A sensitive balance for stability tests on permanent magnets

In order to be able to carry out long-term stability tests on permanent magnets of the type used in moving-coil instruments a special balance has been developed in which the electromagnetic force on a conductor in the air-gap of the magnet is weighed against the force of gravity on a fixed mass. An optical system is used to detect out-of-balance. Various errors which had to be eliminated are discussed, and typical test results are given which indicate an overall maximum error of ± 0.03%.