It is shown that an electromagnetic seismograph may be calibrated by observing the deflections of the galvanometer when an electrical signal is injected through a suitable coupling circuit. A specially designed version of the Maxwell Impedance Bridge is used for this purpose, the seismometer being connected in one arm and the galvanometer across the output. The bridge is balanced when the seismometer is clamped. The seismometer is then unclamped, and the galvanometer starts to swing with an amplitude equal to that which would be produced by a ground acceleration proportional to the bridge current. The constant of proportionality is found by injecting a “substitution e.m.f.” across the ratio arm of the bridge. By carrying out the substitution experiments in their most complete form it is possible to determine the mass, the suspension stiffness, and the damping constant of the seismometer, either including or excluding the effects of the galvanometer reaction. Typical calibration curves for Benioff, Willmore, and Sprengnether seismometers are included.
