without a constant supply of energy from some source or other.

[Illustration: Fig. 1.]

The last of these three principles, involving the relation of electric
currents to the work they can do and to the energy expended in their
production, will be treated of separately and later. Meantime we resume
the task of showing how such currents can be produced mechanically, and
how magnetism comes in in the process.

[Illustration: Fig. 2]

Surrounding every magnet there is a "field" or region in which the
magnetic forces act. Any small magnet, such for example as a compass
needle, when brought into this field of force, exhibits a tendency to
set itself in a certain direction. It turns so as to point with its
north pole toward the south pole of the magnet, and with its south pole
toward the north pole of the magnet; or if it cannot do both these
things at once, it takes up an intermediate position under the joint
action of the separate forces and sets in along a certain line. Such
lines of force run through the magnetic "field" from one pole of the
magnet to the other in curves. If we define a line of force as being the
line along which a free north-seeking magnetic pole would be urged, then
these lines will run from the north pole of the magnet round to the
south pole, and pass through the substance of the magnet itself. In Fig.
1 a rough sketch is given of the lines of magnetic force as they emerge
from the poles of a bar magnet in tufts. The arrow heads show the
direction in which a free north pole would move. These lines of forces
are no fiction of the imagination, like the lines of latitude and
longitude on the globe; they exist and can be rendered visible by the