  In this paper, we introduce the experiment based on a sensitive torsion
pendulum for measuring and calibrating small forces at 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
the part of torsion pendulum and two source masses on a rotation table. Two
force generation mechanisms, optical force from radiation pressure and
electrostatic force by capacitive actuation unit, are designed and will be
calibrated by the gravitation force. We present our recent results of radiation
pressure measurements, and describe the design of capacitive displacement
sensing/actuating unit.
