Metre

The metre is one of the seven fundamental units of measurement. 1 metre is equal to the path length covered by light in a given time in a vacuum, which is 1/299,792,458 of a second.

Originally, the unit metre was defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along the great circle, so the Earth's circumference is around 40,000 km.

Further, in 1799, the metre was redefined as a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a specific number of wavelengths of a certain emission line of krypton-86.

The current definition of metre was adopted in 1983 and improvised in around 2002 to clarify that the metre is a measure of proper length.

This article will help you understand what is a metre and how to perform metre measurement using the following two devices:

• Metre gauge

• Metre scale

Metre Definition: A Fundamental Unit

The SI unit of length, the metre (m), takes its name from Greek and French nouns for “measure.”

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The metre, alongside the kilogram, was one of the first units of the metric system. It was originally described (in 1793, at the time of the French Revolution) as one ten-millionth of the distance on the earth of the meridian line that runs from the north pole, through Paris, to the equator. For practical use, a sequence of platinum-iridium metre bars was cast.

The modern-day definition of the metre dates from 1983, and it fixes the metre in terms of the second and the speed of light. In practice, the metre is identified by measuring the frequency of vacuum wavelength of certain types of lasers. It comes after that the speed of light in vacuum (c0) is precisely 299 792 458 metres per second (m/s).

The 1983 definition defines the metre in form of a fundamental constant, the speed of light in a vacuum. After the restatement of the SI that took impact on 20 May 2019, this link became more explicit. All seven units are described by giving exact numerical values to physical constants.

Metre Measurement 

The metre definition can be practically realised in  the following ways:

• Time of flight, where a pulse of light is transmitted over the length that is to be measured.

• Interferometry, in which the length can be measured in terms of the vacuum wavelength (λ) of a light source of recognised frequency (f), via the relation:

λ =c0/f

The frequency of a light source (radiation) used in interferometry requires to be set on in terms of the second or the frequency of radiation of the cesium atom. This entails comparing the frequency of the radiation used for length interferometry (typically seen to near-infrared 430 nm to 900 nm, 330 THz to 700 THz) with a cesium clock of frequency of 9.1 GHz. However, prior to the invention of optical frequency combs, this was technically very testing, however is now achievable in one step.

To further circulate the realisation of the metre, the International Committee for Weights and Measures (CIPM) and Technical Committees for Length, Time and Frequency maintain a merged list of “Recommended values of standard frequencies for applications incorporating the practical recognition of the metre and secondary representations of the second one.” This listing consists of lasers and other sources, and the frequency they will generate if operated according to the specified parameters.

The recommended visible sources consist of highly-accurate lasers, together with Helium-Neon lasers locked to a hyperfine component of the absorption spectrum of iodine. When operated correctly, the frequency of those lasers may be as fine as 2.5 parts in 1011. The listing also consists of lower-accuracy light sources, such as an unstabilised Helium-Neon laser at 633 nm. Despite their decreased accuracy (1.5 x 10-6), unstabilised lasers can be useful in a few measurements. Since radiation in the advocated values is a primary realisation of the metre, such a laser does not require calibration while the related uncertainty is suited for the purpose.

Now, let us understand what is a metre scale and its uses.

Facts of a Metre Scale

Metre Scale (Metre Rule): A metre scale can measure the distance and length of different items up to 1mm.

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A metre scale has 100 divisions, where each division is of 10 centimetres and further each centimetre is divided into small units of 10 millimetres. However, apart from measuring distances, we can use a metre rule in drawing straight lines as well as cutting blades.

What is a Metre Gauge?

Talking about a metre gauge, this is a combination of two words, which are ‘metre’ and ‘gauge’.

Here, a metre is used as a unit for measuring things; a standard or a standard of measure, while a gauge is an instrument to determine dimensions, distance, or capacity; a standard.