general form of
the typical machines, but I fear that want of time will prevent me
from explaining each of them.

In 1762 Wilcke described a simple apparatus which produced electrical
charges by influence, or induction, and following this the great
Italian scientist Alexander Volta in 1775 gave the electrophorus the
form which it retains to the present day. This apparatus may be viewed
as containing the germ of the principle of all influence machines yet
constructed.

Another step in the development was the invention of the doubler by
Bennet in 1786. He constructed metal plates which were thickly
varnished, and were supported by insulating handles, and which were
manipulated so as to increase a small initial charge. It may be better
for me to here explain the process of building up an increased charge
by electrical influence, for the same principle holds in all of the
many forms of influence machines.

This Volta electrophorus, and these three blackboards, will serve for
the purpose. I first excite the electrophorus in the usual manner, and
you see that it then influences a charge in its top plate; the charge
in the resinous compound is known as negative, while the charge
induced in its top plate is known as positive. I now show you by this
electroscope that these charges are unlike in character. Both charges
are, however, small, and Bennet used the following system to increase
them.

Let these three boards represent Bennet's three plates. To plate No. 1
he imparted a positive charge, and with it he induced a negative
charge in plate No. 2. Then with plate No. 2 he induced a positive
charge in plate No. 3. He then placed the plates Nos. 1 and 3
together, by which combination he had two positive charges within
practically the same space, and with these two charges he induced a
double charge in plate No. 2. This process was continued until the
desired degree of increase was obtained. I will not go through the
process of actually building up a charge by such means, for it would
take more time than I can spare.

In 1787 Carvallo discovered the very important fact that metal plates
when insulated always acquire slight charges of electricity; following
up those two important discoveries of Bennet and Carvallo, Nicholson
in 1788 constructed an apparatus having two disks of metal insulated
and fixed in the same plane. Then by means of a spindle and handle, a
third disk, also insulated, was made to revolve near to the tw