The Moving-Coil Meter: A Coil, a Spring and a Radial Field

A needle that swings across a scale in proportion to the current: the moving-coil meter was the standard electrical instrument for a century. Edward Weston's patents of 1888 made it portable and accurate. A light coil of fine wire on jewelled pivots sits between the poles of a permanent magnet, around a fixed iron cylinder; hair springs hold it at zero and carry the current in; the coil is wound on a copper frame that damps its swing so the needle settles instead of oscillating. The iron cylinder is the clever part. It makes the magnetic field in the gap radial, so the coil's torque does not change as it turns, and the scale comes out evenly spaced. This rung shows why in a notebook, then builds a thread-suspended galvanometer and demonstrates the damping.