A Determination of the Dielectric Constant of Air by a Discharge Method

Abstract

Two air condenser systems, one containing the test condenser and the other the balancing condenser, are charged to equal opposite potentials, the opposite charges are mixed and discharged through a galvanometer. The two condenser systems are adjusted until the galvanometer deflection is zero. A special form of rotating commutator was devised for which the contact resistances are small and uniform. This commutator has three pairs of make and break contacts, two for charging and discharging the two condensers, and one pair connected so that a single battery is used to charge both condensers. The capacity of the test condenser is obtained in terms of readings on a condenser which forms part of the balancing condenser system. The ratio of the capacities of the test condenser, with a vacuum and with air for dielectric is then obtained. The calibration for the readings is described. This calibration is made with the apparatus in place, by simple changes of connections. Possible errors from time lag, thermal expansions, and deformations from pressure changes are discussed. The average of thirteen separate measurements gives 1.000594 for the dielectric constant of air at 0°C and 760 mm Hg pressure. The thirteen separate readings agree in the second significant figure of the decimal part of the result.