# Metro

The meter (symbol: m) is the unit of measurement for length in the International System of Units. It is defined as "the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second".

## History

The origin of the word meter is the Greek term μέτρον (metron) which means measure.
The idea of a unified measurement system was first implemented in France at the time of the French Revolution. The existence of different measurement systems was one of the most frequent causes of disputes between traders, citizens and tax collectors. With the country unified, a single currency and a national market also unified, there was a strong economic incentive to break with this situation and standardize a system of measures. The constant problem was not just the different units, but mainly the different sizes of the units. Rather than simply standardizing the size of existing units, the leaders of the French National Constituent Assembly decided that a completely new system should be adopted.
The French Government asked the French Academy of Sciences to create a system of measurements based on a non-arbitrary constant. After this request, a group of French researchers, composed of physicists, astronomers and surveyors, began this task, defining that the unit of length meter should correspond to a certain fraction of the Earth's circumference and also corresponding to an interval of degrees. of the terrestrial meridian.
On June 22, 1799, two prototypes of iridium platinum, representing the meter and the kilogram, were deposited in the Archives of the Republic in Paris, and which are still preserved today at the Bureau International des Poids et Mesures (Bureau International des Poids et Mesures). ) in France. On May 20, 1875, an international treaty known as the Convention du Mètre (Meter Convention) was signed by 17 States and established the creation of the Bureau International des Poids et Mesures (BIPM), a permanent laboratory and world center of scientific metrology, and the Conférence Générale des Poids et mesures (CGPM), which in 1889, in its 1st edition, defined the international prototype of the meter. quadrant of a terrestrial meridian. However, the growing demand for more precision in the reference and the possibility of its more immediate reproduction led the parameters of the basic unit to be reproduced in the laboratory and compared to another constant value in the universe, which is the speed of electromagnetic propagation. Thus, the ten-millionth part of the quadrant of a terrestrial meridian, measured in the laboratory, corresponds to the linear space traversed by light in vacuum during a time interval corresponding to 1/299,792,458 of a second, which remains the standard meter.
Note: The total path traveled by light in a vacuum in one second is called a light second. The adoption of this definition corresponds to fixing the speed of light in a vacuum at 299 792 458 m/s.

## Multiples

The main unit of length is the meter, however there are situations where this unit is no longer practical. If we want to measure large extensions it is very small. On the other hand, if we want to measure very "small" extensions, the unit meter is too "large".
The multiples and submultiples of the meter are called secondary units of length.
In the International System of Measurements (SI) multiples and divisions of the meter are used:
There is also the ångström, which is equivalent to 10−10 meters, used mainly in physics to deal with quantities of the order of the atom and which is not part of the SI.

## Equivalences in the SI

1 000 mm
100 cm
10 dm
0.1 dam
0.01 hm
0.001 km

## Equivalences in other units

## See also