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A Dictionary of Units

By Sandra Evans,2014-10-27 11:52
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A Dictionary of Units by Frank Tapson This provides a summary of most of the units of measurement to be found in use around the world today (and a few of historical interest), together with the appropriate conversion factors needed to change them into a 'standard' unit of the SI. The units may be found either by looking under the in which they are used, (length energy e..

    A Dictionary of Units

    by Frank Tapson

This provides a summary of most of the units of measurement to be found in use

    around the world today (and a few of historical interest), together with the

    appropriate conversion factors needed to change them into a 'standard' unit of the

    SI.

     The units may be found either by looking under the category

    in which they are used, (length energy etc.)

     or by picking one unit from an alphabetically ordered list of units.

     There is an outline of the S I system,

    a list of its 7 basic definitions,

    some of its derived units,

    together with a list of all the S I prefixes,

    and some of the rules and conventions for its usage.

    On the subject of measures generally, there is a short historical note.

    Then there are descriptions of the Metric system,

    and the U K (Imperial) system,

    followed by statements on the implementation of 'metrication' in the U K,

    and then the U S system of measures.

    At the bottom of this document is a list of other sources,

    and also some links to other Web sites.

    Finally there are some notes on this material .

    A more extensive (3-part) version of this dictionary will be found at

    www.ex.ac.uk/trol/dictunit/

The Systeme International [S I]

    Le Systeme international d'Unites officially came into being in October 1960 and has been officially recognised and

    adopted by nearly all countries, though the amount of actual usage varies considerably. It is based upon 7 principal units,

    1 in each of 7 different categories -

    Category Name Abbrev.

    Length metre m

    Mass kilogram kg

    Time second s

    Electric current ampere A

    Temperature kelvin K

    Amount of substance mole mol

    Luminous intensity candela cd

    Definitions of these basic units are given. Each of these units may take a prefix. From these basic units many other units are derived and named.

    Return to the top of this document

    Definitions of the Seven Basic S I Units

    metre [m]

    The metre is the basic unit of length. It is the distance light travels, in a vacuum, in 1/299792458th

    of a second.

    kilogram [kg]

    The kilogram is the basic unit of mass. It is the mass of an international prototype in the form of a

    platinum-iridium cylinder kept at Sevres in France. It is now the only basic unit still defined in

    terms of a material object, and also the only one with a prefix[kilo] already in place.

    second [s]

    The second is the basic unit of time. It is the length of time taken for 9192631770 periods of

    vibration of the caesium-133 atom to occur.

    ampere [A]

    The ampere is the basic unit of electric current. It is that current which produces a specified force

    between two parallel wires which are 1 metre apart in a vacuum.It is named after the French

    physicist Andre Ampere (1775-1836).

    kelvin [K]

    The kelvin is the basic unit of temperature. It is 1/273.16th of the thermodynamic temperature of the

    triple point of water. It is named after the Scottish mathematician and physicist William Thomson

    1st Lord Kelvin (1824-1907).

    mole [mol]

    The mole is the basic unit of substance. It is the amount of substance that contains as many

    elementary units as there are atoms in 0.012 kg of carbon-12.

    candela [cd]

    The candela is the basic unit of luminous intensity. It is the intensity of a source of light of a

    specified frequency, which gives a specified amount of power in a given direction.

    Return to the top of this document

    Derived Units of the S I

    From the 7 basic units of the SI other units are derived for a variety of purposes. Only a few of are explained here as

    examples, there are many more.

    farad [F]

    The farad is the SI unit of the capacitance of an electrical system, that is, its capacity to store

    electricity. It is a rather large unit as defined and is more often used as a microfarad. It is named

    after the English chemist and physicist Michael Faraday (1791-1867).

    hertz [Hz]

    The hertz is the SI unit of the frequency of a periodic phenomenon. One hertz indicates that 1 cycle

    of the phenomenon occurs every second. For most work much higher frequencies are needed such

    as the kilohertz [kHz] and megahertz [MHz]. It is named after the German physicist Heinrich

    Rudolph Hertz (1857-94).

    joule [J]

    The joule is the SI unit of work or energy. One joule is the amount of work done when an applied

    force of 1 newton moves through a distance of 1 metre in the direction of the force.It is named

    after the English physicist James Prescott Joule (1818-89).

    newton [N]

    The newton is the SI unit of force. One newton is the force required to give a mass of 1 kilogram

    an acceleration of 1 metre per second per second. It is named after the English mathematician and

    physicist Sir Isaac Newton (1642-1727).

