Metrication is the conversion to the metric system of weights and measures. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in the 1790s and spread widely during the following two centuries, to be the internationally dominating measurement system and legal standard. The process is sometimes called metrification.
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While the CIA Factbook in 2006 stated that the "US is the only industrialized nation that does not mainly use the metric system", (in addition to non-industrialised Burma/Myanmar, Liberia), the actual situation, however, is more complicated than the CIA Factbook would suggest - in the United Kingdom, for example, although metric is the official system for most regulated trading by weight or measure purposes, the pint is a permitted unit for milk in returnable bottles and for draught beer and cider in British pubs, and miles, yards and feet remain the official units for road signage, but not road design.
Some sources identify Liberia as metric while reports from Burma suggest that that country is planning to adopt the metric system. However, they all have adopted metric measures to some degree through international trade and standardisation for example, Sierra Leone switched to selling fuel by the litre in May 2011. The United States mandated the acceptance of the metric system in 1866 for commercial and legal proceedings, without displacing their customary units.
In 1971 the National Bureau of Standards completed a 3-year study of the impact of increasing worldwide metric use on the U.S. The study ended with a report to the Congress entitled A Metric America - A Decision Whose Time Has Come. In the last few years metric use has been increasing rapidly in the U.S., principally in the manufacturing and educational sectors. Public Law 93-380, enacted 21 Aug 1974, states that it is the policy of the U.S. to encourage educational agencies and institutions to prepare students to use the metric system of measurement with ease and facility as a part of the regular education program. On 23 December 1975, President Gerald Ford signed Public Law 94-168, the Metric Conversion Act of 1975. This act declares a national policy of coordinating the increasing use of the metric system in the U.S. It established a U.S. Metric Board whose functions as of 1 October 1982 were transferred to the Dept of Commerce, Office of Metric Programs, to coordinate the voluntary conversion to the metric system.
Both Liberia and Myanmar are substantially metric countries, trading internationally in metric units. Visiting advocates of metrication also claim that they use metric units for many things internally with exceptions such as old petrol pumps in Myanmar, calibrated in British Imperial gallons.
A number of jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom, which still uses many imperial measures, such as miles and yards for road-sign distances, road speed limits in miles per hour, pints of beer, and inches for clothes. Most countries have adopted the metric system officially over a transitional period where both units are used for a set period of time. Some countries such as Guyana, for example, have officially adopted the metric system, but have had some trouble over time implementing it. Antigua, also 'officially' metric, is moving toward total implementation of the metric system, but slower than expected. The government of Antigua and Barbuda have announced that they have plans to convert their country to the metric system by the first quarter of 2015. Other Caribbean countries such as Saint Lucia are officially metric but are still in the process toward full conversion.
In the European Union, the European Council (of Ministers) used the Units of Measure Directive to attempt to achieve a common system of weights and measures and to facilitate the European Single Market. Throughout the 1990s, the European Commission helped accelerate the process for member countries to complete their metric conversion processes. During these negotiations, the United Kingdom secured permanent exemptions for the mile and yard in road markings, and (with Ireland) for the pint (Imperial) of draught beer sold in pubs (see Metrication in the United Kingdom). In 2007, the European Commission also announced that (to appease British public opinion and to facilitate trade with the United States) it was to abandon the requirement for metric-only labelling on packaged goods, and to allow dual metric-imperial marking to continue indefinitely.
Other countries using the imperial system completed official metrication during the second half of the 20th century or the first decade of the 21st century. The most recent to complete this process was the Republic of Ireland, which began metric conversion in the 1970s and completed it in early 2005.
The United States and the United Kingdom have some active opposition to metrication, particularly where updated weights and measures laws would make obsolete historic systems of measurement. Other countries, like France and Japan, that once had significant popular opposition to metrication now have complete acceptance of metrication.
Before the metric system 
In the Roman empire a foot measure pes was used, divided in 12 parts unciae ("inches"). The libra ("pound") was an other of many measures that had wide effect on European weight and currency long after Roman times, e.g. lb, £. The meaning came to differ a lot with time. As several others, Charlemagne launched reform programmes of various kinds, and intended to standardise units for measure and currency in his empire, but there was no real general breakthrough.
