A large number of electromagnetic seismographs are now in operation at the different stations of the world network of seismological observatories and valuable data have been collected through them. Various attempts have earlier been made for the precise estimation of the instrumental constants as well as the magnification curves, but such estimations were not precise either due to approximations introduced in the analysis or due to difficulties of precision measurements during the experiments suggested for the determination of some of the instrumental constants.
In the present paper theoretical results have been given for the response of the seismographs to different tests required in the calibration of the electromagnetic seismographs, and also to sinusoidal ground motion. The “reaction of the seismometer and galvanometer” has been retained. Final results have also been given for the special types of seismographs in which the seismometer period and galvanometer period are equal and the galvanometer is critically damped. The variation of the magnification curve as well as that of the response to different tests, with changes in the reaction and seismometer damping, have been obtained. Methods have been indicated by which the instrumental constants can be determined and the seismographs can be adjusted to any prescribed condition. The results of calibration of a Sprengnether seismograph in operation at the Central Seismological Observatory, Shillong have been given. The methods suggested can also be used in the precise estimation of the instrumental constants and the magnification of seismographs of the Galitzin type even in their past operating condition, if the routine test data, particularly the response to tapping test and sudden impulse test are available.