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February 23, 2007, 3:30 pm
September 12, 2011, 11:19 pm
Source: NIST
SI multiples of meter
Multiple Name Symbol Multiple Name Symbol
100 meter m
101 decameter dam 10–1 decimeter dm
102 hectometer hm 10–2 centimeter cm
103 kilometer km 10–3 millimeter mm
106 megameter Mm 10–6 micrometer µm
109 gigameter Gm 10–9 nanometer nm
1012 terameter Tm 10–12 picometer pm
1015 petameter Pm 10–15 femtometer fm
1018 exameter Em 10–18 attometer am
1021 zettameter Zm 10–21 zeptometer zm
1024 yottameter Ym 10–24 yoctometer ym

The meter is the unit of length in the International System of Units (SI). It is one of the seven SI base units. The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some suggested defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the Earth's meridian along a quadrant (one-fourth the circumference of the Earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the Earth, affecting the period of the pendulum.

Thus, the meter was intended to equal 10-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator. However, the first prototype was short by 0.2 millimeters because researchers miscalculated the flattening of the Earth due to its rotation. Still this length became the standard. In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice. In 1927, the meter was more precisely defined as the distance, at 0°, between the axes of the two central lines marked on the bar of platinum-iridium kept at the International Bureau of Weights and Measures (BIPM), and declared Prototype of the meter by the 1st General Conference on Weights and Measures (CGPM), this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.

Engraving of the casting of the platinum-iridium alloy called the '1874 Alloy.'

The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m·s-1. The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under the conditions specified in 1889.

Further reading

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(2011). Meter. Retrieved from