Metric system

The **metric system** is a number of different systems of measurement with length based on the metre, mass on the gram, and volume on the litre. This system is used around the world. It was developed in France and first introduced there in 1795, 2 years after the execution of Louis XVI. The metric units are based on decimal groups (multiples of ten). At first the metric system was based on two quantities: length and weight. The basic units were called the *metre* and the *gramme*.

In 1866, the United States started to use the metric system, and is widely used except by the public. By 1875, many countries in Europe and in Latin America had changed to using the metric system. In 1875, seventeen countries signed the Metre Convention agreeing to share responsibility for defining and managing the metre and kilogram standards. The prototype copies of the metre and of the kilogram were called the "international prototype metre" and "international prototype kilogram". A new organization called the International Bureau of Weights and Measures (BIPM) was set up. The international prototype metre and kilogram were kept at the BIPM headquarters. In 1960, the rules for the metric system were revised. The revised system was called the "International System of Units" (which is often called  for short). The definition of SI also included rules for writing SI quantities. These rules are the same for all countries. In the 1970s, many people in the United Kingdom and the rest of the Commonwealth started using the metric system in their places of work.

## Description 
. The sign uses the international symbol "km" for "kilometres".]]
 as the volume of a cube with edges of .  One kilogram is about the mass of one litre of water at the melting point of ice.]]
 calibrated in degrees Celsius. Water freezes at  and boils at .]]
The metric system is a decimal-based system of measurement. The system has units of measure for each quantity. The names of most units of measure in the metric system have two parts. One part is the unit name and the other part is the prefix. For example, in the name "centimetre", the word "centi" is the prefix and the word "metre" is the unit name. Sometimes, as with metre, litre and gram, there is no prefix.

In the metric system, all units have a "symbol". Symbols are a shorthand way of writing the names of units. All the countries in the world use the same symbol for a unit, even though they might have different ways of writing out the unit name in full. For example
*People write "*kilometre*" in the United Kingdom.
*People write "*kilometer*" in the United States.
*People write "*quilómetro*" in Portugal.
*People write "*một kí lô mét*" in Vietnam.
*People write "*کیلومیتر*" in Saudi Arabia.
*People write "*χιλιόμετρα*" in Greece.
*People write "*километр*" in Russia.
*People write "公里", "千米", or "粁" in China.
*People write "キロ" or "㌔"　in Japan.
*People write "킬로미터" in South Korea.
*Everybody uses the symbol "km" for "kilometre".

### Unit names
The metric system was first developed in France during the French Revolution. A French law passed in 1795 defined five units of measure. Three of these names are still in use today. They are the metre which is the unit of length, the gram which is the unit of mass and the litre which is the unit of volume.  The most commonly used units of measure in the metric system are listed below.

***Length**
:In the metric system, length is measured in metres. The symbol for the metre is the letter "m". The metre was originally defined as being  of the distance between the North Pole and the Equator on the meridian that passed through Paris. In 1799, a platinum bar that was equal to this length was made and became the "prototype metre"

***Volume**
:In the metric system, volume is measured in litres. The symbol for the litre is "L". In 1795 the French Government defined one litre as being the same volume as the volume of a cube which had sides that were .

***Mass** 
:In the metric system, mass is measured in grams.  The symbol for the gram is the letter "g". In 1795 the French Government defined the gram as the mass of one cubic centimetre of water at the freezing point of ice.

The definitions of the units are often being changed. In 1960 the definition of the metre was changed. Since then it has been defined in terms of the speed of light.

### Prefixes 
If the numbers are too big or too small, the metric system uses prefixes to make it easier to understand the numbers. 
* To set up an inter-governmental organisation to administer the treaty. This organization was called the International Bureau of Weights and Measures (BIPM). 
* France was to have responsibility for acquiring suitable premises for the BIPM. These premises would become neutral territory. The BIPM offices and laboratories would be located on the site.
* To make 40 identical copies of the kilogram. One was chosen as the *prototype* (or primary) copy. This copy was known as the "International Prototype Kilogram". It replaced the *Kilogramme des archives* as the world's primary copy of the kilogram. The *Kilogramme des archives* would be kept at the BIPM premises.
* To make 30 identical copies of the metre. One was chosen as the *prototype* (or primary) copy. This copy was known as the "International Prototype Metre". It replaced the *Metre des archives* as the world's primary copy of the metre. The *Metre des archives* would be kept at the BIPM premises.
* To give one copy of the metre and one copy of the kilogram to each country. These would be called "national prototype metres" and "national prototype kilograms". 
* To compare the national prototype metres and kilograms against the international prototypes at regular intervals.  
* To promote the use of the metric system.

