Cavendish took a six-foot wooden rod and attached metal spheres to each end, and then suspended it from a wire. Two 350-pound lead spheres placed nearby exerted just enough gravitational force to tug at the end-weights, causing the bar to twist.
To prevent air currents from interfering, Cavendish set up the apparatus in a wind-proof room and did the measurement using telescopes.
From the twisting force in the wire and the known masses of the spheres, Cavendish was able to calculate the value of the gravitational constant. Since the force of the gravitational attraction of the earth for an object of known mass could be measured directly, the measurement of the gravitational constant allowed the mass of the earth to be calculated for the first time. This in turn allowed the calculation of the masses of the sun and moon, and therefore the planets.