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In this paper, we introduce an experiment based on a sensitive torsion pendulum for measuring and calibrating small forces at the nanonewton scale. The force standard for calibration is the universal gravitation between four masses separated by known distances. It is realized by two test masses suspended as part of the torsion pendulum and two source masses on a rotation table. Two force-generation mechanisms, namely, the optical force from the radiation pressure and the electrostatic force by the capacitive actuation unit, are designed and will be calibrated by the gravitational force. We present our recent results on radiation pressure measurements and describe the design of the capacitive displacement sensing/actuating unit.