In medieval Europe, local laws on weights and measures were set by trade guilds on a city-by-city basis. For example, the ell or elle was a unit of length commonly used in Europe, but its length varied. It was 40.2 centimetres in one part of Germany, 70 centimetres in The Netherlands and 94.5 centimetres in Edinburgh. A survey of Switzerland in 1838 revealed that the foot had 37 different regional variations, the ell had 68, there were 83 different measures for dry grain, 70 measures for fluids and 63 different measures for "dead weights". When Isaac Newton wrote Philosophiae Naturalis Principia Mathematica in 1687, he quoted his measurements in Parisian feet so readers could understand the size. Examples of efforts to have local intercity or national standards for measurements include the Scottish law of 1641, and the British standard Imperial unit system of 1845, which is still commonly used in the United Kingdom. At one time Imperial China had successfully standardised units for volume throughout its territory, but by 1936 official investigations uncovered 53 dimensions for the chi varying from 200 millimetres to 1250 millimetres; 32 dimensions of the cheng, between 500 millilitres and 8 litres; and 36 different tsin ranging from 300 grams to 2500 grams. However, revolutionary France was to produce the definitive International System of Units which has come to be used by most of the world today.
The desire for a single international system of measurement derives from growing international trade and the need to apply common standards to goods. For a company to buy a product produced in another country, they need to ensure that the product will arrive as described. The medieval ell was abandoned in part because its value could not be standardised. It can be argued that the primary advantage of the International System of Units is simply that it is international, and the pressure on countries to conform to it grew as it became increasingly an international standard. However, it also has simplified learning as the units are based on the metre, just using decimal based prefixes to cover all dimensions, instead of new names with ratios of 3, 12 etc., as the figures are kept the same and easy to add or multiply. Also scientific measurement and calculation are greatly simplified as units for electricity, force etc. are metric based according to the SI system (e.g. 1 Joule=1 kgm2/s2=1 VAs). Standardisation of measures has contributed significantly to the industrial revolution and technological development in general[according to whom?]. SI is not the only example of international standardisation; several powerful international standardisation organisations exist for various industries, such as the International Organisation for Standardisation (ISO), the International Electrotechnical Commission (IEC), and the International Telecommunication Union (ITU).
Forerunners of the metric system 
Decimal numbers are an essential part of the metric system, with only one base unit and multiples created on the decimal base, the figures remain the same, simplifying calculation. Although the Indians used decimal numbers for mathematical computations, it was Simon Stevin who in 1585 first advocated the use of decimal numbers for everyday purposes in his booklet De Thiende (old Dutch for 'the tenth'). He also declared that it would only be a matter of time before decimal numbers were used for currencies and measurements. His notation for decimal fractions was clumsy, but was overcome by the introduction of the decimal point, generally attributed to Bartholomaeus Pitiscus who used this notation in his trigonometrical tables (1595).
In his Essay towards a Real Character and a Philosophical Language, published in 1668, John Wilkins proposed a system of measurement that was very similar in concept to today's metric system. He proposed retaining the second as the basic unit of time and proposed that the length of a pendulum which had a period of one second should be the base unit of length. This length, for which he proposed the name "standard", would have been 994 mm. His base unit of mass, which he proposed calling a "hundred", would have been the mass of a cubic standard of distilled rainwater. The names that he proposed for decimal multiples and subunits of his base units of measure were the names of units of measure that were in use at the time.
In 1670, Gabriel Mouton published a proposal that was in essence similar to Wilkins' proposal, except that his base unit of length would have been 1/1000 of a minute of arc (about 2.04 m) of geographical latitude. He proposed calling this unit the virga. Rather than using different names for each unit of length, he proposed a series of names that had prefixes, rather like the prefixes found in SI.
In 1790, Thomas Jefferson submitted a report to the United States Congress in which he proposed the adoption of a decimal system of coinage and of weights and measures. His proposed base unit of length was the foot which he suggested should be either 3/10 or 1/3 of the length of a pendulum that had a period of one second – that is 3/10 or 1/3 of the "standard" proposed by Wilkins over a century previously. His base unit of mass was to be the bushel, which would have been the mass of a cubic foot of water. Like Wilkins, the names that he proposed for multiples and subunits of his base units of measure were the names of units of measure that were in use at the time.
