team-engine, any ordinary
intelligence soon grasps the principles which govern the use of steam in
cylinders or turbines. The diagrams show, it is hoped, quite plainly
"how it works." But electricity is elusive, invisible; and the greatest
authorities cannot say what goes on at the poles of a magnet or on the
surface of an electrified body. Even the existence of "negative" and
"positive" electricity is problematical. However, we see the effects,
and we know that if one thing is done another thing happens; so that we
are at least able to use terms which, while convenient, are not at
present controverted by scientific progress.

FORMS OF ELECTRICITY.

Rub a vulcanite rod and hold one end near some tiny pieces of paper.
They fly to it, stick to it for a time, and then fall off. The rod was
electrified--that is, its surface was affected in such a way as to be in
a state of molecular strain which the contact of the paper fragments
alleviated. By rubbing large surfaces and collecting the electricity in
suitable receivers the strain can be made to relieve itself in the form
of a violent discharge accompanied by a bright flash. This form of
electricity is known as _static_.

Next, place a copper plate and a zinc plate into a jar full of diluted
sulphuric acid. If a wire be attached to them a current of electricity
is said to _flow_ along the wire. We must not, however, imagine that
anything actually moves along inside the wire, as water, steam, or air,
passes through a pipe. Professor Trowbridge says,[11] "No other agency
for transmitting power can be stopped by such slight obstacles as
electricity. A thin sheet of paper placed across a tube conveying
compressed air would be instantly ruptured. It would take a wall of
steel at least an inch thick to stand the pressure of steam which is
driving a 10,000 horse-power engine. A thin layer of dirt beneath the
wheels of an electric car can prevent the current which propels the car
from passing to the rail, and then back to the power-house." There
would, indeed, be a puncture of the paper if the current had a
sufficient voltage, or pressure; yet the fact remains that _current_
electricity can be very easily confined to its conductor by means of
some insulating or nonconducting envelope.

MAGNETISM.

The most familiar form of electricity is that known as magnetism. When a
bar of steel or iron is magnetized, it is supposed that the molecules in
it turn and arrange themselves w