After celebrating the arrival of decimal currency in the UK 50 years ago, Metric Views now goes back to basics on decimal measurement. No, we shall not be looking at nautical distances or Gunter’s chain!
UKMA often describes the metric system as a single, rational, consistent, simple and coherent measurement system. This article examines the rationale for these descriptions and demonstrates how the metric system fits together. It will also look at common base units and prefixes and their common uses.
At its core, the metric system is made up of basic units and prefixes. The modern metric system, known as the International System of Units or SI for short, is based on the principle that all measurable phenomena covered by the system – pretty well everything in the known natural world! – have one basic named unit. However, there are also some metric units outside SI that are approved for use with SI and they are all compatible with SI. All the multiples and subdivisions then follow the same logical structure using prefixes. All units and prefixes in the metric system have common language-independent symbols. These symbols are case-sensitive and are not abbreviations. In the metric system, each prefix always means the same multiple or subdivision of a unit wherever it is used. They are all simple multiples of or divisions by ten (and powers of ten) and they all apply to every basic named unit. Unlike the imperial system, there are no measurement tables to learn.
The USMA poster above shows the links between various metric units.
The metre (symbol: m), the metric unit of length, comes from “metron”, the Greek word for measure. It was originally defined as the ten millionth part of the distance from the North Pole to the equator but is now defined as the distance travelled by light in a vacuum in a given fraction of a second.
The litre (symbol: l or L), the metric unit of volume, comes from “litra”, a Sicilian monetary unit. It is equal in volume to the tenth part of a metre cubed. There are 1000 litres in one cubic metre. Litres are widely used on drink labels and, for example, to describe dustbin and fridge capacity.
The kilogram (symbol: kg), the metric unit of mass, is the only base unit in SI that contains a prefix. The gram (symbol: g), from which the kilogram is derived, comes from late Latin and Greek “gramma”, meaning a small weight. The kilogram was originally defined in 1795 as the mass of a litre of water. However, several problems were subsequently discovered with this definition so the kilogram was eventually redefined as the mass of the international prototype of the kilogram. For all practical purposes, a litre of distilled water at a certain temperature has a mass of approximately 1 kilogram. The tonne (symbol: t) is equal to 1000 kilograms.
The hectare (symbol: ha), the metric unit of area, is made up of “hecto-” plus “are” (pronounced “air”). The “hecto” part of hectare comes from the Greek “hekaton”, meaning hundred. The “are” part of hectare comes from the Latin “area”, which literally means “vacant piece of level ground”. The hectare is equal to 10 000 square metres, which is equivalent to a square that is 100 metres on each side. The hectare is used for land registration across the European Union and widely used for referring to large land areas.
The newton (symbol: N), named after Sir Isaac Newton, is the metric unit of force. The newton is defined as the force which will accelerate a mass of 1 kilogram at a rate of 1 metre per second squared. The newton is not seen very much outside scientific contexts but is mentioned here because several common metric units are derived from the newton.
The joule (symbol: J), named after English physicist James Prescott Joule, is the metric unit of energy. The joule is defined as the energy required to accelerate a mass of 1 kilogram at a rate of 1 metre per second squared over a distance of a metre (= 1 N m). The joule is widely seen in the form of kilojoules (1000 joules) on food nutrition labels.
The watt (symbol: W), named after Scottish engineer James Watt, is the metric unit of power. The watt is defined as the rate of expenditure of energy equal to 1 joule per second. The watt is commonly used for smaller electrical appliances.
The ampere (symbol: A), or amp for short, the metric unit for electric current, is named after the French physicist André-Marie Ampère. Its definition is based on a constant current of a pair of parallel wires placed exactly one metre apart in a vacuum that would produce a force equal to a fixed number of newtons per metre of length. This is the familiar ‘amp’ we associate with things like fuses and electric cable. It is the fundamental base unit for electricity and magnetism.
The volt (symbol: V), named after Italian physicist Alessandro Volta, is the metric unit of electric potential difference. The volt is defined as one watt per ampere. The volt is the familiar unit used for batteries, electricity supplies, power lines, and electrical devices.
The pascal (symbol: Pa), named after French mathematician and physicist Blaise Pascal, is the metric unit of pressure. The pascal is defined as one newton per square metre. The bar (symbol: bar), is equal to 100 000 pascals and is widely used in weather mapping.
The hertz (symbol: Hz), named after German physicist Heinrich Rudolf Hertz, is the metric unit of frequency. The hertz is defined as one cycle per second and is used for such things as radio channels, sound (pitch) and computer processors (clock speed where 1 ‘tick’ is a cycle).
Here are the diagrams of the links between metric units:
|Simplified Diagram||Full Diagram|
Here are the most common prefixes used in the metric system:
|giga||G||billion||Greek “gigas”, meaning giant|
|mega||M||million||Greek “megas”, meaning great|
|kilo||k||thousand||Greek “khilioi”, meaning thousand|
|deci||d||tenth||Latin “decimus”, meaning tenth|
|centi||c||hundredth||Latin “centum”, meaning hundred|
|milli||m||thousandth||Latin “mille”, meaning thousand|
|micro||μ||millionth||Greek “micros”, meaning small|
|nano||n||billionth||Greek “nanos”, meaning dwarf|
Prefixes are combined with units to form multiples and fractions of units. The prefix tells you the multiple or fraction of the unit that the name represents. It is best to demonstrate this fact with common examples. Here are some common combinations of prefixes and units:
|gigawatt||GW||a billion watts||Power station output|
|gigahertz||GHz||a billion hertz||Computer processor speed|
|megawatt||MW||a million watts||Power station output|
|megahertz||MHz||a million hertz||Computer processor speed|
|kilometre||km||a thousand metres||Marathon distances, European road speeds and distances, etc.|
|kilogram||kg||a thousand grams||Goods sold by weight, heavy objects, etc.|
|kilojoule||kJ||a thousand joules||Food nutrition labels|
|kilowatt||kW||a thousand watts||Electrical heaters, consumer and business power consumption, boilers, car engines, etc.|
|decilitre||dl or dL||a tenth of a litre||Blood sugar levels|
|centimetre||cm||a hundredth of a metre||DIY products, furniture, etc.|
|centilitre||cl or cL||a hundredth of a litre||Wine bottles|
|millimetre||mm||a thousandth of a metre||Construction industry, dimensions of small objects, etc.|
|millilitre||ml or mL||a thousandth of a litre||Drinks|
|milligram||mg||a thousandth of a gram||Drug doses, drink driving tests, blood sugar levels, etc.|
|millibar||mbar||a thousandth of a bar||Weather mapping reports|
|micrometre||μm||a millionth of a metre||Precision engineering, paper thickness|
|microgram||μg||a millionth of a gram||Very small drug doses|
|nanometre||nm||a billionth of a metre||Widths of transistors on computer chips|
(1) Reproduced with kind permission of the US Metric Association (USMA). Website: http://www.us-metric.org/
(2) Reproduced from the NIST website: www.nist.gov
Sources for etymologies of units: Oxford English Dictionary, Wikipedia
This article uses the term “billion” in the sense of 1000 million. This is the normal meaning of the word as used in the media.