Units of measurement

Geometric and physical quantities (also size unit or physical unit ) specified in units which have a unique (in practice, fixed, well-defined ) value. All other values ​​of the respective size are given as multiples of the unit.

Known units are, for example, meter, kilogram, and second.

Properties

Units can be defined for all size as well for non-physical quantities such as the perceptual variables critical band rate or loudness.

To avoid measuring values ​​with very large or very small numbers resolutions can be used for units of measurement ( exceptions eg in degrees Celsius or minute) with most units.

Dimensionless variables have the unit 1 (one). This unit will be awarded for clarity often additional names ( auxiliary units), such as rad or steradian. However, auxiliary units can also be omitted. For units of measure, for example, 1 % (percent), ‰ ( parts per thousand ) or ppm ( millionth) above are used.

Unit systems

Units can be combined into systems of units such as the International System of Units. A system of units has certain base units from which arise by deriving additional units.

Unit characters

Unit symbols are used to represent the unit name. They are mostly Latin letters or groups of letters, but also Greek letters or other characters. For old units and unit symbols were in use, do not belong to the alphabet. Unit symbols are not italicized - not even when the surrounding text is in italics. For measurements is between the number and the unit is a space; separation by line break should be avoided.

For more information on the correct use can be found here: spelling of quantities, numerical values ​​and units.

Conversion of units

The value of a physical quantity is generally the product of a number and a physical unit. To illustrate this value with another unit (of the same size type ), you can form this product and use known relationships between the units.

Example: A table has a height of 75 cm. It is known that 1 m = 100 cm. This can be transformed to: 75 cm = 0.75 × 100 cm = 0.75 m.

Often, a unit is a multiple of the other (the " multiples " is not limited here to integer ratios ), in some cases, the relationship is, however, differently, for example is true for temperatures are in degrees Celsius, and in Kelvin: the two temperature scales have different zero points.

If a unit is a multiple of the other, one can perform the conversion by multiplying with 1 to give 1 writes as a quotient of two identical units in the two units, so that cancels out the first unit and the second stops.

The conversion from the above example can thus also perform this:

If a unit is a product or quotient of other units, such conversions can be applied to the latter. If the direct relationship between the two units is not known, but the relationship with each of a third unit, for example a SI unit, the conversion can be carried out by conversion in the third unit, and which are linked to this in the target unit.

Example: 463 feet (ft) per minute (min) are in knots ( kn) to be converted. It is known that 1 ft = 0.3048 m, 1 min = 60 s, 1 kn = 1 sm / h, 1 sm = 1852 m, 1 h = 3600 s

Values ​​of the resolutions for measurement units in the corresponding article. Conditions more recognized units are given in the respective physical quantity, for example, energy or pressure, or in a linked article there on units of this size, such as length dimension. Even with a unit can their relationship to an SI unit to be specified, eg in Langley.

History

In earlier times, units were mostly defined by material measures that had the corresponding property. It's quite possible this is eg of length, volume and mass units, as these are represented by metal rods, spheres or hollow vessels. Fixed at a representative point, often built into the facade of City Hall, enabled such a degree each to calibrate their own instruments. In the SI system of units, the kilogram is the only unit that is defined in this way. Units were previously set very arbitrary and often mutually unrelated, but on practical considerations such as length dimensions of the human body.

More abstract units had earlier in the day only a subordinate role. Such units must be defined via measurement rules that are relatively easy to reproduce with high accuracy. It is necessary to distinguish between " definition " and " realization rule"; the appropriate implementation procedures often differ from the one specified in the definition process. Which method is suitable depends on the accuracy requirements. For example, much more effort can be operated as during calibration of commercial scales for the "presentation " of a unit as a national standard. Depending on the accuracy requirement still embodied dimensions may be relevant today.

Examples

In the International System of Units The kilogram is defined by the mass of Urkilogramms in Paris. All masses are given as multiples of this mass. For example, " 5.1 kg " as much as " 5.1 times as large as the mass of the mass Urkilogramms in Paris " means the specification.

The speed unit meter / second is a class derived from the base units meter and second unit in the SI. However, the Mach number, the unit 1

Examples of old units:

• Horsepower ( hp): 75 kg in the gravitational field of the earth to lift power that is needed in a second meter.
• Torr (or mm Hg ) pressure which corresponds to a mercury column of 1 mm.
• Kilogram-force ( kgf ): Weight force of mass 1 kg in the gravitational field of the earth.