The United Kingdom and the Netherlands went to the conference but did not sign the treaty at that time. After further consideration, the United Kingdom did sign the treaty in 1884

In 1889 the copies of the kilogram and the metre were ready to be given to the different countries that signed the treaty. The United Kingdom signed the treaty 1884. Neither country passed laws making it compulsory to use the metric system.

In 1921 the Metre Convention was extended to include all physical measurements including time, electricity and temperature.

In 1960 the BIPM published the "*International System of Units*" (or SI). SI clarified a number of areas of the metric system, particularly in science and in engineering. The BIPM also standardized the way in which SI was written making it the same for all languages.

## History 
There are 16 US fluid ounces in a US pint but there are 20 imperial [UK] fluid ounces in an imperial pint. The US fluid ounce is larger than the imperial fluid ounce, but the imperial pint is larger than the US pint.  In the 1700's this type situation was common across Europe. Each country  measured length, weight/mass and volume in its own way. Sometimes different countries or cities used the same name for different measurements.   Sometimes different cities in the same country had different ways of measuring things. In 1789 there were a quarter of a million different units of weight and measure in France.

### French Revolution
During the French Revolution, French scientists decided that it would be better to have a new system of weights and measures. The system would be the same in all French provinces and cities. They also decided that it would be easier if the new system used 10's instead of 12's, 16's or 20's, because people normally count in 10's. The new system became the official system of measurement in France in 1799. One of the French leaders, the Marquis de Condorcet declared that  "[the metric system] is for all people for all time". In 1798 the French scientists made a bar of platinum that was exactly one metre long.  They stored this bar in the French archives. It was called the *metre des archives*. People who made one metre rulers were able to check that their rulers were the same length as the *metre des archives*. Other scientists made a kilogram weight from platinum which was also put in the archives. This weight was called the *kilogram des archives*.

During this time, the metric system was still the official system of measurement in France. And it still had simplified unit names. But the French people continued to use the measures they were used to. The French government tried to persuade the people to convert. They mass-produced metric rulers. They tried to teach the people to use metric measures, and commanded the police to punish people who would not cooperate. Eventually the government stopped trying and withdrew the metric system.

#### United States
In 1866, after most of the South American countries started to use the metric system, the United States passed a law that allowed people to use either the metric system or United States customary units for trade. Before 1893 the yard was defined as the length of the "standard yard" which was kept by the United States Treasury. The pound was defined as being the mass of the "standard pound". In 1893 the United States Congress passed the Mendenhall Order. This order defined the yard as being exactly  metres and the pound as being exactly 0.4535924277 kilogram. The order only changed the definitions of the pound and the yard. It had no other effect on people's lives.

In 1975 the Metric Conversion Act started a formal metrication process.  Metrication was to be voluntary. It was to be coordinated by the U.S. Metric Board. In 1988 the Omnibus Trade and Competitiveness Act said that metric units had to be used for all federal projects.  The Act did not apply to state projects. Some states demanded that metric units be used but other states did not. Some industries changed to using metric units but others did not.  Soft drinks are sold in metric quantities. Milk is sold in customary units.  Metric units are widely used in the design of motor cars.  Aircraft such as the Boeing 787 Dreamliner were designed using mainly customary units.

Some people in the United States want to complete the change-over to the metric system. They say that it will make things easier for everybody. Other people say that it will cost too much money.
Some people want to use the metric system because it will make it easier to export goods. Other people say that metrication can only work if all fifty states metricate at the same time. This will not happen unless the Federal Government takes the lead.

However, in the U.S. territories of Puerto Rico and Guam, the metric system is already officially used and is dominant. The metric system was introduced to both territories, when they were Spanish colonies, and has remained official and dominant after they became U.S. territories.

#### United Kingdom
In 1897 the United Kingdom passed a law allowing people to use either the metric system or Imperial units for trade. By the late 1960s three quarters of British exports were to countries that used the metric system. However people in the United Kingdom still used imperial units. The Metrication Board was set up in 1969 to help Britain change to the metric system. Each company had to pay their own expenses. Some companies saved a lot of money by changing to the metric system because they could make the same goods for export as they made for sales in the United Kingdom. For example, almost all motor cars use metric-sized nuts and bolts.  Other companies lost money because they had to make many changes but did not have any benefit from the changes.

When the Metrication Board was closed down in 1981 most of government and industry had changed to the metric system but a lot of everyday things like road signs had not been changed. A survey taken in 2013 showed that metric units and imperial units were both widely used by British people in their private lives.

## Related pages 
* Conversion of units
* International System of Units, describes more advanced aspects of the metric system

## Notes
## References