_Seat of the Electric Charge._--So far we have spoken of electric charge as if it resided on the conductors which are electrified. The work of Benjamin Franklin, Henry Cavendish, Michael Faraday and J. Clerk Maxwell demonstrated, however, that all electric charge or electrification of conductors consists simply in the establishment of a physical state in the surrounding insulator or dielectric, which state is variously called _electric strain_, _electric displacement_ or _electric polarization_. Under the action of the same or identical electric forces the intensity of this state in various insulators is determined by a quality of them called their _dielectric constant_, _specific inductive capacity_ or _inductivity_. In the next place we must notice that electrification is a measurable magnitude and in electrostatics is estimated in terms of a unit called the _electrostatic unit_ of electric quantity. In the absolute C.G.S. system this unit quantity is defined as follows:--If we consider a very small electrified spherical conductor, experiment shows that it exerts a repulsive force upon another similar and similarly electrified body. Cavendish and C.A. Coulomb proved that this mechanical force varies inversely as the square of the distance between the centres of the spheres. The unit of mechanical force in the "centimetre, gramme, second" (C.G.S.) system of units is the _dyne_, which is approximately equal to 1/981 part of the weight of one gramme. A very small sphere is said then to possess a charge of one electrostatic unit of quantity, when it repels another similar and similarly electrified body with a force of one dyne, the centres being at a distance of one centimetre, provided that the spheres are _in vacuo_ or immersed in some insulator, the dielectric constant of which is taken as unity. If the two small conducting spheres are placed with centres at a distance d centimetres, and immersed in an insulator of dielectric constant K, and carry charges of Q and Q' electrostatic units respectively, measured as above described, then the mechanical force between them is equal to QQ'/Kd² dynes. For constant charges and distances the mechanical force is inversely as the dielectric constant.