A moving coil galvanometer of extreme sensitivity

A sensitive moving coil galvanometer is described which is stable enough, under ordinary laboratory conditions, to allow a 20- to 50-fold magnification of its movements. The magnified deflexion is read on a standard microammeter. Negative feedback from the output of a photoelectric amplifier to the galvanometer circuit allows the sensitivity to be diminished as required, with corresponding gain in stability. The galvanometer is strongly overdamped, but its short natural period gives it a reasonably short deflexion time (a few seconds). Tests of performance show that the average error of a single deflexion, in spite of the manifold disturbances of a busy laboratory, corresponds to about 0.002-0.004 μV. In a quiet place it would be considerably less.