A Current can be measured absolutely in the electro-magnetic system of units either by means of the action of the current on a magnet, or of the current on a current. The former method has the disadvantage that at least two independent measurements are necessary. For example, in using an electro-magnetic balance, the strength of the magnet acted on by the electric circuit has to be determined, as well as the force exerted on the magnet by the circuit. In galvanometers, either of the sine or tangent type, the magnetic field produced by the electric circuit is compared with the earth’s horizontal field, the strength of which is determined independently. Further, as the strength of artificial magnets cannot be regarded as truly constant, and the earth’s field is subject to diurnal and secular variations, this class of measurement is not ideal. In the electrodynamic class of measurement the mutual action between two or more coils carrying current takes the form of a torque, as in electrodynamometers, or a direct force, as in current weighers. In electrodynamometers the torque may be measured with a bifilar suspension, the torsion of a wire or spring, or by means of a gravity balance. Current weigher measurements are almost always made by direct comparison with gravity, which is believed to be constant, and is known to a higher degree of accuracy than the strengths of any magnet or magnetic field that has yet been measured.
XII. A new current weigher and a determination of the electromotive force of the normal Weston cadmium cell
