The length of an object represents the distance between two of its ends, most distant possible. When the object is thread-like or in the shape of lace, its length is often that of the completely developed object.

The length of an object makes it possible to appreciate its size. To determine the length of an object, we compare his size with that of another object taken as unit. For example, the length of a wall can be evaluated in meters and in this case we determine the number of times that we can place a Standard meter along this wall on the basis of an end and without exceeding the other end; and to have a more precise measurement we can evaluate the remaining distance by in the same way comparing it with tenth of meter, and continue so on…

There exist several units of length: the meter, the decimetre, the centimetre, the millimetre and decametre, the kilometer… Which belong to the international Système.

The length of a physical object is not an intrinsic property; this one can depend on the temperature, of time…

The length can in certain situations, to represent one duration, as in the length of the days, or the expression “with length of day” which means during all the day or in “trailing in length” which wants to say to last too a long time.

In Data-processing, the length of a word written in an unspecified alphabet corresponds to the number of letters which compose the Of the same mot., the length of a character string corresponds to the number of characters which constitute the chain.

Measure a length

The distance between two points is measured with a right rule (a Toise) which can be graduated
  • under multiples (decimetre, centimetre, millimetre, micrometer)
  • or multiples (one decametre, a measuring land chain, a mileage odometer)

In on this side micrometer (nanometer, picometer, femtometer), one cannot use visual method any more (problem of Diffraction, the Wavelength of the visible light being about 500 Nm). Other radiations thus should be used. Nature provides structures being able to be used as standards: atoms, cores, particles which have parameters invariants.

Beyond the kilometer: measurements are made by the measurement of time that the light or more generally the electromagnetic waves puts to traverse the “right” line which separates two objects: the distance ground sun is of approximately 8 light minutes = 480 second light = 8 × 60 × 1 S × 300.000 km/s.

One makes correspond two points of the object which one wants to measure the distance which separate them by making them coincide with points of the rule; of course is needed that the object and the rule is rigid, indeformable.

the equality then is written: Length = × measurement the unit.

example: let us measure a page of paper with a formed rule of 3 decimetres graduated in mm; the page has for width ( L ) 21 centimetres and as a length ( L ) 29,7 centimetres (or 29.7 centimetres according to whether one represents the numbers with a comma or a point to position the passage of the units to the tenth of unit).

One notes in short:

L = 21 cm = 21 × 1 cm = 21 × 0,01 × 1 m = 0,21 m
and
L = 29,7 cm = 29,7 × 1 cm = 29,7 × 0.01 × 1 m = 0,297 m

It is impossible to measure the thickness of the sheet with the same rule; on the other hand one can measure the thickness of a pile of 500 sheets (a oar) and to note that 500 × E = 5 cm and to deduce from it that the thickness of a sheet must be of a tenth of millimetre.

The measurement lengths frequently utilizes the Trigonométrie, with the technique of Triangulation.

Measuring instruments

Also see

length The international metric system: The imperial system Great Britain: Old units of length: Other units:

External bonds

  • Conversion of units of length

Zh-min-nan: Tn̂g-t-piece

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