, acts feebly on a magnetic needle. 
Not feeling satisfied with this vague explanation, I had again recourse to ex¬ 
periment. 
Exp. VII. Having soldered copper wires to several plates in a common gal¬ 
vanic trough, I connected a single pair with the galvanometer, and observed 
the deflecting force. By connecting the extreme plate and the others in suc¬ 
cession with the galvanometer, the effect, within certain limits, was observed 
to be nearly constant. When the number of plates was increased to forty or 
fifty, a slight diminution of power was observed. 
Since the deflecting force of a single pair is inversely as the square root of 
the distance between them, which is the whole length of the battery, and 
since the effects of all the other plates are directly as the square root of their 
number, it follows that within narrow limits the compound effect must be 
constant *. 
* The supposed analogy between common and voltaic electricity, which was so eagerly traced after 
the invention of the pile, completely fails in this case, which was thought to afford the most striking 
resemblance. 
