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Unit of measurement

real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (International vocabulary of metrology)
This cube can hold one litre of a liquid
Beer mugs, at Oktoberfest: They hold one litre of beer

Units of Measurement provide standards so that the numbers from our measurements refer to the same thing. Measurement is a process that uses numbers to describe a physical quantity. We can measure how big things are, how warm they are, how heavy they are, and lots of other features as well.

For example, the metre is a standard unit to measure length. Before 1982, it was defined as the distance between two markers on a special rod. Now scientists define the metre by using the speed of light. Saying something has a length of two metres means that it is exactly twice as long as that rod used to define the metre.

In past centuries many different units were used in different countries. Today, most units of measure fall into one of three systems:

The older two, the British imperial system and the closely related US customary system use the foot as a measure of length, the pound as a measure for weight, and the second as a measure for time. They use other units as well. The number of smaller units that make the bigger units in these two systems varies: For example, there are 12 inches in a foot and 16 ounces in a pound.

The newest of the three systems is the metric system or SI system which use 10, 100 or 1000 of a smaller unit to make a bigger one. For instance, there are 100 centimetres in one metre or 1000 grams in one kilogram. This system uses the metre for length, the kilogram for weight.

The measurement of time does not follow this pattern. The second is the basis for time measurement, and it is based on the sexagesimal system: 60 seconds make one minute, and 60 minutes make one hour.

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Number and unit of measurementEdit

The property of the thing being measured is given as a number of units of measure. The number only has sense when the unit of measurement is also given.

For example, The Eiffel Tower in Paris, France is 300 meters tall.[1] That is, the distance from the top to the bottom of the Eiffel Tower is 300 meters. The property of the Eiffel Tower being measured is a distance. The number measured is 300. This number does not make sense without the unit of measurement. The unit of measurement is the metre.

Measurement standardsEdit

Standards are special objects that are used to make measurements. A metre stick is an example of a standard. When you measure something with a metre stick, you can compare that measurement to anything else that is also measured with a metre stick. This makes measurement easier and comparisons between measurements easier.

Science, medicine and engineering use smaller units of measurement to measure small things with less error. It is easy to measure large things using larger units of measurement. Astronomical measurements like the width of a galaxy use light years and parsecs.

Small measurements like the mass of an atom use special units of measurement.

Systems of units of measurementEdit

There are many different standards and units used all over the world. Some became less used during the 19th and 20th centuries.

Metric SystemEdit

see SI and Metric system

The metric system is a system of measurement used in most of the world. It is also called the International System of Units, or SI.

Units of measure in the metric system include:

  • The unit of volume is the litre. It is used for measuring an amount of liquid. A millilitre (abbreviated as ml) is the amount of liquid that would fill up a cube that measures 1 centimetre on each side. One l litre of liquid would fill up a cube that is 10 cm on each side.
  • The unit of mass is the kilogram. A kilogram (kg) weighs the same as a litre of water (at normal temperature, and pressure). 1 gram (g) is the weight of 1 millilitre of water at 0 degrees Celsius. The metric tonne is 1000 kilograms or a million grams.

British imperial UnitsEdit

Imperial units were defined in the United Kingdom in 1825. These units were based on similar units that were in use before 1825. Imperial units were used in countries that were part of the British Empire. While many of these countries, including the United Kingdom, have officially adopted SI, the older system of units are still used.

US customary unitsEdit

US customary units are the official units used in the US. These are similar to the British imperial units and also based on the units used in the United Kingdom from before American Independence. Some of the units are different to the British ones. For example, there are 20 imperial fluid ounces in an imperial pint, but 16 US fluid ounces in a US pint. Additionally, the US fluid ounce is slightly bigger than the imperial fluid ounce. The result is that US pints and gallons are smaller than imperial pints and gallons. In the United States, the metric system has been legal for trade since 1866 but other measurements such as the gallon, inch, and the pound are still widely used.

Imperial and US units of measurement include:

  • Length - inch (in), foot (ft), yard (yd), and mile.
    • 1 foot = 12 inches
    • 1 yard = 3 feet (plural of foot) = 36 inches
    • 1 mile = 1760 yards = 5280 feet
  • US volume - US fluid ounces (fl oz), US cup (cp), US pint (pt), US quart (qt), and US gallon (gal).
    • 1 US cup = 8 US fluid ounces
    • 1 US pint = 2 US cups = 16 US fluid ounces
    • 1 US quart = 2 US pints = 4 US cups = 32 US fluid ounces
    • 1 US gallon = 4 US quarts = 8 US pints = 16 US cups
  • Weight and mass are measured in ounces (oz) and pounds (lb), and stone (st) in imperial only.
    • 1 pound = 16 ounces
    • 1 stone = 14 pounds

The ounces for weight and volume are different. Even when measuring water, the number of ounces of weight is not the same as the number of fluid ounces.

Converting between systemsEdit

Metric to US
  • 1 metre = 1.09 yards = 39.37 inches.
  • 1 liter = 33.3 fluid ounces = 1.76 pints = .26 US gallons.
  • 1 kilogram = 35.32 ounces = 2.2 pounds
US to metric
  • Length
    • 1 inch = 2.54 centimetres
    • 1 foot = 30.48 centimetres
    • 1 yard = .914 metres
    • 1 mile = 1.61 kilometres
  • Volume
    • 1 fluid ounce = 29.6 millilitres
    • 1 pint = 473.1 millilitres
    • 1 gallon = 3.79 litres
    • 1 cup = 16 ounces
  • Mass
    • 1 ounce = 28.35 grams
    • 1 pound = .45 kilograms

Other units of measurementEdit

The unit of time is the second. The minute (60 seconds) and hour (60 minutes or 3600 seconds) are larger units. A day is usually said to be 24 hours, but is actually a little bit longer than that. This difference is corrected at the end of some years with what is called a leap second. A week (7 days) and month are also standard units.

A unit of measurement that applies to money is called a unit of account. This is normally a currency issued by a country. For instance, the United States use dollars. Each dollar is 100 cents. The United Kingdom uses pounds. Each pound is 100 pennies or pence. The European Union uses the Euro. There are 100 cents fin the Euro.

The units for electricity, magnetism and radiation were mostly invented in the 19th century when scientists learned how to measure them. Most were originally given imperial systems, but it is usual to use metric systems for them today.

NotesEdit

  1. A person can also say "The Eiffel Tower's height is 300 meters".