    ohm [Ω ]

    The ohm is the SI unit of resistance of an electrical conductor. Its symbol, is the capital Greek letter

    'omega'. It is named after the German physicist Georg Simon Ohm (1789-1854).

    pascal [Pa]

    The pascal is the SI unit of pressure. One pascal is the pressure generated by a force of 1 newton

    acting on an area of 1 square metre. It is a rather small unit as defined and is more often used as a

    kilopascal [kPa]. It is named after the French mathematician, physicist and philosopher Blaise

    Pascal (1623-62).

    volt [V]

    The volt is the SI unit of electric potential. One volt is the difference of potential between two

    points of an electical conductor when a current of 1 ampere flowing between those points dissipates

    a power of 1 watt. It is named after the Italian physicist Count Alessandro Giuseppe Anastasio

    Volta (1745-1827).

    watt [W]

    The watt is used to measure power or the rate of doing work. One watt is a power of 1 joule per

    second. It is named after the Scottish engineer James Watt (1736-1819).

    Note that prefixes may be used in conjunction with any of the above units. Return to the top of this document

The Prefixes of the S I

    The S I allows the sizes of units to be made bigger or smaller by the use of appropriate prefixes. For example, the

    electrical unit of a watt is not a big unit even in terms of ordinary household use, so it is generally used in terms of 1000

    watts at a time. The prefix for 1000 is kilo so we use kilowatts[kW] as our unit of measurement. For makers of electricity, or bigger users such as industry, it is common to use megawatts[MW] or even gigawatts[GW]. The full

    range of prefixes with their [symbols or abbreviations] and their multiplying factors which are also given in other forms

    is

    yotta [Y] 1 000 000 000 000 000 000 000 000 = 10^24

    zetta [Z] 1 000 000 000 000 000 000 000 = 10^21

    exa [E] 1 000 000 000 000 000 000 = 10^18

    peta [P] 1 000 000 000 000 000 = 10^15

    tera [T] 1 000 000 000 000 = 10^12

    giga [G] 1 000 000 000 (a thousand millions = a billion)

    mega [M] 1 000 000 (a million)

    kilo [k] 1 000 (a thousand)

    hecto [h] 100 (a hundred)

    deca [da]10 (ten)

     1

    deci [d] 0.1 (a tenth)

    centi [c] 0.01 (a hundredth)

    milli [m] 0.001 (a thousandth)

    micro [µ] 0.000 001 (a millionth)

    nano [n] 0.000 000 001 (a thousand millionth)

    pico [p] 0.000 000 000 001 = 10^-12

    femto [f] 0.000 000 000 000 001 = 10^-15

    atto [a] 0.000 000 000 000 000 001 = 10^-18

    zepto [z] 0.000 000 000 000 000 000 001 = 10^-21

    yocto [y] 0.000 000 000 000 000 000 000 001 = 10^-24

    [µ] the symbol used for micro is the Greek letter known as 'mu'

    Nearly all of the S I prefixes are multiples (kilo to yotta) or sub-multiples (milli to yocto) of 1000.

    However, these are inconvenient for many purposes and so hecto, deca, deci, and centi are also used.

    deca also appears as deka [da] or [dk] in the USA and Contintental Europe. So much for standards! Return to the top of this document

    Conventions of Usage in the S I

    There are various rules laid down for the use of the SI and its units as well as some observations to be made that will

    help in its correct use.

    ; Any unit may take only ONE prefix. For example 'millimillimetre' is incorrect and should be written

    as 'micrometre'.

    ; Most prefixes which make a unit bigger are written in capital letters (M G T etc.), but when they

    make a unit smaller then lower case (m n p etc.) is used. Exceptions to this are the kilo [k] to avoid

    any possible confusion with kelvin [K]; hecto [h]; and deca [da] or [dk]

    ; It will be noted that many units are eponymous, that is they are named after persons. This is always

    someone who was prominent in the early work done within the field in which the unit is used. Such

    a unit is written all in lower case (newton, volt, pascal etc.) when named in full, but starting with a

    capital letter (N V Pa etc.) when abbreviated. An exception to this rule is the litre which, if written

    as a lower case 'l' could be mistaken for a '1' (one) and so a capital 'L' is allowed as an alternative. It

    is intended that a single letter will be decided upon some time in the future when it becomes clear

    which letter is being favoured most in use.

    ; Units written in abbreviated form are NEVER pluralised. So 'm' could always be either 'metre' or

    'metres'. 'ms' would represent 'millisecond'.

