A type of instrument in which the electromagnetic forces resulting from the passage of a current through a set of coils are balanced against the force of gravity. In one form two coils B, E at each end of a balanced rod are suspended between four fixed coils, A, F and C, D (see diagram). When current flows through all six coils, i.e. in the order A, B, C, D, E, F, the resulting electromagnetic forces cause each fixed coil to displace the balanced arm in the same direction. The arm is rebalanced by moving a rider along a scale on the arm, calibrated to give a reading of the current. If the current is reversed the direction of the displacement is unaltered since the current flow is reversed in all the coils. Thus both direct and alternating currents can be measured.

Kelvin balance