The development of highly sensitive electromagnetic seismographs and their uses in various problems of seismology and other allied branches have considerably extended the region of their application. The theory of such seismographs was developed earlier under different approximations which are justified as long as it is used only in the study of earthquake waves of frequencies less than one cycle per sec. In the present paper the effect of large inductances in the transducer coils and also of any choke in the galvanometer circuit on the response characteristics has been examined. The response curves for different combinations of seismometers and galvanometers have been obtained, and their variations with the coil inductances have been estimated. It has been shown that considerable errors in the amplitude-period relationship occur if the coil inductances are neglected. This possibly explains the various anomalies which now exist in the interpretations of different results obtained from the analyses of seismograms, obtained with different seismographs. The ‘magnification curve’ for a typical electromagnetic seismograph has been obtained experimentally and this has been compared with the theoretical result derived in the present paper. A number of problems have been indicated, in the study of which precise response curves are required, and it has been suggested that with a proper use of the results derived here a large number of anomalies now existent may disappear.

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