  An astatized symmetrical vertical pendulum is monitoring torque {\Gamma}(M)
resulting of gravitational attractions exerted by two external masses M moving
up down. Local gravity field g produce the main pendulum restoring torque
combined with a very weak variable torque {\Gamma}(c) induced by rotation of
watch needles fixed to the pendulum. Transfer of fundamental units to calibrate
the {\Gamma}(c) torques is obtained by a reference torque {\Gamma}({\mu})
resulting of precise displacements of a well known mass {\mu}. We permanently
monitored ratio between the gravitational effect {\Gamma}(M) and calibrated
{\Gamma}(c) to determine G. Position of the pendulum is measured with a
capacitive bridge. Bias voltages sent to two electrodes set-up at the bottom of
the pendulum allows to feedback pendulum with a controlled electrostatic
torque. We discuss potential interest of our prototype to design a multi
pendulum system to check systematic effects for different geometries and
various kinds of materials.
