what the mass of a gram is and we can reproduce it at any time. A cubic cm. of water at four degrees C. has the mass of one gram. Having thus settled the magnitude of the mass unit we determined the numerical value of the ethereal coefficient. In the electrical case we settle arbitrarily the value of the ethereal coefficient as unity, and determine on this basis the magnitude of unit electric quantity. In our experiment the spheres are at rest, there is no flow of electricity, and the system is in static equilibrium.
The unit of charge thus denned is therefore called the electrostatic unit of electric quantity in the c.g.s. system. In our experiment the room was filled with air. Let us now fill the room with oil. Since oil is an excellent insulator the spheres will retain their charges, but we shall observe a diminution of the force. The charge on each sphere has not altered, but the force acting between them has become smaller. We have settled the magnitude of unit quantity in such way that the coefficient in air shall be unity, but after filling the room with oil we find that this coefficient is only say . Whether it is exactly or some other fraction depends on the particular kind of oil used. To treat the matter quite
