Limit of Useful Amplification of a Galvanometer Deflexion

Abstract

The following observations may be of interest in connexion with the demonstration of a short-period galvanometer by Attree, Downing and myself at the recent Physical Society Exhibition. The extent of useful amplification of this instrument has since been tested at University College on an ordinary afternoon : the limit is set by the disturbances, external and intrinsic, to which it is liable. To measure these disturbances the galvanometer was set up for photoelectric amplification and cathode-ray display, and its sensitivity increased to 2·4 x 10^-10 amp./mm., which is equivalent to a scale distance of 370 m. for direct reading. The period on open circuit was 0·0125 sec., the internal resistance 13 Ω, the resistance (total) for critical damping 230 Ω. With an external resistance of 90 Ω, single sweeps were recorded, each consisting of 50–60 'instantaneous' spots at 0·02 sec. intervals. Eight such sweeps are shown in Fig. 1; they illustrate the basic unsteadiness at extreme sensitivity.