Conversion process 
The metric system was officially introduced in France in December 1799. In the 19th century, the metric system was adopted by almost all European countries: Portugal (1814), Netherlands, Belgium and Luxembourg (1820), Switzerland (1835), Spain (1850s), Italy (1861), Germany (1870, legally from 1 January 1872), Austria-Hungary (1876, but the law was adopted in 1871). Thailand did not formally adopt the metric system until 1923, but the Royal Thai Survey Department used it for cadastral survey as early as 1896. Denmark adopted the metric system in 1907.
Chronology and status of conversion by country 
Links in the country point to articles about metrication in that country.'
|1848||Chile||Spanish||almost entirely complete|
|1852||Mexico||variants of Spanish||United States customary units also in usecomplete, but|
|1852||Spain||Spanish||complete (each region still uses its own measures and some are used nationally)|
|1862||Peru||Spanish||almost entirely complete|
|1899||Paraguay||Variants of Spanish||complete|
|1923||Thailand||various||3rd method, almost entirely complete|
|1925||China||Chinese||almost entirely complete|
|1946||Indonesia||various||almost entirely complete|
|1957||Japan||Japanese||almost entirely complete|
|1965[Note 2]||United Kingdom||Imperial||partially complete|
|1971||South Africa[Note 3]||Imperial||complete|
|1975[Note 4]||United States of America||United States customary units||some adoption|
|indeterminate||Liberia[Note 5]||Imperial, Other[Note 6]||some adoption|
|2011[Note 7]||Burma (Myanmar)||Imperial, Other||some adoption|
- Prior to German Unification in 1871, Germany was a collection of independent states. Many German states, particularly those under French tutelage during the Napoleonic Wars (Rheinbund) adopted the metre 1806–15.
- Phased transition announced in 1965
- Including South-West Africa, now Namibia
- Signing of the Metric Conversion Act
- partners. See also units of measure in Liberia
- The Liberian government has begun transitioning away from use of imperial units to the metric system. However, this change has been gradual, with government reports concurrently using both imperial and metric units.
- In June 2011, the Burmese government's Ministry of Commerce began discussing proposals to reform the measurement system in Burma and adopt the metric system used by most of its trading partners. See also units of measure in Burma
There are three common ways that nations convert from traditional measurement systems to the metric system. The first is the quick, or "Big-Bang" route which was used by India in the 1960s and several other nations including Australia and New Zealand since then. The second way is to phase in units over time and progressively outlaw traditional units. This method, favoured by some industrial nations, is slower and generally less complete. The third way is to redefine traditional units in metric terms. This has been used successfully where traditional units were ill-defined and had regional variations.
The "Big-Bang" way is to simultaneously outlaw the use of pre-metric measurement, metricate, reissue all government publications and laws, and change education systems to metric. India's changeover lasted from 1 April 1960, when metric measurements became legal, to 1 April 1962, when all other systems were banned. The Indian model was extremely successful and was copied over much of the developing world.
The phase-in way is to pass a law permitting the use of metric units in parallel with traditional ones, followed by education of metric units, then progressively ban the use of the older measures. This has generally been a slow route to metric. The British Empire permitted the use of metric measures in 1873, but the changeover was not completed in most Commonwealth countries until the 1970s and 1980s when governments took an active role in metric conversion. Japan also followed this route and did not complete the changeover for 70 years. In the United Kingdom, the process is still incomplete. By law, loose goods sold with reference to units of quantity have to be weighed and sold using the metric system. In 2001 the EU directive 80/181/EEC stated that supplementary units (imperial units alongside metric including labelling on packages) would become illegal from the beginning of 2010. In September 2007 a consultation process was started which resulted in the directive being modified to permit supplementary units to be used indefinitely.
