Who Invented Youngs Modulus

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  PAPER: Beal Paper Who invented Young’s Modulus A. N. Beal, Introduction order. Look at the ictures on this page: what do they ave BSc, CEng, MIStructE, we take for granted t a few srcinal thinkers. Some of The first Fig 1) is hat wonderfully complex organ, the homason Partnership In science and engineering, we owe much of the knowledge in common? them are quite amous: Archimedes and his bath; alileo; Newton, who gave us gravity and the laws of force and motion; Euler who solved he problem of classic strut buck- ling; Rankine who gave us earth pressure theory and lso thermodynamics; and Coulomb who gave s another earth pressure theory and also the unit of electric charge. But who was the Young’ of Young‘s Modulus’? Was e a noted engineer in days gone by, or was he an obscure loner who made the study of elasticity his ife’s work? Who was he, and what lse did he do with his ife? Before answering these questions, a little quiz is in Keywords: biographies, history, Thomas Young, light, engineering, hydraulics, 19th century, elasticity human eye. The fundamental principles ofhow itfocuses light and erceives colours were first worked out by a man called Young. The second Fig 2 is the Rosetta Stone: an engraved stone from Egypt, dated to 196 BC. It caused a sensation when it was discovered in 1799, because it appeared to have a message inscribed on it in three languages: the bottom section in Greek, the middle in an old Egyptian script, and op part in ancient Egyptian ieroglyphs. The Rosetta Stone was seen as the key which would allow scholars to crack the code of the hieroglyphs and read the hieroglyphic inscriptions that had been found on monu- ments around the country. The code was eventually bro- ken, and the irst hieroglyphics were translated by a man named Young. The third Fig ) is a drop ofwater, hich is held togeth- er by surface tension. The equation for this was worked out and its value irst calculated by a man called Young. The fourth Fig 4 is the interference pattern created when light passes through two closely-spaced slits. This classic experiment proved that light is a form of wave ener- gy not a stream of particles. Physicists call the pattern Young‘s Fringes’ and refer to it s Young‘s Experiment’, after the man who devised it. o all these things are associated with people by the name of ‘Young‘. What is not o well known is that all these Youngs’ were actually the same man, and he is also the Young’ of Young‘s Modulus’. Amazingly, although he is almost or- gotten today, the above examples represent only a fraction of his total contribution to human knowledge. In addition to the aspects of his work on vision, light, Egypt and engineering which will be discussed ater, he did many things that can be mentioned only in passing: he ig 3. Fig 1 Fig 4 Volume 78 No 4 8 luly 2000 27  PAPER: eat investigated sound waves and harmonics; he made the first calculated estimate of the size of a molecule, 50 years before anyone else; he invented the mechanism used in recording barometers Fig 5) and made a major contribu- tion to the theory of tides; he was responsible for an Admiralty report which transformed the design of wood- en warships; linguists remember him as the man who introduced the term Indo-European’ to show that the lan- guages from India to Ireland share common root. In his day, he was known as ‘The English Leonardo’. I was actually introduced to Young not through engi- neering but by Daniel Kline, Professor of Physiology at Cincinnati University, who came across his work when studying Egyptology. Kline went on to write a short biog- raphyl. Kline recommends he detailed biography by Wood Oldham2, and this s the source for most of the materi- al presented here. Young cropped up again in a paper by Chapman Buhagiar3 about he buckling of cold formed steel beams n the August 1993 issue of the ICE Proceed- ings which included an interesting summary f Young‘s contribution to structural engineering. This paper ecom- mends the account of Young‘s contributions to structural engineering given by Timoshenko4, and I have also re- ferred to his. So who was ‘Young’? The name of this remarkable man was Thomas Young Fig 61, and his career was actually in medicine, as a doctor in general practice. Of his medical work, it was said that ‘He is ... not celebrated as a medical practitioner; nor did he ever enjoy an extensive practice; but in information upon the subjects of his profession, in depth of research into the history of diseases and the opinions of all who have preceded him it would be difficult to find is eq~al’.~ Thomas Young was a Quaker, who was born n Somerset in 1773 and died in London in 1829. He had an extraordi- nary intelligence, which was noted at a very early age: he could read ’with considerable fluency‘6 at the age f 2 yes, 2 ) and had read the ible twice through before he was 4 years old. He began learning Latin when he was five. By the time he was 13 he had also learnt Greek, Hebrew, Italian, and French, and was learning Chaldee, Syriac, and Samaritan, and he had lso developed an interest in optics and telescope making. In the ears that followed, he studied more languages, read French, Italian and English classics, mathematics, astronomy, natural philosophy, botany, chemistry, and medicine. At the age of 20, Young presented a paper to the Royal Society on how he eye focuses, and he as elected a Fellow of the Society the following year. He was Professor of Physics at the Royal Institution from 1800 to 1803 and, in 1802, he gave a series of 31 lectures there, xpanded to 60 the following year. These were published in 1807. The lec- tures were remarkable not only for heir breadth but lso for their depth f coverage: in many of the subjects includ- ing engineering) Young introduced major advances in knowledge. The first 20 lectures dealt with ‘mechanics’ and covered what would be expected plus also lectures n drawing, writing, and measuring, as well as architecture and carpentry. The second set of 20 dealt with ‘hydrody- namics’ and covered the