A New Moving-Magnet Instrument for Direct Current

A new type of d-c instrument is described in this paper. It comprises a diametrically magnetized cylindrical moving- magnet element, a hollow copper damping cylinder surrounding the moving element, a fixed coil, control magnets having a very high coercive force, and an enclosing magnetic shield made of mumetal. The field of the control magnet and the field of the fixed coil have a relatively large angular displacement. The angular position of the vector sum of these fields varies with the current through the coil, and the moving element follows this vector. The scale distribution is nearly uniform for a 90-degree scale. The characteristics compare favorably in many respects with those of a D'Arsonval instrument.