The surface force apparatus developed by Israelachvili, and variants thereof, rely on the measurement of the deflection of a weak spring in order to detect the force between two surfaces. Mechanical instabilities occur when the gradient of the surface force overcomes the spring constant and regions of the force curve become inaccessible. We report here a technique which partly overcomes thisrestriction. It is based on a combination of the bimorph force sensor (Parker, to be published) and a magnetic force transducer (Stewart and Christenson, 1990) which allows the deflection of the sensor to be both controlled and measured with a resolution of 0.05 nm and a force sensitivity of 10−8 N. A force is applied to the spring with the magnetic ttransducer in order to compensate the surface force. This has been achieved with a simple feedback system which allows the spring constant to be increased and strong attractive and repulsive forces to be measured easily.