[1] In which the following additional units appear: siemens (S), watt (W), second (s), farad (F), henry (H), weber (Wb), joule (J), coulomb (C), kilogram (kg), and meter (m).

In the US, a double vowel in the prefixed units "kiloohm" and "megaohm" is commonly simplified, producing "kilohm" and "megohm".

The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent, consistent, and international system of units for electrical quantities.

Telegraphers and other early users of electricity in the 19th century needed a practical standard unit of measurement for resistance.

Various artifacts, such as a length of wire or a standard electrochemical cell, could be specified as producing defined quantities for resistance, voltage, and so on.

The absolute-unit system related magnetic and electrostatic quantities to metric base units of mass, time, and length.

These units had the great advantage of simplifying the equations used in the solution of electromagnetic problems, and eliminated conversion factors in calculations about electrical quantities.

In 1860 Werner Siemens (1816–1892) published a suggestion for a reproducible resistance standard in Poggendorff's Annalen der Physik und Chemie.

One proposal was to devise a unit based on a mercury column that would be coherent – in effect, adjusting the length to make the resistance one ohm.

A legal ohm, a reproducible standard, was defined by the international conference of electricians at Paris in 1884 as the resistance of a mercury column of specified weight and 106 cm long; this was a compromise value between the B.

The international ohm is represented by the resistance offered to an unvarying electric current in a mercury column of constant cross-sectional area 106.3 cm long of mass 14.4521 grams and 0 °C.

In 1908, this definition was adopted by scientific representatives from several countries at the International Conference on Electric Units and Standards in London.

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.

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.

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.

This method avoids overlooking the decimal point, which may not be rendered reliably on components or when duplicating documents.