Both the performance and efficiency of an astatic magnetometer have been greatly improved by using a negative feedback system. The magnetometer has been converted into a multisensitivity instrument. A much accelerated response time permits one to measure directly the directions (declination and inclination) and intensity of magnetization of a rock specimen rather than computing them from the measurement of three orthogonal components. A greater sensitivity permits one to measure intensities ten times smaller than was possible without negative feedback. The instrument can be switched through eleven steps from a sensitivity of 0.56 × 10−9 Oe/mv and a period of 5.1 s down to a sensitivity of 1.2 × 10−7 Oe/mv and a period of 1.2 s. This permits the measurement of rock specimens magnetized to an intensity as high as 5 × 10−3 emu and as low as 1 × 10−7 emu. On an average, twelve specimens are measured in one hour.