physical properties of liquids and gases, discussion of reservoirs, canals, piers and harbours, the theory of the sailing boat, water pump and air pump, in addition to acoustics and optics. The third set of 20 cov- ered astronomy, gravitation, tides, cohesion, heat, elec- tricity, climate nd wind, vegetation, and animal ife. According to a 1934 article in Nature ‘The greatest and most srcinal of all general lecture courses was Young‘s “Lectures of Natural Philosophy and the Mechanical Arts” ,7 Between 1817 and 1825, a number of supplements to the fourth edition of Encyclopaedia Britannica were pub- lished, and Young was a major contributor to these. He cov- 8 Fig 5 Forerunner of recording barometer Fig 6 Thomas Young ered annuities, bathing, bridge, carpentry, chromatics, cohesion, clouble refraction, Egypt, fluents integrals), Herculaneum, hydraulics and tides, languages, life pre- servers, roadmaking, steam engine, weights and meas- ures, as well as 45 biographies of leading scholars, with bibliographies of their work. He was asked t do articles The Structural ngineer  PAPER: Beal on ‘blasting and oring’ and ‘mining and stone cutting’ ut turned these down as he felt hat he did not know enough about them. As well as his work as a doctor and all this theoretical work and writing, he worked on the theory of mortality tables for life assurance and was employed by what is ow Eagle Star insurance company as inspector of Calcula- tions. He also found time to serve as Editor of Nautical Almanac Secretary of the Board of Longitude, and Secretary of the Parliamentary Commission which efined British standards of measurement. To cover all Thomas Young’s attainments properly would take far more than this short paper, so I have selected some of the more interesting examples, which give nsights into his approach and methods. It should be mentioned that, although referred o only occasionally n biographies, Young’s wife Eliza was clearly a highly intelligent person in her own right and took a close interest in his work.) Wave theory of light Young’s work on light is one of the few achievements for Light source Slits Screen Bright Fig 7. Interference: Young’s double-slit method Dark Bright Dark Bright Fig 8. Ripple tank Fig 9 he eye which he received proper recognition. He worked out the basic theory and devised the key experiment that proved that Newton’s corpuscular theory of light was wrong and that light is, in fact, a orm of wave energy. The Young’s Slits’ experiment requires recision and care to carry out, but it is wonderfully simple and decisive see Fig 7). If light is a stream of particles, there should be just two bright lines, one in line with each slit. However, instead of two bright bands, when light passes through Young’s Slits’, a pattern of light and dark bands is seen: roof that waves are spreading out from the two slits and interfering with each other as they cross. Not only did this experiment prove Young’s wave theory of light, but he also noticed that the pacing of the fringes varied with he colour of the light: he therefore used this to calculate correctly) the actual wavelengths of light of different colours and cor- rectly inferred hat the adiation continued o higher and lower wavelengths in he infrared and ultraviolet ranges. His interest in wave behaviour led him also to invent the ripple tank, still used today to analyse and illustrate wave behaviour. Fig 8 shows his design: ‘An apparatus for observing the motions of waves excit- ed, in a luid poured into the trough AB by the vibrations of the elastic wire C, loaded with the movable weight D; the shadow of the waves being thrown on a screen E and by the lamp F, through the bottom of the trough, which is of glass,. It is a simple, classic iece of engineering which seems as right for its purpose today as when it was invented. The eye Young‘s paper ‘The mechanism of the eye’, presented to the Royal Society in 1800, s one of his most notable achieve- ments, laying the foundations for most of what we know about the operation of the eye Fig 9 . In his researches or it, he devised a new form of optometer, which measures the focusing of the eye; he carried out direct measurements n the dimensions of his own eyeballs which must have een very difficult and uncomfortable); and, along he way, he discovered astigmatism, which gives different focus to hor- izontal and vertical lines. Young also established conclusively for the first time / A s the outer transparent cover, the cornea B s a space filled with a fluid called the aqueous humour s the crystalline ens an elastic, jelly-like body D s a space filled with another fluid, the vitreous humour E s the sensitive coating of the inside of the eye, the retina F s the pupil an adjustable diaphragm which regulates the amount of light enetering the system G are the ciliary muscles which adjust the power of the lens, but whose function was unknown to Young H s the blindspot where the optic nerve enters the retina Volume 78/No 74 78 luly 2000 9  PAPER: Beal how the eye focuses. His experiment on its focusing gives Sighting an interesting insight into his methods. t the time, there holes was dispute about whether the eye focused by changing -----_____ crystalline lens. How could these two effects be distin- guished, as both are lenses and both refract the light? Young reasoned that, if the eye was immersed in water, ts cornea would then have water on both sides of it, which Apparent the curvature f the cornea or by hanging the shape f the ----____ -----____ ___--- _____------ .__ would neutralise its contribution. He found that, if he immersed his eye in water, he could not focus sharply, which is why we do not see very well underwater. However, he then tried holding a lens equal in ower to the cornea in the water in front of his eye, and found that he could once again focus accurately on objects, even though the water was stopping his cornea having any effect on the light. In this ay he proved that it must e the crystalline lens that does the focusing. In 1801, he presented ‘On the Theory of Light and Colours’. This established the basis of the modern under- standing of colours and colour vision, where all colours are based on the three primary olours of red, green, and blue: ‘Now, as it is lmost impossible to conceive each sensi- tive point of the retina to contain an infinite number of particles, each capable f vibrating in erfect unison with every possible undulation, it becomes necessary to suppose the number limited, for instance, to the three principal colours ... and each sensitive filament of the nerve may consist of three portions, one for each principal c010ur.’