    ; An abbreviation (such as J N g Pa etc.) is NEVER followed by a full-stop unless it is the end of a

    sentence.

    ; To make numbers easier to read they may be divided into groups of 3 separated by spaces (or half-

    spaces) but NOT commas.

    ; The SI preferred way of showing a decimal fraction is to use a comma (123,456) to separate the

    whole number from its fractional part. The practice of using a point, as is common in English-

    speaking countries, is acceptable providing only that the point is placed ON the line of the bottom

    edge of the numbers (123.456) and NOT in the middle.

    Return to the top of this document

    A Brief History of Measurement

    One of the earliest types of measurement concerned that of length. These measurements were usually based

    on parts of the body. A well documented example (the first) is the Egyptian cubit which was derived from the length of the arm from the elbow to the outstretched finger tips. By 2500 BC this had been standardised

    in a royal master cubit made of black marble (about 52 cm). This cubit was divided into 28 digits (roughly a finger width) which could be further divided into fractional parts, the smallest of these being only just over a millimetre.

    In England units of measurement were not properly standardised until the 13th century, though variations (and abuses) continued until long after that. For example, there were three different gallons (ale, wine and corn) up until 1824 when the gallon was standardised.

    In the U S A the system of weights and measured first adopted was that of the English, though a few differences came in when decisions were made at the time of standardisation in 1836. For instance, the wine-gallon of 231 cubic inches was

    used instead of the English one (as defined in 1824) of about 277 cubic inches. The U S A also took as their standard of dry measure the old Winchester bushel of 2150.42 cubic inches, which gave a dry gallon of nearly 269 cubic inches.

    Even as late as the middle of the 20th century there were some differences in UK

    and US measures which were nominally the same. The UK inch measured 2.53998 cm while the US inch was 2.540005 cm. Both were standardised at 2.54

    cm in July 1959, though the U S continued to use 'their' value for several years in

    land surveying work - this too is slowly being metricated.

    In France the metric system officially started in June 1799 with the declared intent of being 'For all people, for all time'. The unit of length was the metre which was defined as being one ten-millionth part of a quarter of the earth's circumference. The production of this standard required a very careful survey to be done which took several years. However, as more accurate instruments became available so the 'exactness' of the standard was called into question. Later efforts were directed at finding some absolute standard based on an observable physical phenomenon. Over two centuries this developed into the S I. So maybe their original slogan was more correct than anyone could have foreseen then.

    Return to the top of this document

    Metric System of Measurements

     Length Area

     10 millimetres = 1 centimetre 100 sq. mm = 1 sq. cm

     10 centimetres = 1 decimeter 10 000 sq. cm = 1 sq. metre

     10 decimetres = 1 metre 100 sq. metres = 1 are

     10 metres = 1 decametre 100 ares = 1 hectare

     10 decametres = 1 hectometre 10 000 sq. metres = 1 hectare

     10 hectometres = 1 kilometre 100 hectares = 1 sq. kilometre

     1000 metres = 1 kilometre 1 000 000 sq. metres = 1 sq. kilometre

     Volume Capacity

     1000 cu. mm = 1 cu. cm 10 millilitres = 1 centilitre

     1000 cu. cm = 1 cu. decimetre 10 centilitree = 1 decilitre

     1000 cu. dm = 1 cu. metre 10 decilitres = 1 litre

     1 million cu. cm = 1 cu. metre 1000 litres = 1 cu. metre

     Mass

     1000 grams = 1 kilogram

     1000 kilograms = 1 tonne The distinction between 'Volume' and 'Capacity' is artificial and kept here only for historic reasons.

    A millitre is a cubic centimetre and a cubic decimetre is a litre. But see under 'Volume' for problems with the litre.

    Return to the top of this document

    The U K (Imperial) System of Measurements

     Length Area

     12 inches = 1 foot 144 sq. inches = 1 square foot

     3 feet = 1 yard 9 sq. feet = 1 square yard

     22 yards = 1 chain 4840 sq. yards = 1 acre

     10 chains = 1 furlong 640 acres = 1 square mile

     8 furlongs = 1 mile

    5280 feet = 1 mile

    1760 yards = 1 mile Capacity

     20 fluid ounces = 1 pint

     Volume 4 gills = 1 pint

    1728 cu. inches = 1 cubic foot 2 pints = 1 quart

     27 cu. feet = 1 cubic yard 4 quarts = 1 gallon (8 pints)

     Mass (Avoirdupois)

