r conductors nearby it, as between plate
_A_ and plate _B_ of Fig. 120, is called _electrostatic induction_,
and it plays an important part in telephony. It is found that the
ability of a given charged conductor to induce charges on other
neighboring conductors varies largely with the insulating medium or
dielectric that separates them. This quality of a dielectric, by which
it enables inductive action to take place between two separated
conductors, is called _inductive capacity_. Usually this quality of
dielectrics is measured in terms of the same quality in dry air, this
being taken as unity. When so expressed, 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