The third method is to redefine traditional units in terms of metric values. These redefined "quasi-metric" units often stay in use long after metrication is said to have been completed. Resistance to metrication in post-revolutionary France convinced Napoleon to revert to mesures usuelles (usual measures) and to some extent the names remain throughout Europe. In 1814, Portugal adopted the metric system but with the names of the units substituted by Portuguese traditional ones. In this system the basic units were the mão-travessa (hand) = 1 decimetre (10 mão-travessas = 1 vara (yard) = 1 metre), the canada = 1 liter and the libra (pound) = 1 kilogram. In the Netherlands, 500 g is informally referred to as a pond (pound) and 100 g as an ons (ounce), and in Germany and France 500 g is informally referred to respectively as ein Pfund and une livre ("one pound"). In Denmark, the re-defined pund (500 g) is occasionally used, particularly among older people and (older) fruit growers, since these were originally paid according to the number of pounds of fruit produced. In Sweden and Norway a mil (Scandinavian mile) is informally equal to 10 km, and this has continued to be the predominantly used unit in conversation when referring to geographical distances. In the 19th century, Switzerland had a non-metric system completely based on metric terms (e.g. 1 Fuss (foot) = 30 cm, 1 Zoll (inch) = 3 cm, 1 Linie (line) = 3 mm). In China the jin now has a value of 500 g and the liang is 50 g.
It is difficult to judge the degree to which ordinary people change to using metric in their daily lives. In countries that have recently changed, older segments of the population tend to still use the older units. Also, local variations abound in which units are round metric quantities or not. In Canada, for example, ovens and cooking temperatures are usually measured in degrees Fahrenheit and Celsius. Except for in cases of import items all recipes and packaging includes both Celsius and Fahrenheit so Canadians are typically comfortable with both systems of measurement. This extends to manufacturing where companies are able to use both imperial and metric since major export is for the U.S. but metric is required for both domestic and for nearly all other export. This may be due to the overwhelming influence of the neighbouring United States; similarly, still many English Canadians (unlike most French Canadians) often use non-metric measurements in day-to-day discussions of height and weight, and for clothing sizes, which are invariably measured in inches, though most driver's licences and other official government documents record weight and height only in metric (Saskatchewan driver licences, prior to the introduction of the current one-piece licence, indicated height in feet and inches but have switched to centimetres following the new licence format). In Canadian schools though metric is the standard except when it comes up in recipes, where both are included, or in practical lessons involving measuring wood or other materials for manufacturing. In the United Kingdom, Fahrenheit is seldom encountered (except when some people talk about hot summer weather) while other metric units are often used in conjunction with older measurements, and road signs use miles rather than kilometres. Another example is "hard" and "soft" metric. Canada converted liquid dairy products to litres, 500 g and 250 g sizes, which caused some complaining at time of conversion, as a litre of milk is 35 imperial ounces, while the former imperial quart used in Canada was 40 ounces. This is a "hard" metric conversion. Conversely, butter in Canada is sold primarily in a 454 g package, which converts to one Imperial pound. This is considered "soft" metric. Such countries could be said to be "semi-metric". However unlike in the rest of Canada, metrication in the Francophone province of Quebec has been more implemented and metric measures are more consistently used in Quebec than elsewhere in Canada.
As of 2007, in most countries of the world the metric system officially dominates; but traditional units are still used in many places and industries. For example:
- Automobile tyre pressure is measured as psi in countries such as Brazil, Argentina, Australia and Chile which are otherwise completely metric.
- Automotive engine power is usually measured in horsepower (rather than in kW) in Russia and most other ex-USSR countries.
- In Hong Kong, traditional Chinese and British imperial units are normally used instead of metric units in particular types of trade.
- Amongst construction workers in Northern Europe, planks and nails are often called by their old inch-based names.
- The length of small sail boats is often given in feet in popular conversation.
- Office space is often rented in traditional units, such as square foot in Hong Kong and India, tsubo in Japan or pyeong in Korea.
- In plumbing the diameters of pipes are still measured in inches in some countries (in the United Kingdom all new pipes are metric), some pipes can be named by inches but measured in mm.
- Automotive wheel diameters are still set as whole inch measurements (although tyre widths are measured in millimetres).
- Dots per inch and pixels per inch continue to be used in describing graphical resolution in the computer and printing industry.
- Television and monitor screen diameters are still commonly cited in inches in many countries; however, in Australia and South Africa, centimetres are often used for television sets, whereas CRT computer monitors and all LCD monitors are measured in inches.
- Many large format computer printers, commonly known as plotters, have carriage widths measured in inches. Common widths are 24in, 36in, 44in and 60in. While metric media sizes are often quoted (e.g., A0, A1), rolls of film, plain paper or photographic paper are normally sold in these widths, giving rise to wastage when they are trimmed. Rolls lengths are variously quoted in feet or metres.