    437.5 grains = 1 ounce Troy Weights

     16 ounces = 1 pound (7000 grains) 24 grains = 1 pennyweight

     14 pounds = 1 stone 20 pennyweights = 1 ounce (480 grains)

     8 stones = 1 hundredweight [cwt] 12 ounces = 1 pound (5760 grains)

     20 cwt = 1 ton (2240 pounds)

     Apothecaries' Measures Apothecaries' Weights

     20 minims = 1 fl.scruple 20 grains = 1 scruple

     3 fl.scruples = 1 fl.drachm 3 scruples = 1 drachm

     8 fl.drachms = 1 fl.ounce 8 drachms = 1 ounce (480 grains)

     20 fl.ounces = 1 pint 12 ounces = 1 pound (5760 grains) The old Imperial (now UK) system was originally defined by three standard measures - the yard, the pound and the

    gallon which were held in London. They are now defined by reference to the S I measures of the metre, the kilogram

    and the litre. These equivalent measures are exact.

    1 yard = 0.9144 metres - same in US

    1 pound = 0.453 592 37 kilograms - same in US

    1 gallon = 4.546 09 litres - different in US

    Note particularly that the UK gallon is a different size to the US gallon so that NO liquid measures of the same name

    are the same size in the UK and US systems.

    Also that the ton(UK) is 2240 pounds while a ton(US) is 2000 pounds. These are also referred to as a long ton and short

    ton respectively.

    Return to the top of this document

Metrication in the U K

    There have been three major Weights and Measures Acts in recent times (1963, 1976 and 1985) all gradually

    abolishing various units, as well re-defining the standards. All the Apothecaries' measures are now gone, and

    of the Troy measures, only the ounce remains. The legislation decreed that -

    From the 1st October 1995, for economic, public health, public safety and

    administrative purposes, only metric units were to be allowed EXCEPT that -

    ; pounds and ounces for weighing of goods sold from bulk

    ; pints and fluid ounces for beer, cider, waters, lemonades and fruit juices in RETURNABLE

    containers

    ; therms for gas supply

    ; fathoms for marine navigation

    could be used until 31st December 1999.

    The following could continue to be used WITHOUT time limit -

    ; miles, yards, feet and inches for road traffic signs and related measurements of speed and distance

    ; pints for dispensing draught beer and cider, and for milk in RETURNABLE containers

    ; acres for land registration purposes

    ; troy ounces for transactions in precious metals.

    Sports were exempt from all of this, but most of them have (voluntarily) changed their relevant regulations into

    statements of equivalent metric measures.

    That was how the legislation was framed. In common usage the 'old' units are still

    very apparent.

    Some other dates of note

    1950 The Hodgson Report

    was published which, after arguing all the points for and against, favoured a change to metric.

    1963 Weights and Measures Act

    defined the basic measures of the 'yard' and the 'pound' in terms of the 'metre' and the 'kilogram'. Many of the old

    imperial measures were abolished (drachm, scruple, minim, chaldron, quarter, rod, pole, perch, and a few more)

    1971

    Currency was Decimalised

1985 Weights and Measures Act

    abolished several more imperial measures for purposes of trade, and defined the 'gallon' in terms of the 'litre'.

    Thus, all the measures had been metricated even if the public hadn't!

Return to the top of this document

The U S System of Measurements

    Most of the US system of measurements is the same as that for the UK. The biggest differences to be noted are in

    Capacity which has both liquid and dry measures as well as being based on a different standard - the US liquid gallon is

    smaller than the UK gallon. There is also a measurement known at the US survey foot. It is gradually being phased out

    as the maps and land plans are re-drawn under metrication. (The changeover is being made by putting 39.37 US survey

    feet = 12 metres)

     Length Area

     12 inches = 1 foot 144 sq. inches = 1 square foot

     3 feet = 1 yard 9 sq. feet = 1 square yard

     220 yards = 1 furlong 4840 sq. yards = 1 acre

     8 furlongs = 1 mile 640 acres = 1 square mile

    5280 feet = 1 mile 1 sq.mile = 1 section

    1760 yards = 1 mile 36 sections = 1 township

     Volume

    1728 cu. inches = 1 cubic foot

     27 cu. feet = 1 cubic yard

     Capacity (Dry) Capacity (Liquid)

     16 fluid ounces = 1 pint

     2 pints = 1 quart 4 gills = 1 pint

     8 quarts = 1 peck 2 pints = 1 quart

     4 pecks = 1 bushel 4 quarts = 1 gallon (8 pints)

