Cgs

The centimetre-gram-second system (CGS) is a system of physical units. It is always the same for mechanical units, but there are several variants of electric additions. The system goes back to a proposal made in 1832 by the German mathematician Carl Friedrich Gauss and was in 1874 extended by the British physicists James Clerk Maxwell and William Thomson with a set of electromagnetic units. The sizes (order of magnitude) of many CGS units turned out to be inconvenient for practical purposes, therefore the CGS system never gained wide general use outside the field of electrodynamics and was gradually superseded internationally starting in the 1880s but not to a significant extent until the mid-20th century by the more practical MKS (metre-kilogram-second) system, which led eventually to the modern SI standard units. CGS units are still occasionally encountered in older technical literature, especially in the United States in the fields of electrodynamics and astronomy. SI units were chosen such that electromagnetic equations concerning spheres contain 4π, those concerning coils contain 2π and those dealing with straight wires lack π entirely, which was the most convenient choice for electrical-engineering applications. In those fields where formulas concerning spheres dominate (for example, astronomy), it has been argued that the CGS system can be notationally slightly more convenient. Starting from the international adoption of the MKS standard in the 1940s and the SI standard in the 1960s, the technical use of CGS units has gradually disappeared worldwide, in the United States more slowly than in the rest of the world. CGS units are today no longer accepted by the house styles of most scientific journals, textbook publishers and standards bodies. The units gram and centimetre remain useful within the SI, especially for instructional physics and chemistry experiments, where they match well the small scales of table-top setups. In these uses, they are occasionally referred to as the system of “LAB” units. However, where derived units are needed, the SI ones are generally used and taught today instead of the CGS ones.

Electromagnetic units

While for most units the difference between cgs and SI is a mere power of 10, the differences in electromagnetic units are considerable; so much so that formulas for physical laws need to be changed depending on what system of units one uses. In SI, electric current is defined via the magnetic force it exerts and charge is then defined as current multiplied with time. In one variant of the cgs system, electrostatic units (esu), charge is defined via the force it exerts on other charges, and current is then defined as charge per time. One consequence of this approach is that Coulomb’s law does not contain a constant of proportionality. While the proportional constants in cgs simplify theoretical calcuations, they have the disadvantage that the units in cgs are hard to define through experiment. SI on the other hand starts with a unit of current, the ampere which is easy to determine through experiment, but which requires that the constants in the electromagnetic equations take on odd forms. Ultimately, relating electromagnetic phenomena to time, length and mass relies on the forces observed on charges. There are two fundamental laws in action: Coulomb’s law, which describes the electrostatic force between charges, and Ampère’s law (also known as the Biot-Savart law), which describes the electrodynamic (or electromagnetic) force between currents. Each of these includes one proportionality constant, k₁ or k₂. The static definition of magnetic fields yields a third proportionality constant, α. The first two constants are related to each other through the speed of light, c (the ratio of k₁ over k₂ must equal c²). We then have several choices: There were at various points in time about half a dozen systems of electromagnetic units in use, most based on the cgs system. These include electromagnetic units (emu, chosen such that the Biot-Savart law has no constant of proportionality), Gaussian, and Heaviside-Lorentz units. A key virtue of the Gaussian CGS system is that electric and magnetic fields have the same units, both

\epsilon_0

and

\mu_0

are

1

, and the only dimensional constant appearing in the equations is

c

, the speed of light. The Heaviside-Lorentz system has these desirable properties as well, but is a "rationalized" system (as is SI) in which the charges and fields are defined in such a way that there are many fewer factors of

4 \pi

appearing in the formulas, and it is in Heaviside-Lorentz units that the Maxwell equations take their simplest possible form. Further complicating matters is the fact that some physicists and engineers in the United States use hybrid units, such as volts per centimetre for electric field. However, this also can be seen more as an application of the previously described "LAB" units usage since electric fields near small circuit devices would be measured across distances on the order of magnitude of 1 centimetre. The mantissas derived from the speed of light are more precisely 299792458, 333564095198152, 1112650056, and 89875517873681764. A centimetre of capacitance is the capacitance between a sphere of radius 1 cm in vacuum and infinity. The capacitance C between two spheres of radii R and r is :

\frac

. By taking the limit as R goes to infinity we see C equals r.

Units of measurement

Introduction

The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. Disparate systems of measurement used to be very common. Now there is a global standard, the International System (SI) of units, a form of metric system. The SI has been or is in the process of being adopted throughout the world. The United States of America is almost certainly the last to adopt the system but even there it is increasingly being used. Standards are very important. Each unit is a set size. A distance or length or volume or mass or span of time being measured is described as a certain number of these units. A measurement may be quoted to a certain degree of accuracy. One example of the importance of agreed units is the failure of the NASA Mars Climate Orbiter, which was accidentally destroyed on a mission to the planet Mars in September 1999 instead of entering orbit, due to miscommunications about the value of forces: different computer programs used different units of measurement (newton versus pound force). Enormous amounts of effort, time and money were wasted. In physics and metrology units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method. A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures developed long ago for commercial purposes. Science, medicine and engineering often use larger and smaller units of measurement than those used in day to day life and talk about them more exactly. The judicious selection of the units of measure can aid researchers in both framing and solving the problem.

History

Units of measurement were among the earliest tools invented by humans. Primitive societies needed rudimentary measures for many tasks: constructing dwellings of an appropriate size and shape, fashioning clothing, or bartering food or raw materials. The earliest known uniform systems of weights and measures seem to have all been created sometime in the 4th and 3rd millennia BC among the ancient peoples of Mesopotamia, Egypt and the Indus Valley, and perhaps also Elam in Persia as well. Many systems were based on the use of parts of the body and the natural surroundings as measuring instruments. Our present knowledge of early weights and measures comes from many sources.

Systems of measurement

A number of metric systems of units have evolved since the adoption of the original metric system in France in 1791. The current international standard metric system is the International system of units. Prior to the global adoption of the metric system many different systems of measurement had been in use. Many of these were related to some extent or other. Often they were based on the dimensions of the human body. Both the Imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the British Commonwealth and the former British Empire. US customary units are the main system of measurement in the United States however some steps towards metrication have been made. The above systems of units are based on arbitrary unit values, formalised as standards. Some unit values occur naturally in science. Systems of units based on these are called natural units. Similar to natural units, atomic units (au) are a convenient system of units of measurement used in atomic physics. Also a great number of strange and non-standard units may be encountered. These may include: the ton of TNT, the Hiroshima atom bomb and the weight of an elephant.

