_The same Subject continued._
Three curious and important instruments, which are among the latest
improvements in electricity, deserve a particular explanation, and in
all of them the effect depends upon the general principles mentioned
above, viz. that bodies placed within the influence, or, as it is
usually termed, within the atmosphere, of an electrified body, are
affected by a contrary electricity, and that these two electricities
mutually attract each other. These instruments are the _electrophorus_,
the _condenser_ of electricity, and the _doubler_ of it.
The electrophorus consists of an insulated conducting plate applied to
an insulated electric. If the latter have any electricity communicated
to it, for example the negative, the positive electricity of the former
will be attracted by it, and consequently the plate will be capable of
receiving electricity from any body communicating with the earth; being,
in this situation, capable of containing more electricity than its
natural quantity. Consequently, when it is removed from the lower plate,
and the whole of its electricity equally diffused through it, it will
appear to have a redundance, and therefore will give a spark to any body
communicating with the earth. Being then replaced upon the electric, and
touched by any body communicating with the earth, it will be again
affected as before, and give a spark on being raised; and this process
may be continued at pleasure, the electrophorus supplying the place of
any other electrical machine.
If the conducting plate of the electrophorus be applied to a piece of
dry wood, marble, or any other substance through which electricity can
pass but very slowly, or if the insulated conducting plate be covered
with a piece of thin silk, which will make some resistance to the
passage of electricity, and it be then applied to another plate
communicating with the earth; and if, in either of these cases, a body
with a large surface possessed of a weak electricity be applied to the
conducting plate, the weak electricity not being able to overcome the
obstruction presented to it, so as to be communicated to the other
plate, will affect it with the contrary electricity, and this reacting
on the first plate, will condense its electricity on that part of the
plate to which it is contiguous; in consequence of which its capacity of
receiving electricity will be increased; so that on the separation of
the two plates, that electricity which was before condensed, being
equally diffused through the whole plate, will have a greater intensity
than it had before, attracting light bodies, or even giving a spark,
when the body from which it received its electricity was incapable of
it. For though it contained a great quantity of electricity, it was
diffused through so large a space that its intensity was very small.
This instrument is therefore called a _condenser of electricity_.
If an insulated plate of metal possessing the smallest degree of
electricity be presented very near to another plate communicating with
the earth, it will affect this plate with the opposite electricity; and
this being, in the same manner, applied to a third plate, will put it
into the same state with the first. If then these two plates be joined,
and the first plate be presented to either of them, its own electricity
being attracted by that of the plate presented, that of the other will
be drawn into it, so that its quantity will be doubled. The same process
being repeated, will again double the electricity of this plate, till,
from being quite insensible to the most exquisite electrometer, it will
become very conspicuous, or even give sparks. This instrument is
therefore called a _doubler of electricity_, of excellent use in
ascertaining the quality of atmospherical electricity when ever so
small. If this instrument be so constructed that these three plates can
be successively presented to one another by the revolution of one of
them on an axis, it is called the _revolving doubler_; and in this form
it is most convenient for use.