ed:

A cell has reference to a single vessel. When two or more cells are
coupled together they form a _battery_.

_Resistance_ is opposition to the movement of the current. If it is
offered by the electrolyte, it is designated "Internal Resistance." If,
on the other hand, the opposition takes place, for instance, through the
wire, it is then called "External Resistance."

The electrolyte must be either acid, or alkaline, or saline, and the
electrodes must be of dissimilar metals, so the electrolyte will attack
one of them.

The current is measured in amperes, and the force with which it is
caused to flow is measured in volts. In practice the word "current" is
used to designate ampere flow; and electromotive force, or E. M. F., is
used instead of voltage.

ELECTRO-MAGNETIC ELECTRICITY.--The third method of generating
electricity is by electro-magnets. The value and use of induction will
now be seen, and you will be enabled to utilize the lesson concerning
magnetic action referred to in the previous chapter.

MAGNETIC RADIATION.--You will remember that every piece of metal which
is within the path of an electric current has a space all about its
surface from end to end which is electrified. This electrified field
extends out a certain distance from the metal, and is supposed to
maintain a movement around it. If, now, another piece of metal is
brought within range of this electric or magnetic zone and moved across
it, so as to cut through this field, a current will be generated
thereby, or rather added to the current already exerted, so that if we
start with a feeble current, it can be increased by rapidly "cutting the
lines of force," as it is called.

DIFFERENT KINDS OF DYNAMO.--While there are many kinds of dynamo, they
all, without exception, are constructed in accordance with this
principle. There are also many varieties of current. For instance, a
dynamo may be made to produce a high voltage and a low amperage; another
with high amperage and low voltage; another which gives a direct current
for lighting, heating, power, and electroplating; still another which
generates an alternating current for high tension power, or
transmission, arc-lighting, etc., all of which will be explained
hereafter.

In this place, however, a full description of a direct-current dynamo
will explain the principle involved in all dynamos--that to generate a
current of electricity makes it necessary for us to move a field of
force,