An astatic magnetometer for measuring the susceptibility of materials for magnetic instruments

Abstract

It is well recognized that one serious source of error in the absolute measurement of the Earth's magnetic field lies in the use of materials of high magnetic susceptibility in the construction of magnetometers and other magnetic instruments. A rugged instrument has been developed which allows a rapid and accurate measurement of the absolute susceptibility of any material having a susceptibility of 1×10−8 electromagnetic cgs unit or greater for a wide range of geometrical shapes.The instrument consists of an astatic magnetometer in which the magnets are accurately balanced against each other; the magnets are suspended by a 7‐micron diameter fiber of quartz. The material under test is brought close to one end of the magnet‐system and the deflection noted. The magnetometer is then calibrated by bringing a hollow full‐scale model of the specimen—the model being filled with a chemical of known susceptibility—to the same position with respect to the magnet‐system. Since it has been determined that the deflection of the magnetometer is proportional to susceptibility of the specimen, the two deflections depend only on the respective susceptibilities. A small Helmholtz coil is provided around one magnet in order that a null‐method may be used if the susceptibility is high enough to cause greater than full‐scale deflection. For small cylindrical specimens the sensitivity is as high as 1 × 10−9 electromagnetic cgs unit, while for other shapes it is 1 × 10−8 electromagnetic cgs unit, and can be made as insensitive as may be desired. A single measurement requires only a few seconds, apart from the time required to prepare the model used in calibration.