Base and derived units

Different systems of units are based on different choices of a set of fundamental units. The most widely used system of units is the International System of Units, or SI. There are seven SI base units. All other SI units can be derived from these base units. For most quantities a unit is absolutely necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given. But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the base units. Other units are derived units. Derived units are a matter of convenience, as they can be expressed in terms of basic units. Which units are considered base units is a matter of choice. The base units of SI are actually not the smallest set. Smaller sets have been defined. There are sets in which the electric and magnetic field have the same unit. This is based on physical laws that show that electric and magnetic field are actually different manifestations of the same phenomenon. In some fields of science such systems of units are highly favoured over the SI system.

Calculations with units

Units as dimensions

Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Q is written as the product of a unit [Q] and a numerical factor: :

Q = n \times [Q] = n [Q]

The multiplication sign is usually left out, just as it is left out between variables in scientific notation of formulas. In formulas the unit [Q] can be treated as if it was a kind of physical dimension: see dimensional analysis for more on this treatment. A distinction should be made between units and standards. A unit is fixed by its definition, and is independent of physical conditions such as temperature. By contrast, a standard is a physical realization of a unit, and realizes that unit only under certain physical conditions. For example, the metre is a unit, while a metal bar is a standard. One metre is the same length regardless of temperature, but a metal bar will be one metre long only at a certain temperature.

Guidelines

- Treat units like variables. Only add like terms. When a unit is divided by itself, the division yields a unitless one. When two different units are multiplied, the result is a new unit, referred to by the combination of the units. For instance, in SI, the unit of speed is metres per second (m/s). See dimensional analysis. A unit can be multiplied by itself, creating a unit with an exponent (e.g. m²/s^{2^{).

- Some units have special names, however these should be treated like their equivalents. For example, one newton (N) is equivalent to one kg m/s². This creates the possiblity for units with multiple designations, for example: the unit for surface tension can be referred to as either N/m (newtons per metre) or kg/s² (kilograms per second squared).

- Don't let definitions like density is mass per unit volume obscure your understanding of units. It sounds as if it says:

:D = m/m^{3 (mass divided by the unit of volume) (WRONG)

This is not true. The correct statement is that density is mass divided by volume:

:D = m/V (mass divided by volume, both variables)

Expressing a physical value in terms of another unit

Conversion of units involves comparison of different standard physical values, either of a single physical quantity or of a physical quantity and a combination of other physical quantities.

Starting with:

: $Q = n_i \times [Q]_i$

just replace the original unit [Q]_i with its meaning in terms of the desired unit $[Q]_f$ , e.g. if $[Q]_i = c_ij - [Q]_f$ , then:

: $Q = n_i \times c_ij \times [Q]_f$

Now $n_i$ and $c_ij$ are both numerical values, so just calculate their product.

Or, which is just mathematically the same thing, multiply Q by unity, the product is still Q:

: $Q = n_i \times [Q]_i \times ( c_ij \times [Q]_f/[Q]_i )$

For example, you have an expression for a physical value Q involving the unit feet per second ( $[Q]_i$ ) and you want it in terms of the unit miles per hour ( $[Q]_f$ ):

Find facts relating the original unit to the desired unit:

:1 mile = 5280 feet and 1 hour = 3600 seconds

Next use the above equations to construct a fraction that has a value of unity and that contains units such that, when it is multiplied with the original physical value, will cancel the original units:

:1 = (1 mile) / (5280 feet) and 1 = (3600 seconds) / (1 hour)

Last, multiply the original expression of the physical value by the fraction, called a conversion factor, to obtain the same physical value expressed in terms of a different unit. Note: since the conversion factors have a numerical value of unity, multiplying any physical value by them will not change that value.

See also

- Metric system

- SI

- Natural units

- Planck units

- Geometrized units

- Atomic units

- Imperial unit

- US customary units

- List of strange units of measurement

- Conversion of units

- Historical weights and measures

- History of measurement

- International standard ISO 31: Quantities and units

- Units (computer program)

- Weights and measures

- Mesures usuelles

- Metrified English unit

- CODATA

- Metrication

- Metric system in the United States

- Metrology

- UTC (Coordinated Universal Time)

- Binary Prefixes - used to quantify large amounts of computer data

- Orders of magnitude

- ISO 31 - style guide for units of measurement

External links

- [http://www.eppo.go.th/ref/UNIT-OIL.html Unit Conversion : Oil Industry Conversions]

- [http://www.bipm.org/en/si Official SI website]

- [http://www.hmso.gov.uk/si/si1995/Uksi_19951804_en_2.htm UK - Units of Measurement Regulations 1995]

- [http://laws.justice.gc.ca/en/w-6/108519.html Canada - Weights and Measures Act 1970-71-72]

- [http://193.120.124.98/gen531996a.html Ireland - Metrology Act 1996]

- [http://www.ukma.org.uk UK Metric Association]

- [http://lamar.colostate.edu/~hillger US Metric Association]

- [http://aurora.rg.iupui.edu/UCUM/ucum.html The Unified Code for Units of Measure (UCUM)]

Converters

- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units]

- [http://www.santos.com/ConversionCalculator.aspx?p=73 Santos - Conversion Calculator]

- [http://aurora.regenstrief.org/~schadow/units Regenstrief Unit Converter (for UCUM)]

References
Appendix B of NIST Handbook 44, 2002 Edition

Category:Measurement

-

-
Category:Systems of units
Category:Units of length
Category:Units of area
Category:Units of volume
Category:Units of mass

ja:物理単位

1832

1832 was a leap year starting on Sunday (see link for calendar).

Events

- February 12 - Ecuador annexes the Galapagos Islands

- February 12 – serious cholera epidemic begins in London from the East London. It is declared officially over in early May but deaths continue. At least 3000 victims

- March 24 - In Hiram, Ohio a group of men beat, tarred and feathered Mormon leader Joseph Smith, Jr.

- April 6 - The Black Hawk War begins

- May 7 - The Treaty of London creates an independent Kingdom of Greece. Otto of Wittelsbach, Prince of Bavaria is chosen King.

- May 11 - Greece is recognized as a sovereign nation - Treaty of Constantinople ends the Greek War of Independence next July

- May 27 - War between the Ottoman Empire and Egypt. The Egyptians, aided by Maronites, seize Acre after a seven-month siege

- May 30 - In the German town of Hambach, a demonstration for civil liberties and against the sectionalism that has prevailed in Germany since the Thirty Years War ends with no result.

- June 4 - The Great Reform Bill becomes law in the U.K.

- June 5 - anti-monarchist riot briefly breaks out in Paris

- June 15 - Seizure of Damascus by Egyptian forces

- July 4 - University of Durham founded, the first in England since 1209.

- July 9 - Republic of Indian Stream comes into its brief existence (until 1835)

- July 10 - President Andrew Jackson vetoes a bill that would re-charter the Second Bank of the United States.

- July 24 - Benjamin Bonneville leads the first wagon train across the Rocky Mountains by using Wyoming's South Pass.

