ntity of
electricities. 'Is it right,' he seemed to ask, 'to call this agency
which I have discovered electricity at all? Are there perfectly
conclusive grounds for believing that the electricity of the
machine, the pile, the gymnotus and torpedo, magneto-electricity and
thermo-electricity, are merely different manifestations of one and the
same agent?' To answer this question to his own satisfaction he formally
reviewed the knowledge of that day. He added to it new experiments
of his own, and finally decided in favour of the 'Identity of
Electricities.' His paper upon this subject was read before the Royal
Society on January 10 and 17, 1833.

After he had proved to his own satisfaction the identity of
electricities, he tried to compare them quantitatively together. The
terms quantity and intensity, which Faraday constantly used, need a word
of explanation here. He might charge a single Leyden jar by twenty turns
of his machine, or he might charge a battery of ten jars by the same
number of turns. The quantity in both cases would be sensibly the same,
but the intensity of the single jar would be the greatest, for here the
electricity would be less diffused. Faraday first satisfied himself that
the needle of his galvanometer was caused to swing through the same arc
by the same quantity of machine electricity, whether it was condensed
in a small battery or diffused over a large one. Thus the electricity
developed by thirty turns of his machine produced, under very variable
conditions of battery surface, the same deflection. Hence he inferred
the possibility of comparing, as regards quantity, electricities which
differ greatly from each other in intensity. His object now is to
compare frictional with voltaic electricity. Moistening bibulous paper
with the iodide of potassium--a favourite test of his--and subjecting
it to the action of machine electricity, he decomposed the iodide, and
formed a brown spot where the iodine was liberated. Then he immersed
two wires, one of zinc, the other of platinum, each 1/13th of an inch
in diameter, to a depth of 5/8ths of an inch in acidulated water during
eight beats of his watch, or 3/20ths of a second; and found that the
needle of his galvanometer swung through the same arc, and coloured
his moistened paper to the same extent, as thirty turns of his large
electrical machine. Twenty-eight turns of the machine produced an effect
distinctly less than that produced by his two wires. Now,