68. Specific Inductive Capacity. Faraday found that the charge in a condenser between whose surfaces a constant difference of potential was maintained depended upon the nature of the dielectric between the surfaces, the charge being greater when the interval between the surfaces was filled with glass or sulphur than when it was filled with air.

Thus the ‘capacity’ of a condenser (see Art. 51) depends upon the dielectric between the plates. Faraday's original experiment by which this result was established was as follows: he took two equal and similar condensers, A and B, of the kind shown in Fig. 39, made of concentric spheres; in one of these, B, there was an opening by which melted wax or sulphur could be run into the interval between the spheres. The insides of these condensers were connected together, as were also the outsides, so that the potential difference between the plates of the condenser was the same for A as for B. When air was the dielectric between the spheres Faraday found, as might have been expected from the equality of the condensers, that any charge given to the condensers was equally distributed between A and B. When however the interval in B was filled with sulphur and the condensers again charged he found that the charge in B was three or four times that in A, proving that the capacity of B had been increased three or four times by the substitution of sulphur for air.