- October 8 - Washington Irving and Henry Leavitt Ellsworth arrive at Fort Gibson, I.T. in the late morning hours. They left the fort on October 10, with a small company of Rangers who escorted them to the camp of Captain Jesse Bean who was waiting for them near the Arkansas River. Thus began one of the first steps in the United States effort to remove the Indians from their homes on the east coast in what would become known as the "Trail of Tears" some six years later.

- November - Andrew Jackson defeats Henry Clay in the U.S. presidential election

- December - Skull and Bones secret society of Yale University established.

- December 21 - Battle of Konya. The Egyptians defeat the main Ottoman army in Central Anatolia.

- December 28 - John C. Calhoun becomes the first Vice President of the United States to resign.

- Cholera epidemic in France

- In July and August there is a cholera epidemic in New York City

Births

- January 6 - Gustave Doré, French painter and sculptor (d. 1883)

- January 13 - Horatio Alger, Jr., American Unitarian minister and author (d. 1899)

- January 23 - Edouard Manet, French painter (d. 1883)

- January 27 - Lewis Carroll, English author (d. 1898)

- April 19 - José Echegaray y Eizaguirre, Spanish writer, Nobel Prize laureate (d. 1916)

- May 14 - Charles Peace, British criminal (d. 1879)

- May 28 - Tony Pastor, American vaudeville and theater impresario (d. 1908)

- June 17 - Sir William Crookes, English chemist and physicist (d. 1919)

- July 6 - Emperor Maximilian I of Mexico (d. 1867)

- October 2 - Edward Burnett Tylor, English anthropologist (d. 1917)

- August 8 - King Georg I of Saxony (d. 1904)

- November 29 - Louisa May Alcott, American author (d. 1888)

- December 8 - Bjørnstjerne Bjørnson, Norwegian author, Nobel Prize laureate (d. 1910)

- December 15 - Gustave Eiffel, French engineer (d. 1923)

Deaths

- March 4 - Jean-François Champollion, French Egyptologist (b. 1790)

- March 10 - Muzio Clementi, Italian composer (b. 1752)

- March 13 - Samuel Eells, Founder of Alpha Delta Phi Fraternity (b. 1810)

- March 22 - Johann Wolfgang von Goethe, German writer (b. 1749)

- May 13 - Georges Cuvier, French zoologist (b. 1769)

- June 6 - Jeremy Bentham, English philosopher (b. 1748)

- June 23 - James Hall, Scottish geologist (b. 1761)

- September 2 - Franz Xaver, Baron von Zach, Austrian scientific editor and astronomer (b. 1754)

- September 21 - Sir Walter Scott, Scottish writer (b. 1771)

- November 14 - Charles Carroll of Carrollton, Declaration of Independence signer and U.S. Senator (b. 1737)

Category:1832

ko:1832년

ms:1832

simple:1832

1874

1874 was a common year starting on Thursday (see link for calendar).

Events
January - April

- January 1 - New York City annexes The Bronx

- January 23 - Marriage of the Duke of Edinburgh, second son of Queen Victoria, to Grand Duchess Marie Alexandrovna of Russia, only daughter of Emperor Alexander III of Russia.

- January 23 - Camille Saint-Saëns' composition Danse Macabre is premiered.

- January - Signing of the Pangkor Treaty (also known as the Pangkor Engagement), by which the British extended their control over, first the Sultanate of Perak and later the other independent Malay States.

- February 21 - The Oakland Daily Tribune publishes its first newspaper.

- February 23 - Walter Clopton Wingfield patents a game called "sphairistike" which is more commonly called lawn tennis.

- March 18 - Hawaii signs a treaty with the United States granting exclusive trading rights.

- March - founding of a Young Men's Hebrew Association in Manhattan which still operates today as the 92nd Street Y

May - August

- 9 May - The first horse drawn carriage made its début in the city of Mumbai, plying on two routes.

- May 20 - Levi Strauss and Jacob Davis receive a US patent for blue jeans with copper rivets

- July 1 - the first public zoo in the U.S. opens, at Philadelphia.

- July 24 - Mathew Evans and Henry Woodward patent the first incandescent lamp with an electric light bulb.

September - December

- October 19 - modern University of Zagreb founded in Zagreb

- November 7 - A cartoon by Thomas Nast in Harper's Weekly, is considered the first important use of an elephant as a symbol for the United States Republican Party [http://www.harpweek.com/09Cartoon/CartoonOfTheDay.asp?Year=2003?Month=November?Date=7].

- November 10 - John Ernst Worrell Keely demonstrates his "induction resonance motion motor" (later investigation reveals fraud behind another perpetual motion machine)

- November 25 - The United States Greenback Party is established as a political party made primarily of farmers financially hurt by the Panic of 1873.

Unknown date

- Iceland is granted a constitution and limited home rule.

- Home Rule Movement created to protest British Government control over Ireland. (see History of Ireland)

- First Impressionist exhibition, Paris; name coined in hostile review of Claude Monet's Impression, Sunrise

- Opening of the Agra canal in India.

- Charles Russell's Bible Students group (Now known as Jehovah's Witnesses) first claims this year to be the invisible return of Jesus Christ to earth, before shifting to the currently believed year of 1914.

Births
January to June

- January 1 - Gustav Albin Weißkopf , German-American aviation pioneer (d. 1927)

- January 4 - Josef Suk, Czech composer and violinist (d. 1935)

- January 5 - Joseph Erlanger, American physiologist, Nobel Prize laureate (d. 1965)

- January 16 - Robert W. Service, American poet (d. 1958)

- January 20 - Steve Bloomer, English footballer, cricketer and baseball player (d. 1938)

- January 21 - Frederick Madison Smith, American religious leader and author (d. 1946)

- January 25 - William Somerset Maugham, English author (d. 1965)

- January 29 - John D. Rockefeller Jr., American entrepreneur (d. 1960)

- February 1 - Hugo von Hofmannsthal, Austrian writer (d. 1929)

- February 3 - Gertrude Stein, American writer and patron of the arts (d. 1946)

- February 9 - Amy Lowell, American poet (d. 1925)

- February 11 - Elsa Beskow, Swedish writer (d. 1953)

- February 11 - Fritz Bennicke Hart, English-born Australian composer (d. 1949)

- February 15 - Sir Ernest Shackleton, Irish explorer (d. 1922)

- February 17 - Thomas J. Watson, American computer pioneer (d. 1956)

- February 24 - Honus Wagner, Baseball Hall of Famer (d. 1955)

- March 20 - Börries von Münchhausen, German poet (d. 1945)

- March 24 - Harry Houdini, Hungarian-American magician (d. 1926)

- March 26 - Robert Frost, American poet (d. 1963)

- March 29 - Lou Hoover, First Lady of the United States (d. 1944)

- April 8 - Stanisław Taczak, Polish general, commander-in-chief of the Greater Poland Uprising (1918-1919) against the Germans (d.1960)

- April 15 - Johannes Stark, German physicist, Nobel Prize laureate (d. 1957)

- April 19 - Ernst Rudin, Swiss psychiatrist and geneticist (d. 1952)

- April 25 - Guglielmo Marconi, Italian inventor, recipient of the Nobel Prize in Physics (d. 1937)

- May 3 - François Coty, French perfume manufacturer (d. 1934)

- May 9 - Howard Carter, British archaeologist (d. 1939)

- May 14 - Polaire, French actress and singer (d. 1939)

- May 19 - Gilbert Jessop; English cricketer (d. 1955).

