Design of transversal earth magnetic field compensating coil systems and their comparison with conventional systems

The coercive field strength of a soft magnetic specimen can be determined b y magnetizing and demagnetizing it in a solenoid. However, for a precise me asurement, the transversal component of the earth magnetic field has to be compensated. For this purpose, a coil consisting of eight parallel straight electrical conductors has been designed. The conductors are positioned on the surface of a cylinder. The angles of the first and second conductors ar e 42 degrees and 78 degrees related to the direction of the field to be com pensated. The angles of the other 6 conductors ensue from the symmetry. The coil constant is k = 1,0487/R. An elongated, almost homogeneous field region was obtained. The cross secti on of this region is approximately circular with a diameter of 22% of the t otal diameter of the coil, the relative deviation of the field strength bei ng smaller than 10(-4). A compensation to less than 32 mA/m can be achieved in a cylindrical space 100 mm in diameter and 2 m in length. This degree o f precision requires considerable high accuracy in mechanical construction. For less strict requirements, the formulas for two-conductor and four-cond uctor systems are indicated.