Abstract
Professor R. L. Edwards' design of the ampere balance has been modified as follows. Our balance is made from 38-in. aluminum tubing, not bent to shape but cut and soldered. It has two pairs of parallel fixed conductors, 61 cm long and 3 cm apart. The moving conductor, rectangular in plan view, is gabled in vertical view, and made with a dihedral angle of 150° so that its center of gravity is near the axis of rotation. Steel balls rolling on horizontal glass plates are used as pivots. The mercury cups are apart from the pivots. The moving conductor bears a central Lucite cross rod perpendicular to the fixed conductors. This rod carries a small mirror and two threaded brass rods with adjustable bobs for horizontal and vertical balancing of the moving system. To provide the counterbalancing torque short pieces of fine copper wire or analytical weights are laid on Lucite plugs fitted into the open ends of the moving conductor. The damping device is an aluminum vane in a beaker of machine oil. A beam arrest is provided. With a telescope and scale at a distance of two meters the ampere balance has a weight sensitivity of 5 mg/cm and a current sensitivity of 2amp2/cm. An ammeter indicating a current of 5 amp can be calibrated with this balance to somewhat better than 1%.
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