GALVANOMETERS, PROJECT VT/072

Abstract

Abstract : Air damping in long-period galvanometers was reduced by a moving- magnet fixed-coil design, wherein the moving magnet is operated in an evacuated glass housing. The susceptibility of this arrangement to influence by local magnetic fields was minimized by use of a rotor assembly employing a multiplicity of magnetic poles evenly distributed around two ring magnets. Magnetic polarization of materials in the immediate vicinity of the ring magnets was controlled by continuous high-frequency degaussing. Testing of the moving- magnet galvanometer was not fully completed. Preliminary tests indicate that the influence of local magnetic fields has not been entirely eliminated. There appears to be no advantage to the moving-magnet galvanometer over the usual moving-coil design at free periods up to 100 sec. However, the moving-magnet galvanometer shows promise at periods in excess of 100 sec where the air damping in normal moving-coil designs becomes excessive. For dependable use, the galvanometer needs to be properly oriented in the local magnetic field. A computing or dynamometer-type galvanometer has been constructed and tested for use in seismic data analysis where a power-level indication is of interest and in signal multiplication and correlation.