A student is investigating the force between two charged metal spheres S and T, as shown in Fig. 1.1. [Figure 1.1] Each sphere may be charged by connecting the positive lead from a power supply to the sphere and then removing the lead. The electromotive force (e.m.f.) of the power supply used to charge sphere T is V. The force F between the two charged spheres may be determined by attaching sphere S to a top pan balance. For a constant charge on sphere S, it is suggested that the relationship between F and V is F = αν / r² where r is the distance between the centres of the spheres and α is a constant. Design a laboratory experiment to test the relationship between F and V. Explain how your results could be used to determine a value for α. You should draw a diagram, on page 3, showing the arrangement of your equipment. In your account you should pay particular attention to • the procedure to be followed, • the measurements to be taken, • the control of variables, • the analysis of the data, • any safety precautions to be taken.