Unit

a unit is an instrumentation comparison value with well-defined value.

characteristics

condition for the fact that a size is scalable and a unitfor it to be defined can, is:

• There is a reference value (a reference observation), to which the starting point of the scale (usually the zero point or also another defined point as for example the boiling point) can refer.
• It exists a clear defaultthe gradation and/or. Partitioning of the scale, with which the observed values can be defined.

Units are defined whenever sizes can be scaled. Apart from units for physical dimension there is also units for non-physical sizes, z. B. perception-referred sizeslike the clay/toneness, soundness etc.

An overview of physical units is under physical dimension.

Units are used for the elucidation often also for dimensionless sizes (information size units), although these would be representable as pure numbers also without unit:e.g. Per cent for efficiencies, ppm (= millionths) for small concentrations, radian or degree for angles.

systems of units

one often combines today units into systems of units, in which some basic units must be only defined, and all other unitsfrom these result.

unit symbols

unit symbols are letters or groups of letters, which are used representatively of the names of units; in rare cases it can be also Greek letters. Most unit symbols are internationally uniformly fixed.

Not for all up-to-datecommon units are unit symbols fixed; some units have several equal unit symbols. For old units also unit symbols were common, which do not belong to an alphabet.

history

in earlier times units becamemostly over reference body (so-called. “Measure embodiments”) defines, which had the appropriate characteristic. For it longitudinal dimensions, volume, masses , which are representable over metal bars, balls or hollow containers, are well suitable. To representative place fastens, frequently in the front of the city hall immured,made possible it such a measure for everyone to calibrate its own measuring instruments. In the system of units SI the kilogram is at present the only unit, which is in this way defined. Units became in former times very arbitrary and often without relationship to each other, but afterpractical criteria, like longitudinal dimensions at the human body fixed.

Abstrakterere units had a subordinated meaning at the time in the everyday life only. One must define such units over measuring regulations, which are to be reproduced comparatively simply with high accuracy. - Differentiates with respect to the technical rangeone however between “definition” and “realization regulation”; the suitable realization procedures often differ from the procedure specified in the definition. Which procedure is suitable, depends on the precision demands. For example much higher can for the “representation” of a unit as national normalExpenditure to be operated as when the calibration commercial balances. Depending upon precision demand also today still embodied mass can be current.

See also: History of masses and weights

of examples

in the SI-UNIT system is defined the kilogram by the mass of the Urkilogramms in Paris (also international prototype called). All masses are indicated as multiple of this mass, z. B. means the indication “5 kg” as much as “5 times as large mass as the mass of the Urkilogramms in Paris”. By comparative measurements with nationalCopies of the Urkilogramms one determined that it seems to lose mass with the time, what in the sense of the inventors did not lie naturally. Scientists in all world look now for an accurately reproducible definition of the kilogram, either by atomic counting orFeedback of the kilogram on fundamental constants of physics.

The speed unit meter/second is in SI a unit derived from the fundamental units meters and second. However the speed unit Mach is from other units derived and not into a system of units not merged.

Further exampleswith units partly become outdated:

of examples of old units:

• Horse power (HP): Achievement, thoseis needed, in order to lift 75 kg in the gravity field of the earth in one second a meter.
• Torr (and/or. mm Hg): Pressure, the one mercury column of 1 mm corresponds
• to kilopond (kp): Gewichtskraft der Masse 1 kg im Schwerefeld der Erde

see also

• history of masses and weights
• physical dimension
• list of the physical symbols
• list of the prefixes for units

Web on the left of

Wiktionary: Unit - word origin, synonyms and translations

• on-line units Umrechner: Conversion of very many units
• on-line ones Units Umrechner: Conversion of very many units
• of compact Einheitenumrechner for the PC (Freeware)
• introduction to the units of different fields of activity
• some mass & weights
• 3sat - nano: Race around the kilogram - different methods of the mass regulation