An absolute determination of the ampere
Paul Vigoureux · Philosophical Transactions of the Royal Society of London Series A Mathematical and Physical Sciences · 1936
Abstract An absolute determination of the ampere has been made with the current weigher erected by AYRTON, MATHER, and SMITH in 1905, modified as described below. A detailed description of this instrument was given at the time of its erection. It will be sufficient to recall that it is a physical balance with two similar coil systems, one on each side of the central post, fig. 1, Plate 2. Each system consists of two cylindrical coils, of which only the outer one, which has two similar windings separated by a narrow gap at the central diametral plane, can be seen in fig. 1. The small coil is suspended from the end knife-edge and hangs inside the large coil. In the correct position the axes of the two coils are vertical and coincident, and their mean diametral planes coincide. The current to be measured is maintained in opposite directions in the two parts of the large coil, whereby the small coil is urged up or down, depending on the direction of its own current. The couple is doubled by using the other coil system in the same manner, and the total force is measured by weights in the usual way.