s. They are present in every magnet, and although we do not know
in what direction they move, yet in order to speak definitely about
them, it is agreed to assume that they pass out of every magnet at its
north-seeking pole (or the pole which would point to the magnetic north,
were the magnet free to move as a needle), and, after having traversed
the space surrounding the magnet, reenter at its south-seeking pole,
thus completing what is called the magnetic circuit. Any space traversed
by lines of magnetic force is called a magnetic field.

But it is not only a magnet that is thus surrounded by lines of
magnetic force, or by ether streamings. The same is true of any
conductor through which an electric current is flowing, and their
presence may be shown by means of iron filings. If an active
conductor--a conductor conveying an electric current, as, for example, a
copper wire--be passed vertically through a piece of card-board, or a
glass plate, iron filings dusted on the card or plate will arrange
themselves in concentric circles around the axis of the wire. It
requires an expenditure of energy both to set up and to maintain these
lines of force. It is the interaction of their lines of force that
causes the attractions and repulsions in active movable conductors.
These lines of magnetic force act on magnetic needles like other lines
of magnetic force and tend to set movable magnetic needles at right
angles to the conducting wire.

The setting up of an electric current in a conducting wire is,
therefore, equivalent to the setting up of concentric magnetic whirls
around the axis of the wire, and anything that can do this will produce
an electric current. For example, if an inactive conducting wire is
moved through a magnetic field; it will have concentric circular whirls
set up around it; or, in other words, it will have a current generated
in it as a result of such motion. But to set up these whirls it is not
enough that the conducting wire be moved along the lines of force in the
field. In such a case no whirls are produced around the conductor. The
conductor must be moved so as to cut or pass through the lines of
magnetic force. Just what the mechanism is by means of which the cutting
of the lines of force by the conductor produces the circular magnetic
whirls around it, no man knows any more than he knows just what
electricity is; but this much we do know,--that to produce the circular
whirls or currents in a previousl