coil means 52 and 53 referring to fig3 the exciting coil means 46 and 47 serving as coil means for giving magnetic fields have exciting coil bobbins 46a and 47a and exciting coils 60 and 61 wound around the bobbins 46a and 47a the exciting bobbins 46a and 47a comprise cylindrical portions to which the exciting coils 60 and 61 are wound the cylindrical portions having annular flanges 46b and 46c and annular flanges 47b and 47c projecting outward in the radial direction thereof each of the inner annular flanges 46c and 47c has a thickness which is larger than that of each of the outer annular flanges 46b and 47b furthermore a substantially rectangular projecting portion 50 is formed in a portion of each of the annular flanges 46c and 47c thus arranged each of the above described projecting portions 50 projects outward in the radial direction having on the end surface thereof a pair of exciting coil terminals 48 disposed at a predetermined interval in the circumferential direction thereof substantially similarly to the exciting coils means 46 and 47 the sensing coil means 52 and 53 have sensing coil bobbins 52a and 53a and sensing coils 62 and 63 wound around the bobbins 52a and 53a the sensing bobbins 52a and 53a have cylindrical portions to which the sensing coils 62 and 63 are wound the two end portions of the cylindrical portions have annular flanges 52b and 52c and annular flanges 53b and 53c projecting outward in the radial direction the inner annular flanges 52c and 53c have larger thickness than that of.