Metre

The metre (symbol: m), spelled meter in the United States, is the SI unit of length and one of the seven base units in the International System of Units. The name is derived from the Greek μέτρον (metron) meaning "measure", and gives rise to the term "metric system", a system of units of which the metre is one.

The metre was conceived in the aftermath of the French Revolution (1789) as a replacement for the old units of length that were associated with the ancien régime. Although there was initially considerable resistance to the adoption of the new units in France (including an official reversion to the mesures usuelles ["normal units"] for a period), the metre gained following in continental Europe during the mid nineteenth century, particularly in scientific usage, and was consacrated as an international measurement unit by the Metre Convention of 1875.

Definition

The current definition of the metre was agreed at the 17th General Conference on Weights and Measures (CGPM) in 1983:

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

This definition has the effect that the speed of light in vacuum has an exact, defined value in SI units: 299 792 458 m s⁻¹.

Relativity

The metre is considered to be a unit of proper length. As such, the definition is only adequate in itself for measurements in an area where the local gravitational field can be considered to be uniform, a condition which is met by almost all measurements on the surface of the Earth (the Earth's gravitational field varies by about one part in 10¹⁶ per vertical metre, which is far less than the measurement uncertainty in Earth-based length measurements). For astronomical measurements, the effects of general relativity cannot be ignored and the definition of the metre must be completed with a description of the metric tensor which defines the gravitational field.

Realization

The metre can be realized either directly, by measuring the time taken for light to travel a certain distance, or by interferometry.

Interferometry

Source	Stabilizing line (127I2)	Frequency THz	Wavelength nm	ur
Helium–neon laser	a16 component, R(127) 11-5 transition	473.612 353 604(10)	632.991 212 579(13)	2.1 × 10−11
Nd:YAG laser	a10 component, R(56) 32-0 transition	563.260 223 513(5)	532.245 036 104(5)	8.9 × 10−12

For an unstabilized helium–neon laser (Ne 3s₂→2p₄ transition):

f = 473.612 7 THz

λ = 632.990 8 nm

u_r = 1.5 × 10⁻⁶

History

Universal measure

The standard measures of length in Europe diverged from one another following the fall of the Roman Empire: while measures could be standardized within a given jurisdiction (which was often little more than a single market town), they were numerous varieties of measure between regions. Indeed, as the measures were often used as the basis for taxation (of cloth, for example), the use of a certain measure was associated with the sovereignty of a given ruler and often dictated by law.

Nevertheless, with the increasing scientific activity of the seventeenth century came calls for the institution of a "universal measure" (as Englishman John Wilkins called it) or "metro cattolico" (Italian Tito Livio Burattini), which would be based on a natural phenomenon rather than royal decree, and would also be decimal rather than the various systems of multipliers, often duodecimal, that coexisted at the time.

Wilkins' idea was to choose the length of a "seconds pendulum" (a pendulum with a half-period of one second) as the unit length: such pendulums had recently been demonstrated by Christiaan Huygens, and their length is very close to one modern metre (as well as to length units which were then in use, such as the yard). However, it was soon discovered that the length of a seconds pendulum varies from place to place: French astronomer Jean Richer had measured the 0.3% difference in length between Cayenne (in French Guiana) and Paris.

External links

• meter (metre) at Sizes.com