Abstract
Since 1955, continuous geomagnetic observation has been conducted by the Geographical Survey Institute at the Kanozan Geodetic Observatory (latitude: 35°15'11"N and longitude: 139°57'32"E) in order to obtain epoch reduction data for geomagnetic surveys mainly. However, the electrification of the West-Boso line, the distance of which from the obser-vatory is only 7 km, was recently planned so that magnetic contamination of about 2-3 gammas due to the artificial disturbance became unavoidable on the geomagnetic record. Therefore, the institute constructed a new magnetic observatory named the Mizusawa Geodetic Observatory (latitude: 39°06'30"N and longitude: 141°12'26"E) on an eastern hill of Mizusawa City, Iwate Prefecture in 1968. On this occasion, magnetic instruments were improved so as to obtain digital record of the three components for saving routine duties as well as their analogue record in the new observatory. In principle, proton precessional method enables to make digital processing of not only the total force F but also any other components by using bias coils of known direction and size. At the Kanozan Observatory, for example, since 1960 analogue record of the horizontal component H has been obtained by means of the proton magnetometer combined with a set of Fanselau-Braunbek coils for eliminating the geomagnetic vertical component[1]. In the case of the declination observation, however, the proton precessional method may not be so convenient, while the ordinary suspension magnet type D-variometer is much favorable because of its simplicity in principle. Then, in the Mizusawa Observatory, digital records of the three components are composed of the improved D-valiometer with A-D converter, and proton magnetometers for F and H.
