Theory of the electromagnetic seismograph

Abstract

Although the development of the first moving-coil electromagnetic seismograph and a primitive theory to account for its behavior date back almost fifty years, the responses of only a few simple types of these instruments have had adequate theoretical treatment. This lack of theory restricted, but did not stop, the development of other designs for which no adequate theory existed. Confusion and controversy have arisen over the behavior of these later designs. Some work has been done on obtaining magnification curves for these instruments directly by the use of shaking tables or other artificial driving devices. On the whole, however, instruments of this type now in use simply are not calibrated. An attempt guided by the work of previous investigators has been made to clarify further the behavior of electromagnetic seismographs by extending the theoretical treatment. Three new types of seismographs (more properly “adjustments” of seismographs), each of which includes the classical Galitzin as a limiting case, are proposed. Methods for adjusting and calibrating these instruments are outlined. Since overcritical damping is employed in two of the adjustments proposed, a method is given for determining the damping constant of an overdamped galvanometer or seismometer.