Ohm
The ohm (symbol: Ω, the uppercase Greek letter omega) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861.
This article is about the SI derived unit. For other uses, see Ohm (disambiguation).Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined as an exact value in terms of these constants.
Relation to conductance[edit]
The siemens (S) is the SI derived unit of electric conductance and admittance, historically known as the "mho" (ohm spelled backwards, symbol is ℧); it is the reciprocal of the ohm: 1 S = 1 Ω−1.
Power as a function of resistance[edit]
The power dissipated by a resistor may be calculated from its resistance, and the voltage or current involved. The formula is a combination of Ohm's law and Joule's law:
where P is the power, R is the resistance, V is the voltage across the resistor, and I is the current through the resistor.
A linear resistor has a constant resistance value over all applied voltages or currents; many practical resistors are linear over a useful range of currents. Non-linear resistors have a value that may vary depending on the applied voltage (or current). Where alternating current is applied to the circuit (or where the resistance value is a function of time), the relation above is true at any instant, but calculation of average power over an interval of time requires integration of "instantaneous" power over that interval.
Since the ohm belongs to a coherent system of units, when each of these quantities has its corresponding SI unit (watt for P, ohm for R, volt for V and ampere for I, which are related as in § Definition) this formula remains valid numerically when these units are used (and thought of as being cancelled or omitted).
Realization of standards[edit]
The mercury column method of realizing a physical standard ohm turned out to be difficult to reproduce, owing to the effects of non-constant cross section of the glass tubing. Various resistance coils were constructed by the British Association and others, to serve as physical artifact standards for the unit of resistance. The long-term stability and reproducibility of these artifacts was an ongoing field of research, as the effects of temperature, air pressure, humidity, and time on the standards were detected and analyzed.
Artifact standards are still used, but metrology experiments relating accurately dimensioned inductors and capacitors provided a more fundamental basis for the definition of the ohm. Since 1990 the quantum Hall effect has been used to define the ohm with high precision and repeatability. The quantum Hall experiments are used to check the stability of working standards that have convenient values for comparison.[17]
Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined in terms of these constants.
Symbol[edit]
The symbol Ω was suggested, because of the similar sound of ohm and omega, by William Henry Preece in 1867.[18] In documents printed before Second World War the unit symbol often consisted of the raised lowercase omega (ω), such that 56 Ω was written as 56ω.
Historically, some document editing software applications have used the Symbol typeface to render the character Ω.[19] Where the font is not supported, the same document may be displayed with a "W" ("10 W" instead of "10 Ω", for instance). As W represents the watt, the SI unit of power, this can lead to confusion, making the use of the correct Unicode code point preferable.
Where the character set is limited to ASCII, the IEEE 260.1 standard recommends using the unit name "ohm" as a symbol instead of Ω.
In the electronics industry it is common to use the character R instead of the Ω symbol, thus, a 10 Ω resistor may be represented as 10R. This is part of the RKM code. It is used in many instances where the value has a decimal place. For example, 5.6 Ω is listed as 5R6, or 2200 Ω is listed as 2K2. This method avoids overlooking the decimal point, which may not be rendered reliably on components or when duplicating documents.
Unicode encodes the symbol as U+2126 Ω OHM SIGN, distinct from Greek omega among letterlike symbols, but it is only included for backward compatibility and the Greek uppercase omega character U+03A9 Ω GREEK CAPITAL LETTER OMEGA (Ω, Ω) is preferred.[20] In MS-DOS and Microsoft Windows, the alt code ALT 234 may produce the Ω symbol. In Mac OS, ⌥ Opt+Z does the same.