The whole electrostatic theory has so far been based simply upon Coulomb's Law of the inverse square of the distance. We have supposed that one charge of electricity exerts certain forces upon a second distant charge, but nothing has been said as to the mechanism by which this action takes place. In handling this question there are two possibilities open. We may either assume “action at a distance” as an ultimate explanation–i.e. simply assert that two bodies act on one another across the intervening space, without attempting to go any further towards an explanation of how such action is brought about–or we may tentatively assume that some medium connects the one body with the other, and examine whether it is possible to ascribe properties to this medium, such that the observed action will be transmitted by the medium. Faraday and Maxwell followed the latter course. They refused to admit “action at a distance” as an ultimate explanation of electric phenomena, finding such action unthinkable unless transmitted by an intervening medium.

It is worth enquiring whether there is any valid à priori argument which compels us to resort to action through a medium. Some writers have attempted to use the phenomenon of Inductive Capacity to prove that the energy of a condenser must reside in the space between the charged plates, rather than on the plates themselves–for, they say, change the medium between the plates, keeping the plates in the same condition, and the energy is changed.