- May 29 - Gilbert Keith Chesterton, English author (d. 1936)

- June 11 - Lyman Gilmore, American aviation pioneer (d. 1951)

- June 16 - Arthur Meighen, ninth Prime Minister of Canada (d. 1960)

July to December

- July 14 - Abbas II, last khedive of Egypt (d. 1944)

- July 26 - Serge Koussevitsky, Russian conductor (d. 1951)

- July 29 - J.S Woodsworth, Canadian politician (d. 1942)

- August 6 - Charles Fort, writer and researcher into anomalous phenomena

- August 27 - Carl Bosch, German chemist, Nobel Prize laureate (d. 1940)

- September 13 - Arnold Schoenberg, Austrian composer (d. 1951)

- September 21 - Gustav Holst, English composer (d. 1934)

- October 20 - Charles Ives, American composer (d. 1954)

- October 26 - Martin Lowry, English chemist (d. 1936)

- November 15 - August Krogh, Danish zoophysiologist, recipient of the Nobel Prize in Physiology or Medicine (d. 1949)

- November 29 - Egas Moniz, Portuguese physician and neurologist, recipient of the Nobel Prize in Physiology or Medicine (d. 1955)

- November 30 - Sir Winston Churchill, Prime Minister of the United Kingdom, recipient of the Nobel Prize in Literature (d. 1965)

- November 30 - Lucy Maude Montgomery, Canadian author (d. 1942)

- December 13 - Josef Lhévinne, Russian pianist (d. 1944)

- December 17- William Lyon Mackenzie King, Prime Minister of Canada (d. 1950)

- December 22 - Franz Schmidt, Austrian composer (d. 1939)

Deaths

- January 8 - Abbé Charles-Étienne Brasseur de Bourbourg, French writer and historian (b. 1814)

- January 19 - August Heinrich Hoffmann von Fallersleben, German poet (b. 1798)

- February 8 - David Friedrich Strauss, German theologian (b. 1808)

- March 8 - Millard Fillmore, 13th President of the United States (b. 1800)

- June 20 - John Ruggles, American politician

- June 21 - Anders Jonas Ångström, Swedish physicist (b. 1814)

- July 24 - Gijsbert Haan, Dutch-American religious leader (b. 1801)

- October 6 - Samuel M. Kier, American oil magnate (b. 1813)

- December 7 - Constantin von Tischendorf, German Biblical scholar (b. 1815)

Category:1874

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th:พ.ศ. 2417

William Thomson, 1st Baron Kelvin

The Right Honourable William Thomson, 1st Baron Kelvin, GCVO, OM, PC, PRS (26 June 1824–17 December 1907) was a Scottish-Irish mathematical physicist and engineer, an outstanding leader in the physical sciences of the 19th century. He did important work in the mathematical analysis of electricity and thermodynamics, and did much to unify the emerging discipline of physics in its modern form. He is also credited for the discovery of the atom.

He also enjoyed a second career as a telegraph engineer and inventor, a career that propelled him into the public eye and ensured his fame and honour.

Early life and work
Family
William's father was Dr. James Thomson, the son of a Belfast farmer. James received little youthful instruction in Ireland but, when 24 years old, started to study for half the year at the University of Glasgow, Scotland, while working as a teacher back in Belfast for the other half. On graduating, he became a mathematics teacher at the Royal Belfast Academical Institution. He married Margaret Gardner in 1817 and, of their children four boys and two girls survived infancy.

William, and his elder brother James, were tutored at home by their father while the younger boys were tutored by their elder sisters. James was intended to benefit from the major share of his father's encouragement, affection and financial support and was prepared for a fashionable career in engineering. However, James was a sickly youth and proved unsuited to a sequence of failed apprenticeships. William soon became his father's favourite.

In 1832, the father was appointed professor of mathematics at Glasgow and the family relocated there in October 1833. The Thomson children were introduced to a broader cosmopolitan experience than their father's rural upbringing, spending the summer of 1839 in London and, the boys, being tutored in French in Paris. The summer of 1840 was spent in Germany and the Netherlands. Language study was given a high priority.

Youth
William began study at Glasgow University in 1834 at the age of 10, not out of any precociousness; the University provided many of the facilities of an elementary school for abler pupils and this was a typical starting age. In 1839, John Pringle Nichol, the professor of astronomy, took the chair of natural philosophy. Nichol updated the curriculum, introducing the new mathematical works of Jean Baptiste Joseph Fourier. The mathematical treatment much impressed Thomson.

In the academic year 1839-1840, Thomson won the class prize in astronomy for his Essay on the figure of the Earth which showed an early facility for mathematical analysis and creativity. Throughout his life, he would work on the problems raised in the essay as a coping strategy at times of personal stress.

Thomson became intrigued with Fourier's Théorie analytique de la chaleur and committed himself to study the "Continental" mathematics resisted by a British establishment still working in the shadow of Sir Isaac Newton. Unsurprisingly, Fourier's work had been attacked by domestic mathematicians, Philip Kelland authoring a critical book. The book motivated Thomson to write his first published scientific paper under the pseudonym P.Q.R., defending Fourier, and submitted to the Cambridge Mathematical Journal by his father. A second P.Q.R paper followed almost immediately.

While vacationing with his family in Lamlash in 1841, he wrote a third, more substantial, P.Q.R. paper On the uniform motion of heat in homogeneous solid bodies, and its connection with the mathematical theory of electricity. In the paper he made remarkable connections between the mathematical theories of heat conduction and electrostatics, an analogy that James Clerk Maxwell was ultimately to describe as one of the most valuable science-forming ideas.
Cambridge
William's father was able to make a generous provision for his favourite son's education and, in 1841, installed him, with extensive letters of introduction and ample accommodation, at Peterhouse, Cambridge. In 1845 Thomson graduated as second wrangler. However, he won a Smith's Prize, sometimes regarded as a better test of originality than the tripos. Robert Leslie Ellis, one of the examiners, is said to have declared to another examiner You and I are just about fit to mend his pens.

While at Cambridge, Thomson was active in sports and athletics. He won the Silver Sculls, and rowed in the winning boat of the Oxford and Cambridge Boat Race. He also took a lively interest in the classics, music, and literature; but the real love of his intellectual life was the pursuit of science. The study of mathematics, physics, and in particular, of electricity, had captivated his imagination.

