hand to the hot body?

Logically these expressions are identical; still we have come to
prefer one of them. It is because we have learned that in those bodies
which our fathers called hot, the particles are vibrating with greater
energy than in cold bodies, that we prefer to say that heat is added
and not cold subtracted, when a cold body becomes less cold.

Now to come back to our electrified bodies. Let us suppose that this
gutta percha, and this cat's-skin are not electrified. That means that
their electrical condition is the same as that of surrounding bodies.
Let us also suppose that their thermal condition is the same as
surrounding bodies, ourselves included--that is, they are neither hot
nor cold. We express these conditions in other words by saying
that the bodies have the same electrical _potential_ and the same
temperature.

Temperature in heat is analogous to potential in electricity. As
soon as adjacent bodies are at different temperatures, we have
the phenomena which reveal to us the existence of heat. As soon as
adjacent bodies have different electrical potentials, we have the
phenomena which reveal the existence of electricity. As soon as
adjacent regions in the air are at different pressures, we have
phenomena which reveal the existence of air.

Bodies all tend to preserve the same temperature and also the same
electrical potential. Any disturbances in electrical equilibrium are
much more quickly obliterated than in case of thermal equilibrium,
and we therefore see less of electrical phenomena than of thermal. In
thunder storms we see such disturbances, and with delicate instruments
we find them going on continuously. Changes in temperature occurring
on a large scale in our atmosphere, occurring in these gas jets,
in our fires, in the axles of machinery, and in thousands of other
places, are so familiar that we have ceased to wonder at them.

If we rub these two bodies together, the potential of the two is no
longer the same. We do not know which one has become greater, and in
this respect our knowledge of electricity is less complete than of
heat. We assume that the gutta percha has become negative. If we now
leave these bodies in contact, the potential of the cat's skin will
diminish and that of the gutta percha will increase until they have
again reached a common potential--that of the earth. As in the case of
heat and cold, we may say either that this has come about by a flow of
positive elect