PERSONAGEM DO DIA HENRY CAVENDISH (ENGLISH)

Henry Cavendish 1731-1810

The Law of Gravitation

MORE THAN a century elapsed between Newton's publication of his law of universal gravitation and its experimental proof. Not that physicists held serious doubts about the truth of his hypothesis; its agreement with astro-nomical data left little question of its validity. Nevertheless, as in ali other new concepts in physics, direct experimental evidence was required: a measurement, in the laboratory, of the force of attraction between two masses. It is not surprising that the experiment was performed only after Newton's time; the technical difficulties were rnuch too formidable. The force between any two masses of convenient size, such as could be employed in a laboratory investigation, is extremely minute, demanding great skill in its measurement. There was another reason why such an experiment was con-sidered important: if the gravitational force between two known masses1 were measured, the mass of the earth, and hence its density, would follow directly from such data.

The individual responsible for the first successful measurement of the gravitational force pioneered as well in other áreas, notably chemistry and electricity, although he left unpublished most of his investigations in elec-tricity. One of the wealthiest men of his day, he lived in virtual seclusion, devoting his entire life to science. Henry Cavendish was born on October 10, 1731, apparently at Nice (where his mother had gone for her health), the first son of Lord Charles Cavendish, himself an experimenter of some note. Details of his earliest education are lacking, but one would assume that he was tutored privately.   When he was eleven he became a pupil of the Rev. Dr. Newcombe, master of Hackney Seminary, and in 1749 entered Peterhouse College, Cambridge. At that time, Cambridge was not yet the center of scientific inquiry for which it later became so well known. In England science was, for the most part, privately supported, either by individuais or through the Royal Society. The great universities, Cambridge and Oxford, were noted for the liberal education they provide but lacked the tradition of scholarly research. Not until the middle of the nineteenth century did these universities become the focal points of scientific research in England. It is not surprising, therefore, to find science in the eighteenth century practiced in many cases by nonacademic persons, and even by those with little or no formal preparation.

Cavendish left Cambridge after three years without completing work for his degree and took up residence in London. His secluded life and the obscurity which surrounds his activities make it difncult to determine what led him into science. At any rate he appears to have taken a great interest in mathematics and experimental science. His earliest investigations were in chemistry and heat, but he published nothing until 1766, when he sent to the Royal Society a paper on Factitious Airs. There followed many other papers on chemical investigations, clímaxed in 1681 by his discovery that hydrogen (which he obtained by dissolving metais in dilute sulphuric acid) and oxygen, when burned together, formed water. He showed, further-more, that the weight of water produced was equal to that of the gases that disappeared. Despite these advances, the combustion process was poorly understood; the phlogiston theorys was widely held, and even Cavendish referred to hydrogen as phiogiston or inpammable air. and oxygen as de-phlogisticated air. He is generally regarded also as the discoverer of nitric acid, produced in his combustion chamber.

His investigation of the composition of water led him into a dispute over prior discovery. Cavendish did not publish his Experiments on Air until 1783. In the meanwhile Joseph Priestly (1733-1804) had made a some-what similar observation in 1781, unknown to Cavendish in as much as his retiring nature kept him out of touch with his scientific colleagues. James Watt (1736-1819), claiming ignorance of Priestly's experiments, proposed in 1783 that water was composed of dephlogisticated and inflammable air. For a time there was lively debate on the subject; the final conclusion was that Cavendish and Watt had conducted much the same investigations at about the same time, and came to similar conclusions. Had Cavendish been more communicative, probably the dispute would not have developed.

 Prior to his researches on the composition of water, Cavendish spent several years investigating electrical phenomena, but he published only two papers on the subject, in 1772 and 1776. Much later, when Maxwell edited a volume4 of the unpublished papers of Cavendish, it appeared that the most significant of his results had not been put into print, but that he had antici-pated many of the phenomena later discovered by Michael Faraday (1791-1867) and others. Evidently Cavendish carried out these investigations mainly to satisfy his own curiousity and saw no compelling reason to publish his results.

Regarding his measurement of the density of the earth, there is nothing to indicate how Cavendish became interested in the problem, except that he had some interest in the torsion balance, which was the instrument em-ployed in the experiment, and had discussed the problem with the Reverend Michell. He conducted a remarkably detailed series of measurements and obtained a result within one percent of that presently accepted.

Cavendish continued his investigations in various fields until his death in 1810, when he left behind a considerable fortune, allowed to accumulate during his lifetime, and a large stack of manuscripts which attested to his diverse interests and capabilities. The latter were suitably recognized when Cambridge University, late in the nineteenth century, named its new Cavendish Laboratory for him.

The following extract, published in the Philosophical Transactions, vol. 17 (1798), page 469, describes his measurement of the density of the earth.

Greetings,
Prof. Sérgio Torres

Dicas de Física e Super Interessantes

                                                     Sergio Torres

link Physics group
link Dicas de Física e Super Interessantes
link You Tube
link APOSTILAS 
link FIQUE POR DENTRO 

 lessons on Skype:

Enter your email address:


Delivered by FeedBurner