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The early exploration by the petroleum industry was done with a magnetic field balance. This instrument consists essentially of a horizontal magnet supported on a quartz knife edge. The torque due to the reaction of the magnetic poles with the vertical component of the magnetic field is balanced by the weight of the moving system with its center of gravity being adjusted at the correct distance from the knife edge support to achieve this balance. The position of the moving system is indicated by a scale in the eye piece which is reflected from a mirror on the moving system. As commonly adjusted this instrument can measure changes in the magnetic field with an accuracy of 5 to l0 gammas. Such instruments were used very extensively in magnetic prospecting for petroleum in the years from the beginning of such prospecting in the early nineteen twenties until they were largely superseded by airborne instruments beginning shortly after the second World War. In some later instruments, ligaments or fibers are used instead of the rather fragile quartz knife edge bearing. Also electronic instruments, using the principles from the airborne instruments described later, have been used for magnetic surveys on the ground, particularly for very detailed observations.

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