By Hugo Benioff
The vertical seismograph herein described makes use of a mechanical system of short period in combination with an electro-mechanical transducer of high sensitivity coupled to a recording galvanomenter. In the Galitzin seismograph the transducer consists of a coil which moves in a magnetic field in such a manner as to generate electromotive force proportional to the velocity of the inertia reactor. The present transducer embodies a modification of the common telephone receiver principle and is also of the velocity type.
The mechanical system consists of a cylindrical inertia reactor of 100 kilograms mass supported directly by a helical spring of such stiffness as to provide a period of 0.5 of a second approximately. The pendulum is constrained to move in a vertical line by six bicycle spokes arranged in such a way as to provide substantially a double diaphragm support. Critical damping is maintained by means of a perforated disk moving in a cylindrical container filled with oil. The container is turned out of the top of the inertia reactor. The amount of damping is controlled by varying the separation of the damping disk from the bottom of the container by means of capstan nuts provided for the purpose. Stops in the form of screws are arranged to limit the motion of the reactor during severe disturbances to about 0.5 of a millimeter.
The transducer is placed below the inertia reactor. The permanent magnet is screwed to an adjusting plate which is fastened to the base of the seismometer. Pole-pieces of Armco iron are fastened vertically to the poles of the magnet. A rectangular bar of Armco iron rigidly attached to the lower face of the reactor forms the working air-gaps with the pole-pieces. The length of the gaps is approximately 0.25 to 2 millimeters depending upon the desired sensitivity.
Benioff, H., A new vertical seismograph, Bulletin of the Seismological Society of America, vol. 22, No. 2, June, 1932.