An apparatus with which force is measured between current-carrying conductors. The purpose of the measurement was to establish the value of the ampere in terms of its International System (SI) definition; therefore the relative locations of the conductors had to be measured so that the force between them could be computed from their geometry. The difficulty of this measurement limited the accuracy of even the best instruments to a few parts in a million. Therefore, current balances have now been superseded by other methods.