Cavendish Experiment: Weighing the World


The Cavendish Experiment, was one of his most notable experiments. Cavendish performed the experiment in 1797-1798. The Cavendish Experiment was the first experiment to measure the force between masses in the laboratory. Moreover, the first experiment to produce definitive values for the gravitational constant and the mass density of the Earth. The experiment was originally conceived by John Michell before 1783, however in 1793 Michell died before completing his work. Soon thereafter, Cavendish was given Michell's apparatus for the experiment, which he then re-constructed his own model, while keeping major components similar to Michell's original plan. The results of the Cavendish Experiment was the mass density of the earth, yet others were able to derive the actual value of the gravitational constant from the experiments results. The Cavendish Experiment's purpose is frequently misunderstood to think its goal was to determine the gravitational constant(G). When in fact, Cavendish's only goal was to measure the mass density of the Earth. The gravitational constant does not appear in Cavendish's published paper on the topic, nor is there any indication that he regarded it as a goal of this experiment. Nearly 100 years later when G was first measure in a laboratory, they realized that Cavendish had obtained a value of G that was accurate to 1%.

Below is the procedure of The Cavendish Experiment. Provided by:

The Experiment
The apparatus constructed by Cavendish was a torsion balance made of a six-foot wooden rod suspended from a wire, with a diameter 1.61 pound lead sphere attached to each end. Two 12 inch 348 pound lead balls were located near the smaller balls, about 9 inches away, and held in place with a separate suspension system. The experiment measured the faint gravitational attraction between the small balls and the larger ones.
The two large balls were positioned on alternate sides of the horizontal wooden arm of the balance. Their mutual attraction to the small balls caused the arm to rotate, twisting the wire supporting the arm. The arm stopped rotating when it reached an angle where the twisting force of the wire balanced the combined gravitational force of attraction between the large and small lead spheres. By measuring the angle of the rod, and knowing the twisting force ( torque) of the wire for a given angle, Cavendish was able to determine the force between the pairs of masses. Since the gravitational force of the Earth on the small ball could be measured directly by weighing it, the ratio of the two forces allowed the density of the earth to be calculated, using Newton's law of gravitation.

Cavendish found that the Earth's density was 5.448 ± 0.033 times that of water (due to a simple arithmetic error, found in 1821 by F. Baily, the erroneous value 5.48 ± 0.038 appears in his paper).

To find the wire's torsion coefficient, the torque exerted by the wire for a given angle of twist, Cavendish timed the natural oscillation period of the balance rod as it rotated slowly clockwise and counterclockwise against the twisting of the wire. The period was about 7 minutes. The torsion coefficient could be calculated from this and the mass and dimensions of the balance. Actually, the rod was never at rest; Cavendish had to measure the deflection angle of the rod while it was oscillating.

Cavendish's equipment was remarkably sensitive for its time. The force involved in twisting the torsion balance was very small, 1.47 x 10 –7  N, about 1/50,000,000 of the weight of the small balls or roughly the weight of a large grain of sand. To prevent air currents and temperature changes from interfering with the measurements, Cavendish placed the entire apparatus in a wooden box about thick, tall, and wide, all in a closed shed on his estate. Through two holes in the walls of the shed, Cavendish used telescopes to observe the movement of the torsion balance's horizontal rod. The motion of the rod was only about 0.16 inch. Cavendish was able to measure this small deflection to an accuracy of better than one hundredth of an inch using vernier scales on the ends of the rod.

                              Cavendish Experiment                                                                                                             Torsion Balance Apparatus
                   Exp_Room                                                                Torsion Balance Apparatus                                                                                                                                

About Cavendish
Work with Gases
Work with Electricity
Mass Density of Earth
Proof of Centrifugal Acceleration