AN AUTOMATIC AC BRIDGE FOR MEASURING MAGNETIC-SUSCEPTIBILITY AT LOW-TEMPERATURES

An automatic ac bridge for measuring magnetic susceptibility chi = chi ' + j chi '' is described. An imbalance signal is compensated using a separate coil inductively coupled to the pickup coil. The generation o f current in the compensating coil for both in-phase and quadrature co mponents is controlled by a digital integrating feedback circuit built around a voltage-to-frequeney converter. The sensitivity Limit of the bridge is 3 . 10(-8) CGSM units. The operating frequency band is 20 H z-10 kHz. The modulating field amplitude reaches 15 Oe. The temperatur e stabilization system operates in a range of 4.2 to 250 degrees K. Ty pical measurements are presented.