Unit of measurement
Geometric and physical quantities are specified in units of measurement (also size units or physical units ) that have a clear (in practice, well-defined, fixed) value. All other values of the respective size are given as a multiple of the unit. Well-known units of measurement are, for example, meters , seconds , kilowatt hours , Hertz or kilometers per hour .
Quantities of the dimension number have the unit of measurement 1 (one). This unit is often given additional names ( auxiliary units) for clarity , for example rad or steradian . However, auxiliary units of measure can also be omitted. For proportions of unit 1 are z. B. % (percent), ‰ (per mille) or ppm (millionths) commonly used.
Systems of units
Unit symbols are used to represent the unit names. They are mostly Latin letters or groups of letters, but also Greek letters or other characters. Unit symbols that do not belong to any alphabet were also used for old units of measurement. Unit symbols are not set in italics - not even if the surrounding text is italic. For measurements, there is a space between the number and the unit symbol; a line break separation should be avoided.
Further information on correct use can be found here: Notation of quantities, numerical values and units .
The value of a physical quantity is generally the product of a number and a physical unit. In order to represent this value with a different unit (of the same size type), one can transform this product and use known relationships between the units.
Example: A table has a height of 75 cm. It is well known that 1 m = 100 cm. This can be used to shape: 75 cm = 0.75 × 100 cm = 0.75 m.
Often one unit is a multiple of the other (the “multiple” does not have to be an integer), but in some cases the relationship is different. For example, for temperatures in degrees Celsius and in Kelvin:, the two temperature scales have different zero points.
If one unit is a multiple of the other, the conversion can be carried out by multiplying by 1, whereby 1 is written as the quotient of two equal quantities in the two units, so that the first unit is canceled and the second remains.
The conversion from the example above can also be carried out as follows:
If a unit is the product or quotient of other units, such conversions can be applied to the latter. When the direct relationship between two entities is not known, but the relationship to a third entity, e.g. B. an SI unit, the conversion can be carried out by concatenating the conversion into the third unit and that of this into the target unit.
For values of the prefixes for units of measure see this article.
In earlier times, units of measurement were usually defined using material measures that had the corresponding property. This is quite possible, for example. B. for length , volume and mass units , because these can be represented by metal rods, spheres or hollow vessels. Installed in a generally accessible place, for example walled into the facade of the town hall , such a measure made it possible for everyone to calibrate their own measuring devices . Units of measurement used to be very arbitrary and often unrelated to one another, but based on practical aspects such as length dimensions on the human body.
More abstract units of measurement used to have only a subordinate meaning in everyday life. Such units have to be defined using measurement rules that can be reproduced comparatively easily with high accuracy . A distinction must be made between “definition” and “implementation rule”; the appropriate implementation procedures often differ from the procedure specified in the definition. Which method is suitable depends on the accuracy requirements. For example, much more effort can be made to “represent” a unit of measurement as a national standard than for the verification of commercial scales. Depending on the accuracy requirement, embodied dimensions can still be current today.
In the International System of Units, the kilogram was defined by the mass of the original kilogram in Paris until 2019 . All masses are given as multiples of this mass. For example, the specification "5.1 kg" meant "5.1 times the mass of the original kilogram in Paris".
Examples of old units:
- Horsepower (PS): The power required to lift 75 kg in the earth's gravitational field one meter in one second.
- Torr (or mm Hg ): pressure equivalent to 1 mm of mercury.
- Kilopond (kp): weight of the mass 1 kg in the gravitational field of the earth .
- Friedrich Kohlrausch : General information about measurements and their evaluation . In: Volkmar Kose, Siegfried Wagner (Ed.): Practical Physics . 24. rework. and exp. Edition. tape 3 . BG Teubner, Stuttgart 1996, ISBN 3-519-23000-3 , 9.1 Systems of terms and units, p. 3–19 ( ptb.de [PDF; 3.9 MB ; accessed on November 24, 2018] published by the Physikalisch-Technische Bundesanstalt).
- Hans Dieter Baehr: Physical quantities and their units . An introduction for students, natural scientists and engineers (= study books natural science and technology . Volume 19 ). Bertelsmann University Press , Düsseldorf 1974, ISBN 3-571-19233-8 .
- Hans-Joachim von Alberti: Measure and Weight: Historical and tabular representations from the beginnings to the present. Berlin 1957.
- Gerhardt Hellwig: Lexicon of the measures and weights. Gütersloh 1983.
- Link catalog on the topic of units of measurement at curlie.org (formerly DMOZ )
- Convert units of measure
- List of over 300 metric units of measurement, Olle Järnefors: Metric Units Galore: 311 Named Units with Symbol, Definition and Size. October 4, 2000, accessed August 12, 2017 .
- List of 200 non-metric units of measurement, Olle Järnefors: The British / US Unit Mess. October 4, 2000, accessed August 12, 2017 .