It was the question as to the possibility of producing the
electric charge by mere metallic contact which led Cavallo to
make his experiments upon contact electrification. Thus Cavallo
says in Volume III. of "A Complete Treatise on Electricity,"
published in 1795:

'The above mentioned singular properties, together with some
other facts, which will be mentioned in the sequel induced Mr.
Volta, to suspect that possibly in many cases the motions are
occasioned by a small quantity of electricity produced by the
mere contact of two different metals; though he acknowledges
that he by no means comprehends in what manner this can happen.
This suspicion being entertained by so eminent a philosopher as
Mr. Volta, induced Dr. Lind and myself to attempt some
experiment which might verify it; and with this in view we
connected together a variety of metallic substances in diverse
quantities, and that by means of insulated or not insulated
communications; we used Mr Volta's condenser, and likewise a
condenser of a new sort; the electrometer employed was of the
most sensible sort; and various other contrivances were used,
which it will be needless to describe in this place; but we
could never obtain the smallest appearance of electricity from
those metallic combinations. Yet we can infer to no other
conclusion, but that if the mere combination, or contact, of
the two metals produces any electricity, the quantity of it in
our experiments was too small to he manifested by our
instruments.'

Later, on page 111 of the same volume, he says: