Ohm

Ohm is the SI derived unit of electrical resistance with the unit symbol Ω ( large Greek Omega). It is named after Georg Simon Ohm ( 1789-1854 ). Named after him Ohm's law provides a simple relationship between the applied voltage (unit: V, V ) and the resulting current (unit: ampere, A) for so-called ohmic conductors dar. The reciprocal of the electrical resistance, so the electrical conductance G, has the unit Siemens.

Historically

At the First International Congress of Electricity Ohm name as " practical unity" of the electrical resistance for 1.000.000.000 cm / s was adopted on September 21, 1881; in the assumptions used therein variant of a cgs system (more precisely, electromagnetic cgs units ) is 1 cm / s, the " fundamental" unit of resistance. To realize the unit is one ohm while a column of mercury was introduced with fixed cross-sectional area at zero degrees Celsius. This design had Werner Siemens described in 1860 in the Annals of Physics, with dimensions, according to which a value of about 0.944 ohms yields, which was designated as 1 Siemens or Siemens unit (SE). Siemens made ​​and sold silver wires as secondary standard.

On the 4th International Congress of Electricians to Chicago in 1893 this realization rule was modified and found for the German Reich input in the "Law concerning the electrical units " of 1 June 1898. In the formulation of the International Conference for electrical units and standards in London 1908 contains the following provisions: 14.4521 g of mercury, mercury from 106,300 cm length with consistently the same cross section, constant current, temperature of melting ice. The so- called ohms realized "International Ohm". Improved measurement capabilities and voltage sources with more constant, but slightly different voltage values ​​( standard cells ) resulted over the subsequent period no longer acceptable deviations between practical and international ohm. ( Even more annoying was that a similar discrepancy in the units of mechanical or electrical energy and power ceased, which was greater than that of practical and international ohm. )

Basically had originally intended only as a realization rules definitions of " international " electrical units - even for amps and volts there were such - made ​​independently and were entered as a separate unit system in addition to the Gaussian cgs units with the practical electrical units. Therefore, led in 1948 the 9th CGPM again the absolute ohms, so to speak, now the only ohms. Precision rules were at the time: 1 international ohm is equal to 1.00049 ohms. In this form the Ohm was transferred to the MKSA system and this later integrated into the Système international d' unités.

Quantum Hall effect

In strong magnetic fields and low temperatures by a few Kelvin is shown that the Hall voltage divided by the current I, can not assume arbitrary values ​​when the magnetic field strength is varied. The result is instead always an integer fraction of the von Klitzing constant. The value amounts to approximately RK -90 = 25812.807 Ω and the breaking parts, and so on; at slightly different experimental conditions and values ​​can be as expected. The accuracy with which these quantum Hall effect can be reproduced is sufficiently good that has been defined by international agreement as the standard for the realization of the electrical resistance. Klaus von Klitzing was awarded for this discovery in 1985 the Nobel Prize for physics.

Representation in computer systems - coding

The Om is defined as follows and codes:

According to the Unicode standard, the physical units of ohms to be represented by the Greek capital letter Omega. Although Unicode also includes a character called Om (U 2126: Ω ), this was, however, included only for compatibility with older character encoding standards and should not be used in newly created texts.