Seismological Instruments Developed at C.I.T.

Instrument development has been one of the principal activities of the Seismological Laboratory research program ever since it was started in 1921, under the direction of H. O. Wood, as a minor grant of the Carnegie Institution of Washington. The first instrument to be completed on this program was the torsion seismograph. It was invented by Dr. J. A. Anderson of the Mount Wilson Observatory staff and was developed jointly with H. O. Wood. Essentially, it is a horizontal pendulum in the form of a small mass eccentrically mounted on a taut wire suspension, as shown schematically in Fig. 1. Critical damping of the pendulum motion is provided by a permanent magnet. Horizontal vibration of the ground during the passage of an earthquake wave results in an angular vibration of the pendulum mass. This angular vibration is optically magnified and photographically recorded by means of the mirror attached to the pendulum and the recording drum assembly, as shown in the figure.