A portable string galvanometer for use at moderate frequencies
E.W. Marchant, J K Burkitt, Andrew Langley · Journal of Scientific Instruments · 1931
The instrument described has been designed for use in a portable apparatus. The string galvanometer has been constructed with a frequency of vibration of 150-200 per sec. In order to secure critical damping the string is arranged in an oil-filled case, and to maintain proportionality between deflection and current the string is supported at the ends by springs. With this arrangement the deflection is proportional to the current over the whole working range, and with a magnification of 60-80 photographic records can be obtained with the light from an 18-watt 6-volt lamp, the battery for which is easily transportable. The optical system consists of a low-power microscope objective and eyepiece, and is fitted with a double convex lens and a plano-convex cylindrical lens to obtain a sharp image. The motion of the string is recorded by a film camera. The motor for moving the film is spring driven and a spring-driven time marker is also fitted. In order to identify records a device is fitted for marking notches on the edge of the film. After exposure the film is coiled up in a cylindrical box, a spring-operated knife cutting off the film when the lid of the camera is opened, to enable the film box to be removed.