In 1845 he gave the first mathematical development of Faraday's idea that electric induction takes place through an intervening medium, or "dielectric", and not by some incomprehensible "action at a distance". He also devised a hypothesis of electrical images, which became a powerful agent in solving problems of electrostatics, or the science which deals with the forces of electricity at rest. It was partly in response to his encouragement that Faraday undertook the research in September of 1845 that led to the discovery of the Faraday effect, which established that light and magnetic (and thus electric) phenomena were related.

On gaining a fellowship at his college, he spent some time in the laboratory of the celebrated Henri Victor Regnault, at Paris; but in 1846 he was appointed to the chair of natural philosophy in the University of Glasgow. At twenty-two he found himself wearing the gown of a learned professor in one of the oldest Universities in the country, and lecturing to the class of which he was a freshman but a few years before.
Thermodynamics

philosophy
By 1847, Thomson had already gained a reputation as a precocious and maverick scientist when he attended the British Association for the Advancement of Science annual meeting in Oxford. At that meeting, he heard James Prescott Joule making yet another of his, so far, ineffective attempts to discredit the caloric theory of heat and the theory of the heat engine built upon it by Sadi Carnot and Émile Clapeyron. Joule argued for the mutual convertibility of heat and mechanical work and for their mechanical equivalence.

Thomson was intrigued but skeptical. Though he felt that Joule's results demanded theoretical explanation, he retreated into an even deeper commitment to the Carnot-Clapeyron school. He predicted that the melting point of ice must fall with pressure, otherwise its expansion on freezing could be exploited in a perpetuum mobile. Experimental confirmation in his laboratory did much to bolster his beliefs.

In 1848, he extended the Carnot-Clapeyron theory still further through his dissatisfaction that the gas thermometer provided only an operational definition of temperature. He proposed an absolute temperature scale in which a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T-1)°, would give out the same mechanical effect [work], whatever be the number T. Such a scale would be quite independent of the physical properties of any specific substance. By employing such a "waterfall", Thomson postulated that a point would be reached at which no further heat (caloric) could be transferred, the point of absolute zero about which Guillaume Amontons had speculated in 1702. Thomson used data published by Regnault to calibrate his scale against established measurements.

In his publication, Thomson wrote:

:"... the conversion of heat (or caloric) into mechanical effect is probably impossible, certainly undiscovered"

- but a footnote signaled his first doubts about the caloric theory, referring to Joule's very remarkable discoveries. Surprisingly, Thomson did not send Joule a copy of his paper but when Joule eventually read it he wrote to Thomson on October 6, claiming that his studies had demonstrated conversion of heat into work but that he was planning further experiments. Thomson replied on the 27th, revealing that he was planning his own experiments and hoping for a reconciliation of their two views.

Thomson returned to critique Carnot's original publication and read his analysis to the Royal Society of Edinburgh in January 1849, still convinced that the theory was fundamentally sound. However, though Thomson conducted no new experiments, over the next two years he became increasingly dissatisfied with Carnot's theory and convinced of Joule's. In February 1851 he sat down to articulate his new thinking. However, he was uncertain of how to frame his theory and the paper went through several drafts before he settled on an attempt to reconcile Carnot and Joule. During his rewriting, he seems to have considered ideas that would subsequently give rise to the second law of thermodynamics. In Carnot's theory, lost heat was absolutely lost but Thomson contended that it was "lost to man irrecoverably; but not lost in the material world". Moreover, his theological beliefs led to speculation about the heat death of the universe.

:"I believe the tendency in the material world is for motion to become diffused, and that as a whole the reverse of concentration is gradually going on - I believe that no physical action can ever restore the heat emitted from the sun, and that this source is not inexhaustible; also that the motions of the earth and other planets are losing vis viva which is converted into heat; and that although some vis viva may be restored for instance to the earth by heat received from the sun, or by other means, that the loss cannot be precisely compensated and I think it probable that it is under compensated."

Compensation would require a creative act or an act possessing similar power.

In final publication, Thomson retreated from a radical departure and declared "the whole theory of the motive power of heat is founded on ... two ... propositions, due respectively to Joule, and to Carnot and Clausius." Thomson went on to state a form of the second law:

:"It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects."

In the paper, Thomson supported the theory that heat was a form of motion but admitted that he had been influenced only by the thought of Sir Humphry Davy and the experiments of Joule and Julius Robert von Mayer, maintaining that experimental demonstration of the conversion of heat into work was still outstanding.

As soon as Joule read the paper he wrote to Thomson with his comments and questions. Thus began a fruitful, though largely epistolary, collaboration between the two men, Joule conducting experiments, Thomson analyzing the results and suggesting further experiments. The collaboration lasted from 1852 to 1856, its discoveries including the Joule-Thomson effect, and the published results did much to bring about general acceptance of Joule's work and the kinetic theory.

Transatlantic cable
kinetic theory

Calculations on data-rate
Though now eminent in the academic field, Thomson was obscure to the general public. In September 1852, he married childhood sweetheart Margaret Crum but her health broke down on their honeymoon and, over the next seventeen years, Thomson was distracted by her suffering. On October 16, 1854, Stokes wrote to Thomson to try to re-interest him in work by asking his opinion on some experiments of Michael Faraday on the proposed transatlantic telegraph cable.

:To understand the technical issues in which Thomson became involved, see Submarine communications cable: Bandwidth problems

Faraday had demonstrated how the construction of a cable would limit the rate at which messages could be sent — in modern terms, the bandwidth. Thomson jumped at the problem and published his response that month. He expressed his results in terms of the data rate that could be achieved and the economic consequences in terms of the potential revenue of the transatlantic undertaking. In a further 1855 analysis, Thomson stressed the impact that the design of the cable would have on its profitability.

Thomson contended that the speed of a signal through a given core was inversely proportional to the square of the length of the core. Thomson's results were disputed at a meeting of the British Association in 1856 by Wildman Whitehouse, the electrician of the Atlantic Telegraph Company. Whitehouse had possibly misinterpreted the results of his own experiments but was doubtless feeling financial pressure as plans for the cable were already well underway. He believed that Thomson's calculations implied that the cable must be "abandoned as being practically and commercially impossible."

Thomson attacked Whitehouse's contention in a letter to the popular Athenaeum magazine, pitching himself into the public eye. Thomson recommended a larger conductor with a larger cross section of insulation. However, he thought Whitehouse no fool and suspected that he may have the practical skill to make the existing design work. Thomson's work had, however, caught the eye of the project's undertakers and in December 1856, he was elected to the board of directors of the Atlantic Telegraph Company.

Scientist to engineer
Thomson became scientific adviser to a team with Whitehouse as chief electrician and Sir Charles Tilston Bright as chief engineer but Whitehouse had his way with the specification, supported by Faraday and Samuel F. B. Morse.

