The transformation, in this case, is easily followed by the mind's
eye. First, the weight as a whole is set in motion by the attraction
of gravity. This motion of the mass is arrested by collision with
the earth; being broken up into molecular tremors, to which we give
the name of heat.
And when we reverse the process, and employ those tremors of heat to
raise a weight, as is done through the intermediation of an elastic
fluid in the steam-engine, a certain definite portion of the
molecular motion is destroyed in raising the weight. In this sense,
and this sense only, can the heat be said to be converted into
gravity, or more correctly, into potential energy of gravity. It is
not that the destruction of the heat has created any new attraction,
but simply that the old attraction has now a power conferred upon it,
of exerting a certain definite pull in the interval between the
starting-point of the falling weight and its collision with the earth.
So also as regards magnetic attraction: when a sphere of iron placed
at some distance from a magnet rushes towards the magnet, and has
its motion stopped by collision, an effect mechanically the same as
that produced by the attraction of gravity occurs. The magnetic
attraction generates the motion of the mass, and the stoppage of
that motion produces heat. In this sense, and in this sense only,
is there a transformation of magnetic work into heat. And if by the
mechanical action of heat, brought to bear by means of a suitable
machine, the sphere be torn from the magnet and again placed at a
distance, a power of exerting a pull through that distance, and
producing a new motion of the sphere, is thereby conferred upon the
magnet; in this sense, and in this sense only, is the heat converted
into magnetic potential energy.
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