What is a 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.
FAQs on Metre
1. What is the metre and what system of units does it belong to?
The metre (symbol: m) is the base unit of length in the International System of Units (SI). The SI system, which is the modern form of the metric system, is the globally accepted standard for scientific and technical measurements. The metre serves as the foundation for measuring distance, displacement, and spatial dimensions.
2. How is the modern metre officially defined according to the SI system?
The metre is officially defined based on a universal physical constant: the speed of light. It is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. This definition makes the unit precise and universally reproducible.
3. In India, which spelling is correct for the unit of length: 'metre' or 'meter'?
In India, the correct and standard spelling used in scientific and educational contexts, including CBSE and NCERT textbooks, is 'metre'. This follows the British English spelling, which is also the official international spelling adopted by the International Bureau of Weights and Measures (BIPM). The spelling 'meter' is primarily used in American English.
4. What is the practical use of a metre scale in daily life and in a Physics lab?
A metre scale is a common measuring instrument used for various purposes. In daily life, it's used in carpentry, construction, and tailoring for measuring lengths. In a Physics lab, it is essential for experiments involving:
- Measuring distance and displacement of objects in motion.
- Determining the dimensions of objects to calculate area or volume.
- Measuring the focal length of mirrors and lenses.
5. How are other important physical quantities like area, volume, and velocity derived from the metre?
The metre, as a base unit, is fundamental for deriving other units. For example:
- Area: It is the product of two lengths (length × width), so its SI unit is the square metre (m²).
- Volume: It is the product of three lengths (length × width × height), so its SI unit is the cubic metre (m³).
- Velocity: It is defined as displacement per unit time, so its SI unit is metres per second (m/s).
6. Why did scientists change the definition of a metre from a physical bar to the speed of light?
The definition of the metre was changed from a physical platinum-iridium bar to the speed of light for two main reasons:
- Invariability and Precision: The speed of light in a vacuum is a universal constant that does not change. A physical bar, however, can be damaged, degrade over time, or change slightly in length due to environmental factors, limiting its precision.
- Accessibility: The new definition allows any well-equipped laboratory in the world to reproduce the standard for a metre accurately, rather than having to rely on a single prototype object stored in France. This ensures global consistency.
7. What is parallax error and how can it lead to incorrect readings on a metre scale?
Parallax error is an observational error that occurs when the apparent position of an object changes due to a change in the line of sight of the observer. When using a metre scale, this error happens if your eye is not positioned directly perpendicular to the marking you are reading. Viewing the scale from an angle can make the reading appear shorter or longer than it actually is, leading to inaccurate measurements.
8. What is the difference between the 'metre' used in Physics and the 'meter' found in poetry or music?
While the words can be pronounced similarly, they refer to completely different concepts. The 'metre' in Physics is a fundamental SI unit used to measure physical length or distance. In contrast, the 'meter' in poetry or music refers to the rhythmic structure of a verse or composition, determined by the pattern of stressed and unstressed syllables or beats.
9. What are the seven fundamental SI units, and what is the significance of the metre among them?
The seven fundamental SI units are:
- Metre (m) for length
- Kilogram (kg) for mass
- Second (s) for time
- Ampere (A) for electric current
- Kelvin (K) for temperature
- Mole (mol) for amount of substance
- Candela (cd) for luminous intensity