Thomson sailed on board the cable-laying ship Agamemnon in August 1857, with Whitehouse confined to land owing to illness, but the voyage ended after just 380 miles when the cable parted. Thomson contributed to the effort by publishing in the Engineer the whole theory of the stresses involved in the laying of a submarine cable, and showed that when the line is running out of the ship, at a constant speed, in a uniform depth of water, it sinks in a slant or straight incline from the point where it enters the water to that where it touches the bottom.

Thomson developed a complete system for operating a submarine telegraph that was capable of sending a character every 3.5 seconds. He patented the key elements of his system, the mirror galvanometer and the siphon recorder, in 1858.

However, Whitehouse still felt able to ignore Thomson's many suggestions and proposals. It was not until Thomson convinced the board that using a purer copper for replacing the lost section of cable would improve data capacity, that he first made a difference to the execution of the project.

The board insisted that Thomson join the 1858 cable-laying expedition, without any financial compensation, and take an active part in the project. In return, Thomson secured a trial for his mirror galvanometer, about which the board had been unenthusiastic, alongside Whitehouse's equipment. However, Thomson found the access he was given unsatisfactory and the Agamemnon had to return home following the disastrous storm of June 1858. Back in London, the board was on the point of abandoning the project and mitigating their losses by selling the cable. Thomson, Cyrus Field and Curtis M. Lampson argued for another attempt and prevailed, Thomson insisting that the technical problems were tractable. Though employed in an advisory capacity, Thomson had, during the voyages, developed real engineer's instincts and skill at practical problem-solving under pressure, often taking the lead in dealing with emergencies and being unafraid to lend a hand in manual work. A cable was finally completed in August 5.

Disaster and triumph
Thomson's fears were realised and Whitehouse's apparatus proved insufficiently sensitive and had to be replaced by Thomson's mirror galvanometer. Whitehouse continued to maintain that it was his equipment that was providing the service and started to engage in desperate measures to remedy some of the problems. He only succeded in fatally damaging the cable by applying 2,000 V. When the cable failed completely Whitehouse was dismissed, though Thomson objected and was reprimanded by the board for his interference. Thomson subsequently regretted that he had acquiesced too readily to many of Whitehouse's proposals and had not challenged him with sufficient energy.

A joint committee of inquiry was established by the Board of Trade and the Atlantic Telegraph Company. Most of the blame for the cable's failure was found to rest with Whitehouse. The committee found that, though underwater cables were notorious in their lack of reliability, most of the problems arose from known and avoidable causes. Thomson was appointed one of a five-member committee to recommend a specification for a new cable. The committee reported in October 1863.

In July 1865 Thomson sailed on the cable-laying expedition of the SS Great Eastern but the voyage was again dogged with technical problems. The cable was lost after 1,200 miles had been laid and the expedition had to be abandoned. A further expedition in 1866 managed to lay a new cable in two weeks and then go on to recover and complete the 1865 cable. The enterprise was now feted as a triumph by the public and Thomson enjoyed a large share of the adulation. Thomson, along with the other principals of the project, was knighted on November 10, 1866. Thomson's barony was named after the River Kelvin, which ran through the grounds of the University of Glasgow.

To exploit his inventions for signalling on long submarine cables, Thomson now entered into a partnership with C.F. Varley and Fleeming Jenkin. In conjunction with the latter, he also devised an automatic curb sender, a kind of telegraph key for sending messages on a cable.

Later expeditions
Thomson took part in the laying of the French Atlantic submarine communications cable of 1869, and with Jenkin was engineer of the Western and Brazilian and Platino-Brazilian cables, assisted by vacation student James Alfred Ewing. He was present at the laying of the Pará to Pernambuco section of the Brazilian coast cables in 1873.

Thomson's wife had died on June 17, 1870 and he resolved to make changes in his life. Already addicted to seafaring, in September he purchased a 126-ton schooner, the Lalla Rookh and used it as a base for entertaining friends and scientific colleagues.

In June 1873, Thomson and Jenkin were onboard the Hooper, bound for Lisbon with 2,500 miles of cable when the cable developed a fault. An unscheduled 16-day stop-over in Madeira followed and Thomson became good friends with Charles R. Blandy and his three daughters. On May 2 1874 he set sail for Madeira on the Lalla Rookh. As he approached the harbour, he signalled to the Blandy residence Will you marry me? and Fanny signalled back Yes. Thomson married Fanny, 13 years his junior, on June 24, 1874.

Other activities and contributions
1874
Thomson introduced a method of deep-sea sounding, in which a steel piano wire replaces the ordinary land line. The wire glides so easily to the bottom that "flying soundings" can be taken while the ship is going at full speed. A pressure gauge to register the depth of the sinker was added by Sir William.

About the same time he revived the Sumner method of finding a ship's place at sea, and calculated a set of tables for its ready application.

His most important aid to the mariner is, however, the adjustable compass, which he brought out soon afterwards. It is a great improvement on the older instrument, being steadier, less hampered by friction, and the deviation due to the ship's own magnetism can be corrected by movable masses of iron at the binnacle.

Sir William was an enthusiastic yachtsman. His interest in all things relating to the sea perhaps arose, or at any rate was fostered, by his experiences on the Agamemnon and the Great Eastern. Charles Babbage was among the first to suggest that a lighthouse might be made to signal a distinctive number by occultations of its light; but Sir William pointed out the merits of the Morse code for the purpose, and urged that the signals should consist of short and long flashes of the light to represent the dots and dashes.

Thomson did more than any other electrician up to his time to introduce accurate methods and apparatus for measuring electricity. As early as 1845 he pointed out that the experimental results of William Snow Harris were in accordance with the laws of Coulomb. In the Memoirs of the Roman Academy of Sciences for 1857 he published a description of his new divided ring electrometer, based on the old electroscope of Johann Gottlieb Friedrich von Bohnenberger and he introduced a chain or series of effective instruments, including the quadrant electrometer, which cover the entire field of electrostatic measurement.

A variety of physical phenomena and concepts with which Thomson is associated are named Kelvin:

- Kelvin material

- Kelvin wave

- Kelvin-Helmholtz instability

- Kelvin-Helmholtz mechanism

Geology and theology
Kelvin-Helmholtz mechanism

Thomson remained a devout believer in Christianity throughout his life and saw chapel as part of his daily routine, though he might not identify with fundamentalism if he were alive today. He saw his Christian faith as supporting and informing his scientific work, as is evident from his address to the annual meeting of the Christian Evidence Society, May 23, 1889.

