Cavendish experiment

The Cavendish experiment was a landmark study in physics. It was the first laboratory experiment to measure the force of gravity between two masses.

While earlier scientists like Bouguer and Maskelyne attempted similar measurements, their results were not accurate. Henry Cavendish performed his experiments between 1797 and 1798. His work provided the first highly accurate values for geophysical constants. Today, we often describe his experiment as a measurement of the gravitational constant, known as G. G is the number that defines the strength of gravity between objects.

To understand the mechanism, we must look at the torsion balance. This device consisted of a horizontal wooden rod suspended by a fine wire. Two small lead spheres were attached to the ends of this rod.

Cavendish then used two much larger lead balls, which were suspended separately. These large masses could be moved to either side of the small spheres. The gravitational attraction between the large and small balls created a tiny pull. This pull caused the rod to rotate, which in turn twisted the suspension wire. The rod would rotate until the twisting force of the wire balanced the gravitational pull.

Measuring such a small force required extreme precision and care. The force involved in twisting the balance was only about 0.0177 milligrams. This is a tiny fraction of the weight of the small spheres. To prevent air currents or temperature changes from ruining the data, Cavendish used a large mahogany box. The box was 1.98 meters wide, 1.27 meters tall, and 14 cm thick. He placed this entire setup inside a closed shed on his estate. To observe the rod without touching it, he used telescopes through holes in the shed walls. He measured the deflection of the rod using vernier scales at the ends of the beam.

Cavendish had to account for several technical details to ensure accuracy. He needed to find the torsion coefficient, which is the twisting force of the wire. He did this by timing the natural oscillation period of the balance rod. As the rod swung clockwise and counterclockwise, he timed how long each swing took. In his first three experiments, the period was about 15 minutes. For the next 14 experiments, he used a stiffer wire, which cut the period to 7.5 minutes. He also had to measure the deflection angle while the rod was still oscillating, as it was never truly at rest.

The history of this discovery involves several important scientific figures. The experiment was actually devised by the English geologist John Michell before 1783. Michell built a torsion balance, but he died in 1793 before the work was finished. The apparatus passed to Francis John Hyde Wollaston and then to Cavendish. Cavendish rebuilt the machine but followed Michell's original design closely. Cavendish reported his findings to the Philosophical Transactions of the Royal Society in 1798. His work was so precise that its accuracy was not beaten until 1895 by C. V. Boys.

Cavendish's results had a profound impact on our understanding of the Earth. He originally expressed his findings in terms of the Earth's relative density. He calculated that the Earth's density was about 5.4 times that of water. Although he made a small arithmetic error discovered later by Francis Baily, his value was very close to the truth. The currently accepted density of the Earth is 5.514 g/cm³. His result was 23% larger than previous estimates and much higher than the density of the Earth's outer crust. This provided strong evidence that the Earth has a dense, metallic core made of iron.

Modern science uses Cavendish's work to define the gravitational constant, G. While Cavendish did not use the term G explicitly, his data allows us to calculate it. If we convert his density results into modern SI units, his value for G is very close to the 2014 CODATA value. His experiment was a bridge between Newton's laws and modern geophysics. The torsion balance method became the dominant way to measure gravity for a long time. Even today, contemporary scientists use variations of his original idea to study the physical world.