A linear strain seismograph

The response of hitherto known practical forms of seismographs depends upon the relative motion of a pendulum and the moving ground to which its supporting structure is fastened. The various kinds of seismographs differ in the type of pendulum used, such as gravity, spring, torsion, and/or they differ in the type of magnifying and recording elements which they employ. They all measure or indicate the vibratory motion of the ground at a giver point. In contrast to these earlier forms, the strain seismograph is a nonpendular instrument. It does not respond directly to vibratory movements Its operation depends upon variations in the distance between two points of the ground. Such variations or linear strains are set up by seismic waves. A schematic drawing of the seismometer is shown in figure 1. A and B are two piers separated by a distance of 20 meters. One end of the rod R is rigidly fastened to the pier B. The other end extends to within a short distance of pier A. Earth strains resulting from a seismic wave train produce variations in the separation of the two piers, and these variations are observable as changes in the distance between the free end of the rod and pier A. These small movements of the end of the rod relative to the adjacent pier actuate an electromechanical transducer which generates an e.m.f, proportional to the rate of change of the relative displacement. Recording is accomplished by means of galvanometers. A photograph of the seismometer is reproduced in figure 2