[Illustration:  Fig. 119.  Condenser Plate]

Theory.  Due to the proximity of the plates, the positive electricity on plate A is drawn by the negative charge on plate B towards plate B, and likewise the negative electricity on plate B is drawn to the side towards plate A by the positive charge on that plate.  These two charges so drawn towards each other will, so to speak, bind each other, and they are referred to as bound charges.  The charge on the right-hand side of plate A and on the left-hand side of plate B will, however, be free charges, since there is nothing to attract them, and these are, therefore, neutralized by a further flow of electricity from the battery to the plate.

[Illustration:  Fig. 120.  Theory of Condenser]

Obviously, the closer together the plates are the stronger will be the attractive influence of the two charges on each other.  From this it follows that in the case of plate A, when the two plates are being moved closer together, more positive electricity will flow into plate A to neutralize the increasing free negative charges on the right-hand side of the plate.  As the plates are moved closer together still, a new distribution of charges will take place, resulting in more positive electricity flowing into plate A and more negative electricity flowing into plate B.  The closer proximity of the plates, therefore, increases the capacity of the plates for holding charges, due to the increased inductive action across the dielectric separating the plates.

Condenser Defined.  A condenser is a device consisting of two adjacent plates of conducting material, separated by an insulating material, called a dielectric.  The purpose is to increase by the proximity of the plates, each to the other, the amount of electricity which each plate will receive and hold when subjected to a given potential.

Dielectric.  We have already seen that the capacity of a condenser depends upon the area of its plates, and also upon their distance apart.  There is still another factor on which the capacity of a condenser depends, i.e., on the character of the insulating medium separating its plates.  The inductive action which takes place between a charged conductor and other 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,