- In the electronic industry, where the dominating spacing for components is based on 1/10 inch, and a change would lead to compatibility issues, e.g., for connectors.
- Within the mechanical industry, inch based spare parts can occasionally be kept, e.g., to service American or pre-World War II machines, but at maintenance, screws may be exchanged to metric thread.
- In Ireland, the only exception to the metrication process was the pint in bars, pubs, and clubs; although alcohol sold in any other location is in metric units (usually 330 ml (canned beer), 500 ml (bottled beer), 750 ml (wine), or 1 l (spirit)).
- In Australia, a pint of beer was redefined to 570 ml (see Australian beer glasses).
- In both metric and non-metric countries, racing bicycle frames are generally measured in centimetres, while mountain bicycle and other frames are measured in either or both.
- In Spain and former colonies i.e. Americas and Philippines certain pre-metric units are still used, e.g., the quiñón for land measurement in the Philippines, the fanega, ferrado and atahúlla to name three used in Spain and other former possessions.
- The pulgada (inch) is 23 mm, 2 mm shorter than the English inch.
- In many long-time metric countries, when non metric units are used, it is often to give rough estimates in a short form, while accurate measures always are metric, e.g., "6 feet" may feel less exact and shorter to say than "1,8 metres" or "180 cm". Measurement tools for inches are generally rare to find there, only on the other side of some carpenter´s rulers, and may present a variation between national legacy inches and British/US inches, easily causing significant measurement errors if used.
- The Imperial gallon is used as a unit of measure for fuel in Anguilla, Antigua and Barbuda, Burma, the Cayman Islands, Dominica, Grenada, Montserrat, St Kitts and Nevis and St Vincent and the Grenadines.
- The U.S. gallon is used in the Bahamas, Belize, British Virgin Islands, Columbia, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Liberia, Panama, Peru, Turks and Caicos Islands and the USA, especially for pricing.
- In Latin America, as SI units are standard, litres are used as well, e.g., often regarding fuel economy (km per liter), and the Spanish word galón may occasionally refer to a portable fuel container, often 5-20 L.
- Road distances and speed limits are still displayed in miles and mph in the USA, UK, Burma and various Caribbean nations.
In some countries (such as Antigua, see above), the transition is still in progress. The Caribbean island nation of Saint Lucia announced metrication programmes in 2005 to be compatible with CARICOM.
United Kingdom 
In the United Kingdom the population continue to resist metrication and the traditional imperial measures are preferred by the majority and continue to be in widespread common use. The metric system is used by most businesses and is used for most trade transactions. Metric units must be used for certain trading activities (selling by weight or measure for example) although imperial units may continue to be displayed in parallel.
British law has enacted the provisions of European Union directive 80/181/EEC which catalogues the units of measure that may be used for "economic, public health, public safety and administrative purposes". These units consist of the recommendations of the General Conference on Weights and Measures supplemented by some additional units of measure that may be used for specified purposes. Metric units could be legally used for trading purposes for nearly a century before metrication efforts began in earnest. The government had been making preparations for the conversion of the Imperial unit since the 1862 Select Committee on Weights and Measures recommended the conversion and the Weights and Measures Act of 1864 and the Weights and Measures (Metric System) Act of 1896 legalised the metric system. In 1965, with lobbying from British industries and the prospects of joining the Common Market, the government set a 10-year target for full conversion and created the Metrication Board in 1969. Metrication did occur in some areas during this time period, including the re-surveying of Ordnance Survey maps in 1970, decimalisation of the currency in 1971, and teaching the metric system in schools. No plans were made to make the use of the metric system compulsory, and the Metrication Board was abolished in 1980 following a change in government.
The United Kingdom avoided having to comply with the 1989 European Units of Measurement Directive (89/617/EEC), which required all member states to make the metric system compulsory, by negotiating derogations (delayed switchovers), including for miles on road signs and for pints for draught beer, cider, and milk sales.
In popular conversation, the stone unit is still used for body weight and feet and inches are used to describe height.
