This method to determine the value of the universal gravitational constant
uses two gold spheres each of mass
at either end of a bar suspended in the middle by a torsion wire. The bar is suspended between lead masses A and B each of mass
as shown in the diagram.
The forces acting on the spheres are shown below.
The forces of gravitational attraction between the gold masses and the lead masses causes the bar to twist through an angle
The torque on the beam is given by
and equating this to the restoring torque
where
is the restoring force per radian of turn exerted by the torsion bar gives
then