Describe laboratory method of determining the universal gravitational constant, G. 

Determining gravitational constant

• Two equal lead spheres A and B each of mass, m, are attached to end of a bar AB of length, L.
• The bar AB is suspended from a ceiling.
• Large spheres P and Q are brought towards A and B respectively from the opposite side
• Large spheres P and Q altered small spheres A and B respectively by equal and opposite gravitational forces give rise to gravitational torque, F, which in turn twist the suspended through angle θ.
• A resting torque of the wire opposes the twisting of the wire from equilibrium position

Then

Where

d = distance between the centre of A and P or B and Q.

C = The twisting couple per unit twist (θ = 1)

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