Improvement of a Coulomb's Law Apparatus for Student Use

The improvement of a Coulomb's law apparatus for student use is described. The conventional apparatus, which measures the displacement of a suspended charged ball repelled by a fixed charged ball, has long been used in physics education, but has a disadvantage in that setting the separation between the balls is difficult. The improvement is achieved by adding an inclined device to make the suspended ball return to the equilibrium position. This allows both the repulsive force to be derived from the tangent of the inclination and the separation between the two balls to be set easily. The setup is installed in an enclosure with a transparent cover and a desiccant inside in order to minimize the effect of humidity. Results obtained from the improved apparatus show that the force follows an inverse square law with an exponent $n=-1.99{\pm}0.01$ and is proportional to the product of charges, in agreement with Coulomb's law.