Permittivity, also called electric permittivity, is a constant of proportionality that exists between electric displacement and electric field intensity. This constant is equal to approximately 8.85 x 10^-12 farad per meter (F/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized ε_o. In engineering applications, permittivity is often expressed in relative, rather than in absolute, terms. If _o represents the permittivity of free space (that is, 8.85 x 10-¹² F/m) and ε represents the permittivity of the substance in question (also specified in farads per meter), then the relative permittivity, also called the dielectric constant εr, is given by: ε_r = ε / ε_o = ε (1.13 x 10¹¹). Various substances have dielectric constants εr greater than 1. These substances are generally called dielectric materials, or simply dielectrics. Commonly used dielectrics include glass, paper, mica, various ceramics, polyethylene, and certain metal oxides. Dielectrics are used in capacitors and transmission lines in alternating current (AC), audio frequency (AF), and radio frequency (RF) applications. Also See: Farad per meter, capacitor, and electric field