DC current standards
Active basic electrical standards

Like the volt, the ampere is now realized in the SI indirectly, through the joint realization of the watt and the ohm. The ampere can also be realized directly from current scales, of which the watt scale is a modern emanation. They are based on the principle of the electrodynamometer. Consider two concentric solenoidal coils of axis z, one fixed, the other movable along the axis and suspended from the arm of a balance, and let I _1...