An absolute current-balance having a simple approximate theory
Lewis Fry Richardson, V Stanyon · Proceedings of the Physical Society · 1928
A simple form of current balance has been constructed which, when tested by experimental comparison with a standard ohm and Weston cell, measures currents with a probable error of about 1 part in 1000. The calculation of the force acting on the moving coil of the balance involves no pure mathematics beyond that of the Intermediate B.Sc. The coils are single layers so that they can in the future be made as precise helices. Actually we have had to aim at cheapness rather than at perfection, and so irregularities of shape leave the current uncertain by 5 parts in 1000. A second approximation, depending on a simple deduction from Laplace's equation, corrects the elementary theory by 1.4 in 1000 of current.