The two pistons, through the elastic force of the gas, repel
each other with a force which, according to the law of Mariotte,
varies in inverse ratio to the distance. The method favoured by Ampere
would first of all allow this law of repulsion between the two pistons
to be discovered, even if the existence of a gas enclosed in the
barrel of the pump were unsuspected; and it would then be natural to
localize the potential energy of the system on the surface of the two
pistons. But if the phenomenon is more carefully examined, we shall
discover the presence of the air, and we shall understand that every
part of the volume of this air could, if it were drawn off into a
recipient of equal volume, carry away with it a fraction of the energy
of the system, and that consequently this energy belongs really to the
air and not to the pistons, which are there solely for the purpose of
enabling this energy to manifest its existence.
Faraday made, in some sort, an equivalent discovery when he perceived
that the electrical energy belongs, not to the coatings of the
condenser, but to the dielectric which separates them.
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