In no other sense has the convertibility of force, at present,
any scientific meaning.

By the contraction of a muscle a man lifts a weight from the earth.
But the muscle can contract only through the oxidation of its own
tissue or of the blood passing through it. Molecular motion is thus
converted into mechanical motion. Supposing the muscle to contract
without raising the weight, oxidation would also occur, but the
whole of the heat produced by this oxidation would be liberated in
the muscle itself. Not so when it performs external work; to do
that work a certain definite portion of the heat of oxidation must
be expended. It is so expended in pulling the weight away from the
earth. If the weight be permitted to fall, the heat generated by
its collision with the earth would exactly make up for that lacking
in the muscle during the lifting of the weight. In the case here
supposed, we have a conversion of molecular muscular action into
potential energy of gravity; and a conversion of that potential
energy into heat; the heat, however, appearing at a distance from
its real origin in the muscle. The whole process consists of a
transference of molecular motion from the muscle to the weight,
and gravitating force is the mere go-between, by means of which the
transference is effected.

These considerations will help to clear our way to the conception of
the transformations which occur when a wire is moved across the
lines of force in a magnetic field. In this case it is commonly
said we have a conversion of magnetism into electricity. But let us
endeavour to understand what really occurs. For the sake of
simplicity, and with a view to its translation into a different one
subsequently, let us adopt for a moment the provisional conception