~ Young’s proposal is based essentially on considerations of engineering simplicity. His theory not only explains colour and its erception, it also provided an obvious expla- nation for the mystery of colour blindness: if one set of nerves is paralysed or absent, only two of the three pri- mary colours can be recognised. Many years later, the great James Clerk Maxwell wrote: ‘It seems almost a truism o say that colour is a sensa- tion; and yet Young, by honestly recognising this elemen- tary truth, stablished the first onsistent theory f colour. So far as I know, Thomas Young was the first who, start- ing from the well-known fact that there are three primary colours, sought for the explanation of this fact, not in he nature of light, but in the onstitution of man.’g In fact, Newton thought here were seven primary colours, and it was Young who proposed reducing this to three. Young’s theory of colours was not widely noticed at the time, and it was only when Helmholtz rediscovered it, nearly 50 years later, that it gained general currency. Eriometer/size of blood corpuscle When a point source of light is viewed through a cloud of small drops or particles, coloured rings are seen around t, and the radius f these varies inversely with he average size of the particles. Young took advantage of this effect to make an ingenious instrument called the eriometer, which he used to measure he size of very small particles. The eriometer effectively measures the angle to these rings formed by light shone through a sample. The meas- ured angle can hen be compared with he angle generat- ed by a sample of known size. This allows the size of the particles to be calculated. Its operation is illustrated in Fig 10. Using it, Young measured the size of blood corpuscles as 0*000278in, or 7.1pm. This was the first time this had ever been measured, and his figure compares with the modern textbook value of 7.8pm. Hydraulics In 1808, Young presented a paper entitled ‘Hydraulic investigations’ to the Royal Society, which included new formula for he flow of fluids in pipes, the resistance to flow caused by bends, and he propagation of an impulse through an elastic tube. He had investigated these things in preparation or his paper On the functions of hearts and Distance can be varied to match different sizes of halo Fig 10. Diagram of arteries’ ater hat year. The prevailing view of the time eriometer was that contraction of the walls of arteries was an impor- tant cause of the circulation of blood in the human body, but Young’s paper conclusively disproved this idea, open- ing the way for the correct solution. Egypt The Rosetta Stone was discovered in 1799 and its Greek section translated by 1802. However, after that, progress was slow: by 1814, scholars ad made out only a few words of the ancient Egyptian demotic script, and they had ade no progress at all on the hieroglyphs. Young became nter- ested and set to work on it in the summer of 1814. He decided to attack the demotic script first, and his account of his methodlo is interesting. e noticed hat the Greek section had two references to Alexander and Alex- andria, so he first looked in the demotic script for two well-marked groups of characters resembling each other. Then he looked or a small group of characters that occurred in almost every line, which he thought would probably signify ‘and’. He then noticed a group of charac- ters in the demotic script which appeared about 30 times, and reasoned hat this must correspond to the only equal- ly common word n the Greek, which was ‘king’. y a sim- ilar process, he identified ‘Ptolemy’. He hen wrote out the lines of the two texts side by side, used the words he had identified as markers o divide up the text, and ompared the words in between. Young’s approach was simple, ractical, and effective. By October 1814 he had translated more of the demotic script than anybody else nd he was starting o look at the iero- glyphic section. He noticed strong resemblances between some of the hieroglyphs and some of the demotic charac- ters and deduced that the demotic script must have been derived srcinally from the hieroglyphic. He worked out that some of the hieroglyphic signs represented objects, but that others might be used alphabetically to represent sounds, and proceeded to identify and decode the name ‘Ptolemy’. He hen proceeded to identify other names and used these to help him deduce he meaning of symbols and work out more of the principles on which the script was based. His wrote up his findings in 1818, and they were published in his article ‘Egypt’ in Encyclopaedia Britan nica in 1819. There he showed that he had iscovered the principles of the script and its numerical notation, cor- rectly identified many names, and rovided a hieroglyph- ic vocabulary of 218 words. He had cracked the code and opened the way for others to work out the complete lan- guage. In later years, udgell described Young’s article as ‘practically, he foundation of the science of Egyptology’. Often the credit for decoding Egyptian hieroglyphs is given to the French scholar Champollion, who prepared the first omprehensive hieroglyphic dictionary. However, Champollion made no progress at all until he met oung in 1822,4 years after oung’s srcinal article was written. All his important published work appears to have been done after that date. In later years, Young concentrated his attention on the Egyptian demotic script, and he was completing the first dictionary for this at the ime of his death. 30 The Structural Engineer
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