Specific Inductive Capacity

The ratio of the capacity of a condenser when its plates are separated by any substance to the capacity of the same condenser when its plates are separated by air.

A static accumulator consists of two conducting surfaces separated by an insulator. It is found that the capacity of an accumulator for an electric charge, which varies with or may be rated by the potential difference to which its conductors will be brought by the given charge, varies with the nature of the interposed dielectric, and is proportional to a constant special to each substance. This constant is the specific inductive capacity of the dielectric.

The same condenser will have a higher capacity as the dielectric is thinner, other things being equal. But different dielectrics having different specific inductive capacities, the constant may be determined by ascertaining the relative thicknesses of layers having the same total inductive capacity. The thicker the layer, the higher is its specific inductive capacity.

Thus it is found that 3.2 units thickness of sulphur have the same total inductive capacity as 1 unit thickness of air. In other words, if sulphur is interposed between two conducting plates, they may be separated to over three times the distance that would be requisite to retain the same capacity in air. Hence, sulphur is the better dielectric, and air being taken as unity, the specific inductive capacity of sulphur is 3.2.

The specific inductive capacity of a dielectric varies with the time and temperature. That of glass rises 2.5 per cent. between 12° C.(53.6° F.) and 83° C. (181.4° F.). If a condenser is dischargeddisruptively, it retains a small residual charge which it can part with later. If a metallic connection is made between the plates, the discharge is not instantaneous. Vibration shaking and jarring facilitate the complete discharge. All this shows that the charge is a phase of the dielectric itself, and indicates a strained state into which it is brought.

The following table gives the specific inductive capacity of various substances:

                        Specific Inductive Capacity.
Substance                                        Specific
                                                                   Inductive   Authority
                                                 Capacity.
Vacuum, air at about 0.001 millimeters pressure  0.94about   Ayrton
Vacuum, air at about 5 millimeters               0.9985      Ayrton
                                                 0.99941     Boltzmann
Hydrogen at about 760 millimeters pressure       0.9997      Boltzmann
                                                 0.9998      Ayrton
Air at about 760 millimeters pressure            1.0         Taken as the 
                                                             standard
Carbon Dioxide at about 760 millimeters pressure 1.000356    Boltzmann
                                                 1.0008      Ayrton
Olefiant Gas at about 760millimeters pressure    1.000722    Boltzmann
Sulphur Dioxide at about 760millimeters pressure 1.0037      Ayrton
Paraffin Wax,Clear                               1.92        Schiller
                                                 1.96        Wüllner
                                                 1.977   Gibson and Barclay
                                                 2.32        Boltzmann
Paraffin Wax,Milky                               2.47        Schiller
India Rubber,Pure                                2.34        Schiller
India Rubber,Vulcanized                          2.94        Schiller
Resin                                            2.55        Boltzmann
Ebonite                                          2.56        Wüllner
                                                 2.76        Schiller
                                                 3.15        Boltzmann
Sulphur                                          2.88 to 3.21 Wüllner
                                                 3.84        Boltzmann
Shellac                                          2.95 to 3.73 Wüllner
Guttapercha                                      4.2  
Mica                                             5
Flint Glass, Verylight                           6.57        J. Hopkinson
Flint Glass,Light                                6.85        J. Hopkinson
Flint Glass,Dense                                7.4         J. Hopkinson
Flint Glass, Double extradense                   10.1         J. Hopkinson