The specific inductive capacity of air, which is nearly the same as that of a vacuum, is taken as unity. In terms of this unit the following are some typical values of the dielectric constant: water 80, glass 6 to 10, mica 6.7, gutta-percha 3, India rubber 2.5, paraffin wax 2, ebonite 2.5, castor oil 4.8.
In underground cables for very high pressures, the insulation, if homogeneous throughout, would have to be of very great thickness in order to have sufficient dielectric strength. By employing material of high specific inductive capacity close to the conductor, and material of lower specific inductive capacity toward the outside, that is, by grading the insulation, a considerably less total thickness affords equally high dielectric strength.
Fig. 1,283.—Parallel connection of condensers. Like terminals are joined together. The joint capacity of such arrangement is equal to the sum of the respective capacities, that is C = c + c' + c".
Ques. How are capacity tests usually made?
Ans. By the aid of standard condensers.
Ques. How are condensers connected?
Ans. They may be connected in parallel as in fig. 1,283, or in series (cascade) as in fig. 1,284.
Condensers are now constructed so that the two methods of arranging the plates may conveniently be combined in one condenser, thereby obtaining a wider range of capacity.