Ohm
Symbol

The symbol Ω was suggested, because of the similar sound of ohm and omega, by William Henry Preece in 1867. 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 56.

Historically, some document editing software applications have used the Symbol typeface to render the character Ω. 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 (& ohm;, & Omega;) is preferred. In MS-DOS and Microsoft Windows, the alt code ALT 234 may produce the Ω symbol. In Mac OS, ⌥ Opt + Z does the same.
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