DIELECTRIC. The insulating medium separating the con ductors in a condenser was called by Faraday the insulating dielectric. The term dielectric is now used to denote any insulator when it is regarded as a medium in which electromagnetic stresses may be set up. The dielectric constant for a substance is the ratio of the capacity of a condenser when that substance fills the space between the plates to the capacity of the same condenser when air is the dielectric. (See CONDENSER and ELECTRICITY.)