A BRIEF BIOGRAPHY OF HENRY CAVENDISH

According to his own accounts, Isaac Newton probably formulated his ideas about universal gravitation during the period 1665-1666. Although he was not the only one to have concluded that the force of attraction between the Sun and planets must be an inverse square law - his contemporaries Edmund Halley (1656-1742) and Robert Hooke (1635-1703) had also come to the same conclusion but were unable to prove their hypothesis for elliptical orbits - Newton is always associated with its discovery and verification, at least as far as planetary motion is concerned. However, the first experimental proof (on Earth) did not occur until 1798. It wasn't that scientists did not believe Newton - there was little doubt from astronomical observations - but evidence was required in the laboratory of the force of attraction between two masses. The force between two laboratory masses is extremely small and it would take great skill and determination in measuring the effect. There was another, very important reason for if the gravitational attraction between two objects could be measured, then it would be possible to determine directly the mass (and hence density) of the Earth from a knowledge of the Gravitational constant (G).


The man responsible for carrying out the first successful measurements was Henry Cavendish. He pioneered developments in other areas of science as well, most notably in chemistry and electricity although he left unpublished most of his studies of electricity (and would have been much more famous had he published them since his ideas predated those of Coulomb and Faraday). He was a very wealthy man but chose to live most of his life devoted to science and in seclusion.

Henry Cavendish was born on October 10, 1731 in Nice, France, where it seems his mother had gone for her health. He was the first son of Lord Charles Cavendish, also an experimental scientist of some fame. His early education was probably carried out by private tutors but when he was eleven he became a pupil of the Rev. Dr. Newcombe, master of Hackney seminary, and in 1749 entered Peterhouse College, Cambridge. He left Cambridge in 1753 without completing his degree and, after touring Europe with his brother, he went to live in London. He lived his life frugally, and in relative obscurity, which makes it difficult to follow his progress, but there is little doubt that he took a great interest in mathematics and science. Apparently, he was terrified of women and communicated with his female servants by written notes. His earliest investigations were in chemistry and heat, but his first publication did not appear until 1766 when he sent a paper to the Royal Society - he was an active member of the Society from 1760 until his death - on Factious Airs [1]. He wrote several other papers on chemical investigations and in 1781 showed that hydrogen and oxygen when burned together formed water, and furthermore, that the weight of water produced was equal to that of the gases. The actual processes were not at all well understood; the phlogiston theory [2] was still widely held. Cavendish is also believed to have discovered nitric acid in 1795.

Cavendish's retiring nature, his reluctance to publish and lack of contact with his scientific contemporaries led to an unfortunate dispute over his investigations of the composition of water. He did not publish his results until 1783 but in the meantime Joseph Priestly (1733-1804) and James Watt (1736-1819) had carried out similar investigations. There was a lively debate about who had made the original discovery which, most likely, would have been averted had Cavendish communicated more freely.

Cavendish also spent several years studying electrical phenomena but published only two papers on the subject, in 1772 and 1776. When, about 100 years later, Maxwell edited a volume of the unpublished works of Cavendish [3] it seems that the most significant of his results pre-dated many of the important conclusions and results discovered by Faraday (1791-1867) and Coulomb (1736-1806). His experiments included studies of capacitance and he measured the strength of electrical currents by giving himself a shock and estimating the magnitude of the pain! Apparently, he had carried out all these studies to satisfy his own curiosity and did not feel they should be published.

After measuring the density of the Earth in 1798 (see below) he continued to be interested in a variety of topics. He died in 1810, leaving a large fortune, which had accumulated during his lifetime, and stacks of manuscripts on a wide range of his scientific interests. His qualities as a first-rate experimenter were recognized when late in the 19th century the University of Cambridge named in new Cavendish Laboratory after him.

Measurement of the density of the Earth

It is not clear precisely when Cavendish became interested in the problem of measuring the density of the Earth although in his paper to the Royal Society describing his experiments, which was published in 1798 [4] he does make reference to a brief history of the method and equipment used [5]:

In his paper he demonstrates his superb and careful experimental approach. After carrying out a series of experiments he comments:

He describes various tests including testing the suspension and any effects due to magnetism but finally decides that the changes with time were due to the differences in temperature between the wooden case (shielding and protecting the smaller weights from draughts) and the weights themselves.

Also, at the end of the paper he refers to the differences between his measurements and those from another novel experiment:

It is difficult to conceive of another experiment that allows such a direct measurement of the mass of the Earth. In addition, we should appreciate that the Earth is not spherical; the distance from the center to the poles is less than that at the equator. As a result, the acceleration due to gravity is not constant over the Earth's surface.

FOOTNOTES

[1] Gases in the form of chemical compounds.

[2] Based on the theory that air was the only gaseous element and phlogiston was that which escaped during combustion.

[3] J.C. Maxwell The Electrical Researches of the Hon. Henry Cavendish (Cambridge University, Cambridge, 1879).

[4] H. Cavendish Philosophical Transactions of the Royal Society 17, 469 (1798).

[5] A very sensitive torsion balance.

[6] John Mitchell (1724-1793) was the founder of the science of seismology.

REFERENCES

Books

M. Shamos Great Experiments in Physics (Dover Publications Inc., New York, 1987).

Web-sites