The known equivalence between the Ampère and Biot - Savart force laws, for closed circuits carrying an electric current, is here extended to the case of the force on a part of a circuit and due to the action of the other part of the same circuit. Our theorem invalidates some criticism made to the Biot - Savart law and the experimental results favouring Ampère's law. A recent experiment is in agreement with the here proved theorem.