One of the clearest instances of this interaction is in his estimate of the age of the Earth. Given his juvenile work on the figure of the Earth and his interest in heat conduction, it is no surprise that he chose to investigate the Earth's cooling and to make historical inferences. Thomson believed in an instant of Creation but he was no creationist in the modern sense. He contended that the laws of thermodynamics operated from the birth of the universe and envisaged a dynamic process that saw the organisation and evolution of the solar system and other structures, followed by a gradual "heat death". He developed the view that the Earth had once been too hot to support life and contrasted this view with that of uniformitarianism, that conditions had remained constant since the indefinite past. He contended that "This earth, certainly a moderate number of millions of years ago, was a red-hot globe ... ."

After the publication of Sir Charles Darwin's On the Origin of Species in 1859, Thomson saw evidence of the, relatively short, habitable age of the Earth as tending to contradict an evolutionary explanation of biological diversity. He was soon drawn into public disagreement with Darwin's supporters Tyndall and T.H. Huxley.

Thomson ultimately settled on an estimate that the Earth was 100,000,000 years old but by the time of his death it was becoming apparent that the effects of radioactivity accounted for a much greater age. Though Thomson continued to defend his estimates, privately he admitted that they were most probably wrong.

Honours
radioactivity

- Knighted, (1866);

- Baron Kelvin, of Largs in the County of Ayr, (1892). The title derives from the River Kelvin, which passes through the grounds of the University of Glasgow. His title died with him, as he was survived by neither heirs nor close relations.

- Knight Grand Cross of the Victorian Order, (1896);

- Fellow of the Royal Society, (1851);

- Royal Medal, (1856);

- Copley Medal, (1883);

- President (1890–1895);

- One of the first members of the Order of Merit (1902);

- Privy Counsellor.

- He is buried in Westminster Abbey, London.

- The SI unit of temperature is named for him.

Notes
# P.Q.R (1841) "On Fourier's expansions of functions in trigonometric series" Cambridge Mathematical Journal 2, 258-259
# P.Q.R (1841) "Note on a passage in Fourier's 'Heat'" Cambridge Mathematical Journal 3, 25-27
# P.Q.R (1842) "On the uniform motion of heat and its connection with the mathematical theory of electricity" Cambridge Mathematical Journal 3, 71-84
# , Vol.2, p.301
# Thompson (1910) vol.1, p.98
# Chang (2004), Ch.4
# Thomson, W. (1848) "On an absolute thermometric scale founded on Carnot's theory of the motive power of heat, and calculated from Regnault's observations" Math. and Phys. Papers vol.1, pp100-106
# - (1949) "An account of Carnot's theory of the motive power of heat; with numerical results deduced from Regnault's experiments on steam" Math. and Phys. Papers vol.1, pp113-1154
# Sharlin (1979), p.112
# Ibid
# Thomson, W. (1851) "On the dynamical theory of heat; with numerical results deduced from Mr. Joule's equivalent of a thermal unit and M. Regnault's observations on steam" Math. and Phys. Papers vol.1, pp175-183
# Ibid p.179
# Ibid p.183
# Thomson, W. (1856) "On the thermal effects of fluids in motion" Math. and Phys. Papers vol.1, pp333-455
# - (1854) "On the theory of the electric telegraph" Math. and Phys. Papers vol.2, p.61
# - (1855) "On the peristaltic induction of electric currents in submarine telegraph wires" Math. and Phys. Papers vol.2, p.87
# - (1855) "Letters on telegraph to America" Math. and Phys. Papers vol.2, p.92
# - (1857) Math. and Phys. Papers vol.2, p.154
# Sharlin (1979) p.141
# Ibid p.144
# "Board of Trade Committee to Inquire into … Submarine Telegraph Cables’, Parl. papers (1860), 52.591, no. 2744
# "Report of the Scientific Committee Appointed to Consider the Best Form of Cable for Submersion Between Europe and America" (1863)
# McCartney & Whitaker (2002), reproduced on [http://physicsweb.org/articles/world/15/12/6 Institute of Physics website]
# Sharlin (1979) p.7
# Thomson, W. (1889) [http://wikisource.org/wiki/William_Thomson%27s_account_of_his_Christian_faith Address to the Christian Evidence Society]
# Sharlin (1979) p.169
# Burchfield (1990)

Bibliography
Kelvin's works

-

-

-

-

-

Biography

-

-

-

-

- King, E.T. (1909) Lord Kelvin's Early Home

-

-

- Munro, J. (1891) Heroes of the Telegraph, London: Religious Tract Society

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-

-

-

-

External links

- [http://www.physics.gla.ac.uk/Physics3/Kelvin_online/Kelvin_society/society.htm Kelvin Society of Glasgow]

- [http://www.physics.gla.ac.uk/Physics3/Kelvin_online/ Lord Kelvin Online]

- [http://onlinebooks.library.upenn.edu/webbin/gutbook/lookup?num=979 Heroes of the Telegraph] at Project Gutenburg

-

- [http://zapatopi.net/lordkelvin.html Humorous website devoted to the "worship" of Lord Kelvin]

- [http://physicsweb.org/articles/world/15/12/6 William Thomson: king of Victorian physics] at Institute of Physics website

- [http://www.lse.ac.uk/collections/CPNSS/pdf/DP_withCover_Measurement/Meas-DP%2026%2002%20C.pdf Measuring the Absolute: William Thomson and Temperature], Hasok Chang and Sang Wook Yi (PDF file)

Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thmson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron
Kelvin, William Thomson, 1st Baron

ja:ウィリアム・トムソン

1880s

Events and Trends
Technology

- Development and commercial production of electric lighting

- Development and commercial production of gasoline-powered automobile by Karl Benz, Gottlieb Daimler and Maybach

- First commercial production and sales of phonographs and phonograph recordings.

- First steel frame construction "sky-scrapers"

- American Society of Mechanical Engineers founded 16 February 1880, New York, N.Y.

Science

- Heinrich Hertz discovers the photoelectric effect

- Michelson-Morley experiment, showing that the speed of light is invariant

- James-Lange theory of emotion

War, peace and politics

- First Boer War

- The New Imperialism

Other

- Krakatoa, a volcano in Indonesia, erupts cataclysmically; 36,000 people are killed, the majority by the resulting tsunami

- About 300,000 Swedes emigrate to the United States

People
World Leaders

- Emperor Franz Josef (Austria-Hungary)

- Prime Minister Sir John A. Macdonald (Canada)

- Guangxu Emperor (China)

- Emperor Wilhelm I (Germany)

- Emperor Wilhelm II (Germany)

- Chancellor Otto von Bismarck (Germany)

- King Umberto I (Italy)

- Pope Leo XIII

- Emperor Meiji (Japan)

- Emperor Alexander II (Russia)

- Queen - Empress Victoria (United Kingdom)

- Prime Minister William Ewart Gladstone (United Kingdom)

- Prime Minister Lord Salisbury (United Kingdom)

- President Rutherford B. Hayes (United States)

- President Chester A. Arthur (United States)

- President Grover Cleveland (United States)

Category:1880s

ja:1880年代

SI

The International System of Units (abbreviated SI from the French language name Système International d'Unités) is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science.

