1. In the experiment performed to find the magnitude of gravitational constant (G), Cavendish balance is used.

2. The large spheres in the balance attract the nearby smaller spheres by equal and opposite force \(\overrightarrow{F}\) . 

Hence, a torque is generated without exerting any net force on the bar.

3. Due to the torque the bar turns and the suspension wire gets twisted till the restoring torque due to the elastic property of the wire becomes equal to the gravitational torque.

4. If r is the initial distance of separation between the centres of the large and the neighbouring small sphere, then the magnitude of the force between them is, F = \(G\frac{mM}{r^2}\)

5. If length of the rod is L, then the magnitude of the torque arising out of these forces is τ = FL = \(G\frac{mM}{r^2}\)L

6. At equilibrium, it is equal and opposite to the restoring torque.

∴ \(G\frac{mM}{r^2}\)L = Kθ

Where, K is the restoring torque per unit angle and θ is the angle of twist.

7. By knowing the values of torque τ₁ it and corresponding angle of twist a, the restoring torque per unit twist can be determined as K = τ₁/α.

8. Thus, in actual experiment measuring θ and knowing values of τ, m, M and r, the value of G can be calculated from equation (2).