HENRY CAVENDISH

The man responsible for carrying out the first successful measurements of Isaac Newton Law of Gravitation 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He pioneered developments in other areas of science as well, most notably in chemistry and electricity although he left unpublished most of his studies of electricity (and would have been much more famous had he published them since his ideas predated those of Coulomb and Faraday).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.

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

“These experiments are sufficient to show, that the attraction of the weights on the balls is very sensible, and are also sufficiently regular to determine the quantity of this attraction pretty nearly, as the extreme results do not differ from each other by more than 1/10 th part. But there is a circumstance in them, the reason of which does not readily appear, namely, that the effect of the attraction seems to increase, for half an hour, or an hour, after the motion of the weights; as it may be observed, that in all three experiments, the mean position kept increasing for that time, after moving the weight to the positive position; and kept decreasing, after moving them from the positive to the midway position”.

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.

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.