Electricity, by definition, is the movement of electrons within the molecules of a material. Although electrons can move freely within conductors, they cannot do so within insulators. When a direct current voltage is applied to an insulator, electrons do not separate from molecules, and are divided by an electrical charge (positive or negative) induced at both ends of the material.

In addition, ions themselves may move. Materials in which electrical charges appear at both ends are called dielectric substances. Dielectricity is measured by the relative dielectric constant, a value representing the ratio of the dielectric constant of the material in question and that of a vacuum. The dielectric constant of quartz is 3.8, while that of sapphire (main component: oxidized aluminum) is 9.4. The dielectric constant of barium titanate, a ferroelectric material, is as high as 4,000 to 5,000.

Relative Dielectric Constants