Boys Method for the Determination og G

This method to determine the value of the universal gravitational constantuses two gold spheres each of massat 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 massas 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 byand equating this to the restoring torque whereis the restoring force per radian of turn exerted by the torsion bar givesthen