other sources of the electricity. The one observation which
seemed to offer a definite suggestion as to the possible source
of the electrical charge was the fact that, in general two
different metals must be used to connect the muscle and nerve
before a discharge would take place from the one to the other.
This made Galvani's theory that the metals served merely as
conductors seem improbable. On the other hand, it was sometimes
possible to get the muscular contractions by using a single
bent wire or rod to connect the nerve and muscle, especially if
the two ends were of different degrees of polish, or if one end
was warmer than the other.

Volta was apparently the first to suggest that the electricity
which seemed to be generated in Galvani's experiments might
have its source in the contact of the two metals. Several
writers called attention to an apparent relation between
Galvani's experiments and a phenomenon announced by J. G.
Sulzer, in 1760. Sulzer found that if pieces of lead and silver
were placed upon the tongue separately no marked taste was
produced by either, but that if while both were on the tongue
the metals were brought into contact a strong taste was
produced which he compared to the taste of iron vitriol. Here
was a case of undoubted stimulation of the nerves of taste by
the contact of two metals, and it seemed not improbable that
other nerves might be stimulated in the same manner. In the
meantime Mr. John Robison had increased the Sulzer effect
greatly by building up a pile of pieces of zinc with silver
shillings and placing these in contact with the tongue and the
cheek.