546. One of the best means of storing energy is by setting a heavy body
in rapid motion. This has already been referred to in the case of the
cannon-ball. In order to render this method practically available for
the purposes of machinery, the heavy body we use is a fly-wheel, and
the rapid motion imparted to it is that of rotation about its axis. A
very large amount of energy can by this means be stored in a manageable
form.
547. We shall illustrate the principle by the apparatus of Fig. 72.
This represents an iron fly-wheel B: its diameter is 18", and its
weight is 26 lbs.; the fly is carried upon a shaft (A) of wrought iron
¾" in diameter. We shall store up a quantity of energy in this wheel,
by setting it in rapid motion, and then we shall see how we can recover
from it the energy we have imparted.
548. A rope is coiled around the shaft; by pulling this rope the wheel
is made to turn round: thus the rope is the medium by which my energy
shall be imparted to the wheel. To measure the operation accurately,
I attach the rope to the hook of the spring balance (Fig. 9); and by
taking the ring of the balance in my hand, I learn from the index the
amount of the force I am exerting. I find that when I walk backwards
as quickly as is convenient, pulling the rope all the time, the scale
shows a tension of about 50 lbs. To set the wheel rapidly in motion,
I pull about 20' of rope from the axle, so that I have imparted to
the wheel somewhere about 50 × 20 = 1,000 units of energy. The rope
is fastened to the shaft, so that, after it has been all unwound, the
wheel now rapidly rotating winds it in. By measuring the time in which
the wheel made a certain number of coils of the rope around the shaft,
I find that it makes about 600 revolutions per minute.
[Illustration: FIG. 72.]
549. Let us see how the stored-up energy can be exhibited. A piece of
pine 24" × 1" × 1" of which both ends are supported, requires a force
of 300 lbs. applied to its centre to produce fracture (See p. 190). I
arrange such a piece of pine near the wheel. As the shaft is winding in
the rope, a tremendous chuck would be given to anything which tried to
stop the motion. If I tied the end of the rope to the piece of pine,
the chuck would break the rope; therefore I have fastened one end of a
10' length of chain to the rope, and the other has been tied round the
middle of the wooden bar. The wheel first winds in the rope, then the
chain takes a few turns before it tightens, and crack goes the rod of
pine. The wheel had no choice; it must either stop or break the rod:
but nature forbids it to be stopped, unless by a great force, which
the rod was not strong enough to apply. Here I never exerted a force
greater than 50 lbs. in setting the wheel in motion. The wheel stored
up and modified my energy so as to produce a force of 300 lbs., which
had, however, only to be exerted over a very small distance.
550. But we may show the experiment in another way, which is that
represented in the figure (72). We see the chain is there attached to
two 56 lb. weights. The mode of proceeding is that already described.
The rope is first wound round the shaft, then by pulling the rope the
wheel is made to revolve; the wheel then begins to wind in the rope
again, and when the chain tightens the two 56 lbs. are raised up to
a height of 3 or 4 feet. Here, again, the energy has been stored and
recovered. But though the fly-wheel will thus preserve energy, it
does so at some cost: the store is continually being frittered away
by friction and the resistance of the air; in fact, the energy would
altogether disappear in a little time, and the wheel would come to
rest; it is therefore economical to make the wheel yield up what it has
received as soon as possible.
551. These principles are illustrated by the function of the fly-wheel
in a steam-engine. The pressure of the steam upon the piston varies
according to the different parts of the stroke: and the fly-wheel
obviates the inconvenience which would arise from such irregularity.
Its great inertia makes its velocity but little augmented by the
exuberant action of the piston when the pressure is greatest, while it
also sustains the motion when the piston is giving no assistance. The
fly-wheel is a vast reservoir into which the engine pours its energy,
sudden floods alternating with droughts; but these succeed each other
so rapidly, and the area of the reservoir is so vast, that its level
remains sensibly uniform, and the supplies sent out to the consumers
are regular and unvaried. The consumers of the energy stored in the
fly-wheel of an engine are the machines in the mill; they are supplied
by shafts which traverse the building, conveying, by their rotation,
the energy originally condensed within the coal from which combustion
has set it free.