Henry Cavendish

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Image:Henry cavendish.JPG Image:Cavendish-lab.jpg Henry Cavendish (October 10, 1731 - February 24, 1810) was a British scientist.

Discovery of hydrogen

He is generally credited with having discovered hydrogen, since he had described the density of 'inflammable air', which formed water on combustion, in a paper "On Factitious Airs" that appeared in 1766. Antoine Lavoisier later reproduced his experiment and gave the element its name.

The density of the Earth

Cavendish is also credited with one of the earliest accurate calculations of the density of the earth. He published his results in the Philosophical Transactions of the Royal Society of London<ref>Experiments to determine the Density of the Earth by Henry Cavendish Esq. F.R.S. and A.S, Philosophical Transactions of the Royal Society of London Vol. 88 (1798), 469-526.</ref>. The apparatus used was actually designed and built by John Michell, who died before he could begin the experiment. The apparatus was sent in crates to Cavendish, who completed the experiment in 1797.

The apparatus consisted of a torsion balance to measure the gravitational attraction between two 350 pound lead spheres. There is the common mistake in History of Science to affirm that Cavendish calculated Newton's gravitational constant, G. This mistake has been pointed out by several authors<ref>CLOTFELTER BE (1987), THE CAVENDISH EXPERIMENT AS CAVENDISH KNEW IT , AMERICAN JOURNAL OF PHYSICS 55 (3), 210-213.</ref><ref>Falconer I (1999), Henry Cavendish: the man and the measurement , MEASUREMENT SCIENCE & TECHNOLOGY 10 (6): 470-477.</ref> although it appears in most of the physics textbooks (e.g. <ref>Tipler P.A. and Mosca G. (2003), Physics for Scientists and Engineers: Extended Version, W. H. Freeman ISBN 0716743892.</ref> and <ref>Feynmann R.P.(1970), Feynman Lectures On Physics, Addison Wesley Longman , ISBN 0201021153 </ref>). In reality Cavendish wanted to estimate the Earth's density relative to water's. His accurate results were reinterpreted later to calculate G. The first time that this constant was ever used was almost 100 years after the Cavendish experiment, in 1873<ref>Cornu A. and Baille J.B. (1873), Mutual determination of the constant of attraction and the mean density of the earth, C.R. Acad. Sci., Paris Vol. 76, 954-958.</ref>.

His results were also used to calculate Earth’s mass and remained the accepted standard until the 20th century. The current best estimate for the Earth's mass is 5.9725 billion trillion tonnes, a difference of only about 1% from Cavendish's measurements.

Work on electricity

Cavendish made several remarkable discoveries in electrical science, but they cannot be described as significant because he did not bother to publish them. They did not become known until they were collected and published by James Clerk Maxwell a century later, in 1879, long after other scientists had been credited with the same results. Among Cavendish's discoveries were the following [1]:

• The concept of electric potential, which he called the 'degree of electrification'
• An early unit of capacitance, that of a sphere one inch in diameter
• The formula for the capacitance of a plate capacitor
• The concept of the dielectric constant of a material
• The relationship between electric potential and current, now called Ohm's Law. (1781)
• Laws for the division of current in parallel circuits (now attributed to Charles Wheatstone)
• Inverse square law of variation of electric force with distance (now called Coulomb's Law)

Other work

Additionally, Cavendish established an accurate composition of the atmosphere. He found that 79.167% is "phlogisticated" air (now nitrogen + argon) and 20.8333% is "dephlogisticated" air (now known to be 20.95% oxygen). He also found that 1/120 is a third gas (William Ramsay and Lord Rayleigh established the gas to be argon 100 years later).

Personal life

He was silent and solitary, viewed as somewhat eccentric, and formed no close personal relationships outside his family. By one account, Cavendish had a back staircase added to his house in order to avoid encountering his housekeeper because he was especially shy of women. The contemporary accounts of his personality have led some modern commentators, such as Oliver Sacks, to speculate that he had Asperger's syndrome, though he may have been merely painfully shy.

Because of his asocial and secretive behaviour, he often avoided publishing his work, and much of his findings were not even told to his fellow scientists. It wasn't until the late nineteenth century, after his death, that James Clerk Maxwell looked through his papers. He found that credit to most of his discoveries had already been given to others. Examples of what was included in Cavendish's discoveries or anticipations were Richter's Law of Reciprocal Proportions, Ohm's Law, Dalton's Law of Partial Pressures, principles of electrical conductivity and Charles's Law of Gases.

Some later relatives, especially William Cavendish, 7th Duke of Devonshire (Chancellor of the University from 1861 to 1891), donated money which was used to endow the Cavendish Laboratory at Cambridge University in the 1870s.

References

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Further reading

• Bill Bryson "The Size of the Earth"

A short history of Nearly Everything, 60-62 (2003).

• Dictionary of National Biography, Vol. 3, p.1261.

• Jungnickel and McCormmach, p. 306.ar:هنري كافيندش

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