Newton demonstrated that gravitational forces are dependent upon the masses of the objects involved and the distance between their centers. These relationship was given by Newton's law of universal gravitation as:
Where G represents the Gravitation Constant. In Newton’s day, the value of G was not known. To determine the value of G from the Universal Law,CAVENDISH performed an experiment using torsion balance. The spheres used were made of a dense material such as lead. When the larger spheres were placed close to the smaller spheres at the ends of a rod supported by a thin thread, the thread twisted slightly due to the gravitational attraction between the metallic spheres. The gravitational attraction between the spheres can be quantified if the thread is first subjected to very small known forces and the resulting deflection angles measured carefully. Since the masses of the spheres and the distances between them can be easily measured, the value of G can be calculated.this calculated value of G was very close to the acceptable value for G ie.6.67 into 10 power-11.
Interestingly, Cavendish wasn’t attempting to measure the value of G. He was instead attempting to determine the density of the Earth. The value of G wasn’t used in a published calculation until 100 years or so after Cavendish’s work.