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Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Definitions of the Seven Basic S I Units

Meter [m]

The meter 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 meter 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.

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 meter 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 meter 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 George 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 meter. 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 electrical 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).

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

 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 meter, the kilogram and the liter. These equivalent measures are exact.

1 yard = 0.9144 meters - same as UK
1 pound = 0.453 592 37 kilograms - same as UK
1 gallon (liquid) = 3.785 411 784 liters
1 bushel = 35.239 070 166 88 liters

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.

Errors

Whilst every care has been taken in the compilation of this document, and many checks have been carried out, the possibility of an error is always present in a work like this and that must be borne in mind by all users. The author would be glad to be told of any errors detected.

Accuracy

In a general dictionary like this it is impossible to know just what accuracy is needed by any particular user. Where the given value is an exact one then it has been signaled. In most cases other values are accurate to at least the number of significant figures shown. In some cases it might be more than that as trailing zeros have not been included.

Presentation

The conversion factors have mainly been presented as multipliers, but exceptions to that have been made for two reasons. First, it is easier to convey the exact value 'divide by 60' rather than the approximation 'multiply by 0.0166667' and it is more likely to be keyed in without errors if a calculator is being used. Second, most calculators accept only 8 digits, which means that 'multiply by 0.000 084 666' will become '0.000 0846' (3 significant figures) whereas 'divide by 11 811' will give the result to 6 significant figures. The appearance of a '1' needs no operator but shows that the named unit is exactly equivalent to the standard unit.

Inverse usage

In nearly all cases the conversion factors have been given to change 'non-standard' units into standard units of the SI. For those cases where it is necessary to do a conversion the other way it is only a matter of reversing the operation. For example to convert feet into meters you multiply by 0.3048 so, to convert meters into feet you divide by 0.3048. Following on from this it can be seen how conversions can be made between non-standard units, changing first into the standard unit and then back into the required unit.

Author's Note

A guiding principle behind the writing and presentation of this document has been that of clarity for non-specialist readers. To that end I have been guilty of breaking "the rules" in a few places. I am sorry that these transgressions may offend some readers but I have done so in the belief that it will be a little bit easier for many, and also help the flow of a continuous narrative.
This dictionary is not meant to be encyclopedic in its coverage, and there are many many more units which are not touched upon, but it is hoped that all 'ordinary' needs are covered. The many references to other sources, both in books and on-line should take care of anything beyond that.
Finally, I must thank all of those who wrote with suggestions (and corrections!) after reading the earlier editions.