A Sensitive Induction Magnetograph

Abstract

A SENSITIVE induction magnetograph for measuring the time-rate dH/dt of the horizontal component of the earth's magnetic field is described by H. Nagaoka and T. Ikebe in Scientific Papers of the Institute of Physical and Chemical Research (Tokyo) of August 1939. The magnetograph embodies a design of a special shape for collecting the lines of force into a permalloy core enclosed in a cylindrical coil. A record is given of complicated magnetic disturbances caused by electric trams. Even at a distance of several hundred kilometres from a great city, the disturbances can be detected as minute ripples on the traces. The selection of an observing station for observing true geophysical phenomena is therefore a difficult problem. The observed effect of weak earthquake shocks is discussed. Apart from the purely mechanical effects of shock, there are probably some true magnetic effects due to the varying magnetic susceptibility of erupting lava (the subject of study being the Asawa volcano). The authors think that it may be possible to predict violent eruptions half an hour before they occur. It is also anticipated that the new magnetograph will provide data to test whether short-period disturbances of a few seconds’ duration exist during magnetic storms. Provided that a suitable site can be obtained, the magnetograph has other applications in the study of the characteristics of the E- and F-layers of the ionosphere with respect to the height of origin of the current system producing the diurnal variation of the earth's magnetic field.