     Mass

    437.5 grains = 1 ounce Troy Weights

     16 ounces = 1 pound (7000 grains) 24 grains = 1 pennyweight

     14 pounds = 1 stone 20 pennyweights = 1 ounce (480 grains)

    100 pounds = 1 hundredweight [cwt] 12 ounces = 1 pound (5760 grains)

     20 cwt = 1 ton (2000 pounds)

     Apothecaries' Measures Apothecaries' Weights

     60 minims = 1 fl.dram 20 grains = 1 scruple

     8 fl.drams = 1 fl.ounce 3 scruples = 1 dram

     16 fl.ounces = 1 pint 8 drams = 1 ounce (480 grains)

     12 ounces = 1 pound (5760 grains)

    As with the UK system these measures were originally defined by physical standard measures - the yard, the pound, the

    gallon and the bushel.They are now all defined by reference to the S I measures of the metre, the kilogram and the litre.

    These equivalent measures are exact.

    1 yard = 0.9144 metres - same as UK

    1 pound = 0.453 592 37 kilograms - same as UK

    1 gallon (liquid) = 3.785 411 784 litres

    1 bushel = 35.239 070 166 88 litres

    Note particularly that the US gallon is a different size to the UK gallon so that NO liquid measures of the same name

    are the same size in the US and UK systems. Also that the ton(US) is 2000 pounds while a ton(UK) is 2240 pounds. These are also referred to as a short ton and long

    ton respectively.

    Note than in matters concerned with land measurements, for the most accurate work, it is necessary to establish whether

    the US survey measures are being used or not. Return to the top of this document

Categories of Units

     length density, area power

    area density, line pressure

    volume or capacity density, volume speed

    mass energy spread rate (by mass)

    temperature force spread rate (by volume)

    fuel consumption stress

    mass per unit length torque

    mass per unit area

    mass per unit volume

    Return to the top of this document

List of Units

    Units are listed in alphabetical order. Scanning can be speeded up by selecting

    the initial letter of the unit from these individual letters or groups

    A - B - C - D - E - F - G - H - IJ - K - L - M

    N - O - PQ - R - S - T - UVW - XYZ

    A to K

    A E IJ

    acres ells (UK) inches

    angstroms ems (pica) inches of mercury or water

    ares ergs (energy) inches of rain (by mass)

    astronomical units ergs (torque) inches of rain (by volume)

    atmospheres inches per minute etc.

    joules B F joules per hour etc. barleycorns Fahrenheit

    K barrels (oil) fathoms

    bars feet Kelvin British thermal units feet of water kilocalories Btu/hour etc. feet per hour etc. kilocalories per hour etc. bushels fluid ounces kilograms-force

    foot pounds-force kilogram-force metres (energy) C foot pounds-force per minute etc. kilogram-force metres (torque) calories foot poundals kilogram-force metres per hour etc. calories per hour etc. furlongs kilogram-force per area carats, metric kilograms G Celsius kilograms per area centigrade gallons kilograms per metre centigrade heat units gallons per area kilograms per volume centilitres gigajoules kilojoules centimetres gigawatts kilojoules per hour etc. centimetres of mercury or water grains kilometres centimetres per minute etc. grains per gallon kilometres per hour etc. chains (surveyors') grams kilometres per litre circular inches gram-force centimetres kilonewton per square metre cubic (+ any units) grams per area kilonewtons cubic measures per area grams per cm kilopascals cubits grams per (any volume) kilowatts

    kilowatt hours D H kips (force) decilitres hands kips per square inch denier hectares knots drex hides

    dynes horsepower

    horsepower hours

    hundredweights

    L to Z

    L O T

    leagues ounces tex

    light years ounces per inch therms

    links (surveyors') ounces per area tonnes

    litres ounces per volume ton-force metres

    litres per area tonnes-force PQ tonnes-force per area M parsecs tonnes per hectare Mach number pascals tonnes per km megajoules perch (=rods or poles) tonnes per volume meganewtons picas ton-force feet meganewtons per square metre pints tons megawatts points (printers') tons-force metres poundals tons-force per area poundals per square foot tons per acre metres of water pounds tons per mile metres per second etc. pounds per area tons per volume microns (=micrometres) pounds per foot townships miles pounds per volume troy ounce miles per gallon pounds-force

    UVW miles per hour etc. pound-force inches

    millibars pounds-force per area watt second

    milligrams per cm quarts watt hours

    milligrams per (any volume) watts

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