3- Law of universal Gravitation

As we have seen, two bodies placed in presence of one another exert a force of attraction between them. This property is revealed by the Cavendish twisting balance, built for this purpose; however, for moderate bodies of mass it acquires very small values.
This scale consists of two spheres ordinarily gold or platinum of known mass, mounted on a light horizontal bar, which is suspended by its center of a quartz thread. By means of a mirror fixed to the thread, the light of a light beam is reflected on a scale. Two large masses of lead are arranged as indicated in the figure below and it is observed that due to the force of attraction in the masses of the system a pair of forces originates, which sets the balance in motion. This movement is recorded by the deviation of the light beam on the scale.