The older metric system included several groupings of units. The SI was developed in 1960 from one of these, the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants.

The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses.

With few exceptions (such as draught beer sales in the United Kingdom), the system is legally being used in every country in the world, and many countries do not maintain official definitions of other units. In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units (e.g. the US and UK) have redefined most of their traditional, non-SI units in terms of SI units.

History
:See main articles: metre, kilogram, second, ampere, Kelvin, and candela.
The metric system was officially adopted in France after the French Revolution. During the history of the metric system a number of variations have evolved and their use spread around the world replacing many traditional measurement systems.

By the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations whilst others were based on the Imperial and American systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the metre, kilogram, second, ampere, Kelvin degree (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name: Le Système International d'Unités. The seventh base unit, the mole, was added in 1970 by the 14th CGPM.

The International System is now either obligatory or permissible throughout the world. It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).

Units
:Main articles: SI base unit, SI derived unit, SI prefix

The international system of units consists of a set of units together with a set of prefixes. The units of SI can be divided into two subsets. There are the seven base units. Each of these base units are dimensionally independent. From these seven base units several other units are derived. In addition to the SI units there are also a set of non-SI units accepted for use with SI.

A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.

SI writing style

- Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". The one exception is the litre, whose original abbreviation "l" is dangerously similar to "1". The NIST recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM. The cursive "ℓ" is occasionally seen, especially in Japan, but this is not currently recommended by any standards body. For more information, see Litre.

- Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs". Pluralization would be language dependent; "s" plurals (as in French and English) are particularly undesirable since "s" is the symbol of the second. Other cases may be marked in a language-dependent manner, e.g. Finnish 25 kg:lla = 25 kilogrammalla "with 25 kg".

- Symbols do not have an appended period (.).

- It is preferable to write symbols in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).

- A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×10² m²", "22 °C" [http://physics.nist.gov/Pubs/SP811/sec07.html]. Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.

- Spaces should be used to group decimal digits in threes, e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems, e.g. 1,000,000 or 1.000.000).

- The 10th resolution of CGPM in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". In practice, the full stop is used in English, and the comma in most other European languages.

- Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), e.g. N m or N·m.

- Symbols formed by division of two units are joined with a solidus (/), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s^-1", "m·s^-1" or $\frac$ . A solidus should not be used if the result is ambiguous, i.e. "kg·m^-1·s^-2" is preferable to "kg/m/s²".
Spelling variations

- Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling. In addition, the official US spelling for the SI prefix 'deca' is 'deka'.

- The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.

Cultural issues

The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with our decimal numeral system.

Cultural differences can be represented in the local everyday uses of metric units. For example, bread is sold in one-half, one or two kilogram sizes in many countries, but you buy them by multiples of one hundred grams in the former USSR. In some countries, the informal cup measurement has become 250 mL, and prices for items are sometimes given per 100 g rather than per kilogram. A profound cultural difference between physicists and engineers, especially radio engineers, existed prior to the adoption of the metre-kilogram-second (MKS) system and hence its descendent, SI. Engineers work with volts, amperes, ohms, farads, and coulombs, which are of great practical utility, while the centimetre-gram-second (CGS) units, which, though appropriate for theoretical physics, can be inconvenient for electrical engineering usage and are largely unfamiliar to householders using appliances rated in volts and watts. People with diabetes test their plasma glucose level regularly. In the U.S., measurement are recorded in milligrams per deciliter (mg/dL); in Europe, the standard is millimole/liter (mmol/L).

The fine-tuning that has happened to the metric base units over the past 200 years, as experts have tried periodically to refine the metric system to fit the best scientific research do not affect the everyday use of metric units. Since most non-SI units, such as the U.S. customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.

See also

- Units of measurement

- Weights and measures

- Mesures usuelles

- Metrified English unit

- History of measurement

- Other systems of measurement:

- Imperial units

- U.S. customary units

- Metre-tonne-second system of units

- Chinese system of units

- Planck units

- Atomic units

- Geometrized units

- CODATA

- Metrication

- Metric system in the United States

- Metrology

- UTC (Coordinated Universal Time)

- Binary prefixes - used to quantify large amounts of computer data

- Orders of magnitude

- ISO 31

External links

Official

- [http://www.bipm.fr/en/si/ BIPM (SI maintenance agency)] (home page)

- [http://www.bipm.org/en/si/si_brochure/ BIPM brochure] (SI reference)

- [http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=5448&ICS1=1 ISO 1000:1992 SI units and recommendations for the use of their multiples and of certain other units], with its price tag of 99 Swiss francs for a 22 page, coverless pamphlet showing why the public is sometimes a little slow to pick up on their recommendations.

Information

- [http://physics.nist.gov/cuu/Units/index.html US NIST reference on SI]

- [http://ts.nist.gov/ts/htdocs/200/202/pub814.htm#chart chart]

- [http://www.aticourses.com/international_system_units.htm SI - Its history and use in science and industry]

- [http://www.unc.edu/~rowlett/units/ A Dictionary of Units of Measurement]

- [http://www.unics.uni-hannover.de/ntr/russisch/si-einheiten.html5 Cyrillic transcription of SI symbols]

- Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 ([http://ts.nist.gov/ts/htdocs/200/202/SP%20447.pdf 46 page PDF file])

- [http://www.france-property-and-information.com/metric_conversion_table.htm Metric system and conversion tables (courtesy French property advice)]

- [http://www.metre.info metre-info - an encyclopaedia of all metric units]
Pro-metric pressure groups

- [http://www.ukma.org.uk/ The UK Metric Association]

- [http://www.metric.org/ The US Metric Association]
Pro-customary measures pressure groups

- [http://www.bwmaonline.com/ The British Weights and Measures Association]

Further reading

- I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838.

Category:SI units
Category:Systems of units
Category:International standards
Category:Dimensional analysis

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Astronomy
:This article is about the science branch. For information about the magazine, see Astronomy (magazine).

Astronomy (magazine) as they circled the Moon in 1969. Located near the center of the far side of Earth's Moon, its diameter is about 58 miles (93 km).]]

Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects and phenomena that originate outside the Earth's atmosphere, such as stars, planets, comets, galaxies, and the cosmic background radiation. It is concerned with the formation and development of the universe, the evolution and physical and chemical properties of celestial objects and the calculation of their motions. Astronomical observations are not only relevant for astronomy as such, but provide essential information for the verification of fundamental theories in physics, such as general relativity theory. Complementary to observational astronomy, theoretical astrophysics seeks to explain astronomical phenomena.

Astronomy is one of the oldest sciences, with a scientific methodology existing at the time of Ancient Greece and advanced observation techniques possibly much earlier (see archaeoastronomy). Historically, amateurs have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where}}}