Page:Electricity (1912) Kapp.djvu/31

Rh generally let us call the fraction $$\frac{1}{K}$$. The force will now be expressed by the formula

$$F = \frac{1}{K} \frac{Qq}{D^2}$$

$$K$$ being a number depending on the medium in which the spheres are suspended. This numeric indicates the degree of attenuation of the force brought about by the presence of an insulating body between the spheres. This body, which separates the two electrified bodies, is called the dielectric. To bring back the force to its old value we must increase the charges. By using a dielectric we have enabled the spheres to hold a greater charge without exerting on each other a greater force. We have increased their capacity for storing a charge, and for this reason $$K$$ is called the specific inductive capacity of the medium, or also the dielectric constant of the medium. The value of $$K$$ is about $$2$$ for oil, $$2$$ to $$3$$ for paper, $$6$$ for mica, and may go up to as much as $$10$$ for glass. The larger $$K$$, the greater is the charge with a given force pushing the electricity on to the conductor. This force must, however, not be confounded with the mechanical force of attraction or repulsion