it is termed specific inductive capacity.  To be more accurate the specific inductive capacity of a dielectric is the ratio between the capacity of a condenser having that substance as a dielectric, to the capacity of the same condenser using dry air at zero degrees Centigrade and at a pressure of 14.7 pounds per square inch as the dielectric.  To illustrate, if two condensers having plates of equal size and equal distance apart are constructed, one using air as the dielectric and the other using hard crown glass as the dielectric, the one using glass will have a capacity of 6.96 times that of the one using air.  From this we say that crown glass has a specific inductive capacity of 6.96.

Various authorities differ rather widely as to the specific inductive capacity of many common substances.  The values given in Table VIII have been chosen from the Smithsonian Physical Tables.

TABLE VIII

Specific Inductive Capacities

+-----------------------+------------------------+
|DIELECTRIC             |  REFERRED TO AIR AS 1  |
+-----------------------+------------------------+
|Vacuum                 |   .99941               |
|Hydrogen               |   .99967               |
|Carbonic Acid          |  1.00036               |
|Dry Paper              |  1.25 to 1.75          |
|Paraffin               |  1.95 to 2.32          |
|Ebonite                |  1.9  to 3.48          |
|Sulphur                |  2.24 to 3.90          |
|Shellac                |  2.95 to 3.73          |
|Gutta-percha           |  3.3  to 4.9           |
|Plate Glass            |  3.31 to 7.5           |
|Porcelain              |  4.38                  |
|Mica                   |  4.6  to 8.0           |
|Glass--Light Flint     |  6.61                  |
|Glass--Hard Crown      |  6.96                  |
|Selenium               | 10.2                   |
+-----------------------+------------------------+

This data is interesting as showing the wide divergence in specific inductive capacities of various materials, and also showing the wide divergence in different observations of the same material.  Undoubtedly, this latter is due mainly to the fact that various materials differ largely in themselves, as in the case of paraffin, for instance, which exhibits widely different specific inductive capacities according to the difference in rapidity with which it is cooled in changing from a liquid to a solid state.

We see then that the capacity of a condenser varies as the area of its plates, as the specific inductive capacity of the dielectric employed, and also inversely as the distance between the plates.

Obviously, therefore, in making a condenser of large capacity, it is important to have as large an area of the plate as possible; to have them as close together as possible; to have the dielectric a good insulating medium so that there will be practically no leakage between the plates; and to have the dielectric of as high a specific inductive capacity as economy and suitability of material in other respects will permit.