System of measurement

Unit system, formerly measurement system is a collection of units, in which each size type is assigned to exactly one unit. In Germany, the International System of Units (SI ) is used in general. Other unit systems are the CGS system or the Anglo-American system of measurement.


Physical quantities can ( short unit ) only as a multiple of a unit can be specified. So the equation for the relationship of place, time and speed is in unaccelerated motion

Where the unit of length, the speed unit and the time unit. is a real constant of proportionality that depends on the choice of units.

By rearranging this equation, one can combine the constants and receives


Is, for example, the location in meters (m ), the time in seconds (s ) and the speed as a multiple of the vacuum light velocity () is given, and said constant is

So if you have for example a speed of 0.5 c and a time of 2 s, we obtain the equation

- A conclusive result.

Since it is impractical to carry such a constant in each equation, choose units suitably so that many constants to be 1. Thus one (ie m / s according to the above example) defines the unit of speed as meters / second, and this results in the above equation the constant to what it is the familiar equation


The constant in this equation thus says something about the unit system used. Many constants of nature are in fact " unit system constants ". Thus, the Boltzmann constant is nothing more than a conversion factor between energy and temperature (which is why the temperature is also often expressed in units of energy ). So you really says nothing about the nature, but just something about the temperature scale used.

Variants of unit systems

While it makes little sense for the sake of intuition to define a unit system in which does not apply, who have been specially established quite different spellings of sizes equations for the physical quantities of electrodynamics. That's about the first Maxwell equation in vacuum in SI units

In Gaussian cgs units

(also called rationalized cgs ) and Heaviside - Lorentz units

These spellings differ from the perspective of SI only in that in the two CGS systems, the constant is arbitrarily set equal to a number. This has the consequence that the electric current of the character of the base size in these systems of units; also can be a size value such as 2.0 cm no longer consider what size is meant - it must be a length not.

Some important unit systems are:

  • The SI unit system ( and its predecessor the MKS system and MKSA System)
  • Technical measurement system
  • CGS unit system
  • Gaussian System of Units
  • Heaviside - Lorentz system of units
  • Geometric units (in the theory of relativity )
  • Natural units (in the high energy physics)
  • Atomic units ( in atomic physics )
  • Various systems of astronomical units
  • Planck units