United States and Canada 
Over time, the metric system has influenced the United States through international trade and standardisation. The use of the metric system was made legal as a system of measurement in 1866 and the United States was a founding member of the International Bureau of Weights and Measures in 1875. The system was officially adopted by the federal government in 1975 for use in the military and government agencies, and as preferred system for trade and commerce. It has remained voluntary for federal and state road signage to use metric units, despite attempts in the 1990s to make it a requirement.
A 1992 amendment to the Fair Packaging and Labeling Act (FPLA), which took effect in 1994, required labels on federally regulated "consumer commodities" to include both metric and U.S. customary units. As of June 2007, all but two US states (New York and Alabama) have passed laws permitting metric-only labels for the products they regulate. Likewise, Canada also legally allows for dual labelling of goods provided that the metric unit is listed first and that there is a distinction of whether a liquid measure is a U.S. or a Canadian (Imperial) unit.
Today, the American public and much of the private business and industry still use U.S. customary units despite many years of informal or optional metrication. At least two states, Kentucky and California, have even moved towards demetrication of highway construction projects.
Air and sea transport 
||The examples and perspective in this article may not represent a worldwide view of the subject. (September 2008)|
Air and sea transport use the nautical mile. This is about one minute of arc of latitude along any meridian arc and is precisely defined as 1852 metres (about 1.151 miles). It is a non-SI unit (although accepted for use in the SI by the BIPM). The prime unit of speed for maritime and air navigation remains the knot (nautical mile per hour).
The prime unit of measure for aviation (altitude, or flight level) is usually estimated based on air pressure values and, in many countries, is still described in nominal feet though many others employ nominal metres. The policies of the International Civil Aviation Organization (ICAO) relating to measurement are:
- there should be a single system of units throughout the world
- the single system should be SI
- the use of the foot for altitude is a permitted variation.
Consistent with ICAO policy, aviation has undergone a significant amount of metrication over the years; for example, runways are usually given in metres. The United States metricated the data interchange format (METAR) for temperature reports in 1996. Metrication is also gradually taking place in cargo weights/dimensions and fuel volume/weight.
Accidents and incidents 
Confusion over units during the process of metrication can sometimes lead to accidents. One of the most famous examples was during Canada's metrication: in 1983, an Air Canada Boeing 767, nicknamed the "Gimli Glider" after the incident, ran out of fuel in mid-flight. The incident was caused, in a large part, by the confusion over the conversion between litres, kilograms and pounds, resulting in the aircraft receiving 22,300 pounds of fuel instead of the required 22,300 kg.
During the reconstruction of Ieper after the First World War, English army engineers drew up the plans for rebuilding a bombed city house in feet. However, the Belgian builders interpreted the numbers in metres. As a result the house had enormous doors and windows. It still exists today.
While not strictly an example of national metrication, the use of two different systems was a contributing factor in the loss of the Mars Climate Orbiter in 1998. NASA specified metric units in the contract. NASA and other organisations worked in metric units but one subcontractor, Lockheed Martin, provided thruster performance data to the team in pound force seconds instead of newton seconds. The spacecraft was intended to orbit Mars at about 150 kilometres (93 mi) altitude but the incorrect data meant that it descended to about 57 kilometres (35 mi) and most likely burned up in the Martian atmosphere.
On 25 September 2009, the British Department for Transport published a draft version of legislation to amend its road signage legislation for comment. Amongst the proposed changes is an amendment to existing legislation to make dual unit height and width warning and restriction signs mandatory. This is justified in Paragraph 53 of the Impact Analysis by the text "... Based on records from Network Rail's incident logs since April 2008, approximately 10 – 12% of bridge strikes involved foreign lorries. This is disproportionately high in terms of the number of foreign lorries on the road network." This proposal was shelved with the change of government in 2010, though many bridges are now dual signed.
Only the United States continues to see significant popular opposition to metrication, the main objections being based in localism, tradition, cultural aesthetics, economic impact, or distaste for measures viewed as "foreign". Japan had significant popular opposition at one time for similar reasons. Popular opposition in the United Kingdom exists to a lesser degree and can be associated with anti-European Union sentiment. In Canada, a change in government led to the abolition of the Metric Commission in 1985 before the metric conversion program could be completed. France, where the measures were largely invented, saw popular opposition during the early 19th century, though not for long. Thus, with the exception of the United States, metrication is now fully or substantially accepted in all countries.
See also 
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- Transcription: Wilkins
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