3. The third principle to be borne in mind is that to do work of any
kind, whether mechanical or electrical, requires the expenditure of
energy to a certain amount. The steam engine cannot work without its
coal, nor the laborer without his food; nor will a flame go on burning
without its fuel of some kind or other. Neither can an electric current
go on flowing, nor an electric light keep on shedding forth its beams,
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.
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