Metre definition

Wave-length measurement and the speed of light. Metre definition. Optical interferometry... 

For historic reasons a number of different units of measurement have evolved to express quantity of the same thing. In the 1960s, many international scientific bodies recommended the standardisation of names and symbols and the adoption universally of a coherent set of units—the SI units (Systeme Internationale d Unites)— based on the definition of five basic units metre (m) kilogram (kg) second (s) ampere (A) mole (mol) and candela (cd). 

While feet and yards are still used in Britain and other countries, the usual length is now the metre. At the time of the French Revolution in the 18th century and soon after, the French Academy of Sciences sought to systemize the measurement of all scientific quantities. This work led eventually to the concept of the Systeme Internationale, or SI for short. Within this system, all units and definitions are self-consistent. The SI unit of length is the metre. 

The SI unit of current I is the ampere (A). An ampere was first defined as the current flowing when a charge of 1 C (coulomb) passed per second through a perfect (i.e. resistance-free) conductor. The SI definition is more rigorous the ampbre is that constant current which, if maintained in two parallel conductors (each of negligible resistance) and placed in vacuo 1 m apart, produces a force between of exactly 2 x 10-7 N per metre of length . We will not employ this latter definition. 

For the more adventurous, parachute rockets are available in 22 and 25 mm calibre, or even festoons or bag rockets with sticks up to three metres in length. The latter are definitely fired individually ... 

According to the modern convention, measurable quantities are expressed in SI (System Internationale) units and replace the centimetre-gram-second (cgs) system. In this system, the unit of length is a metre (m, the unit of mass is kilogram (kg) and the unit of time is second (s). All the other units are derived from these fundamental units. The unit of thermal energy, calorie, is replaced by joule (1 J = 107 erg) to rationalize the definition of thermal energy. Thus, Planck s constant... 

A friction sensitive explosive, though chemically inert, 300 mg scraped with a spatula broke glassware 2 metres distant. Explosion in oxygen definitely produces boron trifluoride and it is presumed formation of this is the driving force. 

Only a few of the studies estimated the TVOC of the microenvironments reported [42,67,71] or focussed on complaint buildings [67,72]. Because different definitions and methods of TVOC were employed to estimate the values quoted, it is difficult to make direct interstudy comparisons nevertheless it appears that TVOC can range from 10 ggm to several thousand micrograms per cubic metre in indoor environments. 

These redox cells can operate on a number of scales that depend on the length of the diffusion path from the point that the oxidised form becomes reduced to the point where it reduces another sediment constituent. In some pelagic cores these diffusion paths can be observed in linear portions of the pore-water profiles (e.g. Sawlan Murray, 1983). Here the sedimentation rate and the carbon burial rate are sufficiently low, relative to diffusion, to extend the processes of early diagenesis over tens of metres into the sediment. In coastal environments the sedimentation rate and the concentration and reactivity of the organic matter is often high, which results in a much more complex pattern. In this case, the distances between the cells are much shorter, since by definition the adjustment must occur more rapidly. Like laminar and turbulent flow, there may come a point where the flow of electrons downwards is better dispersed through eddies , which in this case are transitory micro-environments with small-scale three dimensional diffusion, rather than more stable... 

Calcite-cemented layers typically have thicknesses from around 10 cm to a metre or two, and vary widely in lateral extent. Minimum lateral extent is a matter of definition, as it is really determined by where one chooses to start talking of layers rather than of concretions. Maximum lateral extents are certainly greater than a few kilometres, as such extents can be observed in cliff exposures (Bryant et al., 1988 Walderhaug et al., 1989), and lateral extents of tens of kilometres cannot be excluded. Intermediate lateral extents of tens and hundreds of metres have also been observed (McBride et al., 1995 Walderhaug et al., 1995). 

As measuring techniques became more precise and the demand for accuracy increased, the standards on which people based their units were improved. In the 18 century, the French invented the metric system, based on a more consistent, systematic, and carefully defined set of standards than had ever been used before. For example, the meter (or metre, from the Greek metron, a measure ) became the standard for length. The first definition for the standard meter was one ten-millionth of the distance from the North Pole to the Equator. This became outdated as the precision of scientist s measuring instruments improved. Today, a meter is defined as the distance light travels in a vacuum in 1/299,792,458 second. Technical instruments for measuring length are calibrated in accordance with this very accurate definition. 

Comite Consulatif Pour La Definition du Metre, J. Phys. Chem. Ref. Data, 3, 852,1974. 

The mean free path X of the molecule by definition is the average distance traveled between collisions. In one second, a molecule travels c> x 1 s) metres and makes Z collisions. Dividing the distance traveled by the number of collisions, we obtain the distance traveled between collisions ... 

The molar conductivity is the conductivity the solution would have if there were one mole of the substance in one cubic metre of the solution. Combining this definition with Eq. (31.23), we obtain... 

It was also in 1960 that the first laser was constructed and by the mid-1970s lasers were being used as length standards. In 1983 the krypton-86 definition was replaced and the metre was 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 and tins is done at NTL by an iodine-stabilised hefium-neon laser with an uncertainty of 3 parts in 10 (to the power 11). This is the equivalent to measuring the Earth s mean circumference to about 1 mm. 

F. Bayer-Helms, Documents concerning the new definition of the metre. Metrologia 19,163... 

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