Progress Report, Seismological Laboratory, California Institute of Technology, 1949

Abstract

In addition to those published during 1949 (list at end) the following research activities have been carried forward. Instrumental development under Benioff has included an experimental model seismometer using a capacity transducer. The pendulum is of the torsion type with an inertia reactor of approximately one kg mass. Damping is provided by a coil moving in a magnetic field with leads attached to the upper and lower torsion ribbons which are insulated from each other. In this way damping can be adjusted with an external resistance connected to the coil and any change in period brought about by reaction of the magnetic field on the coil is accounted for. The variable capacity consists of a fixed plate separated about one millimeter from a movable plate attached to a light boom on the cylinder. The mechanical magnification is approximately five. The seismometer capacitor serves to couple the grid and plate circuits of a tuned-grid tuned-plate oscillator. For small movements of the pendulum the amplitude of oscillation varies very nearly linearly with the amplitude of the pendulum displacement. A voltage output of seismic frequency is derived from the oscillator grid leak and is coupled to a one-stage amplifier by means of a resistance-capacity network. The output of the amplifier is also coupled through a resistance-capacity network. The time constant of the coupling circuits are such as to respond to seismic waves up to 60 sec-period. A short-period galvanometer (0.1 to 0.3 sec) is used for recording.

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