Siemens (unit)

<h2 id="definition">Definition</h2>
For an element conducting <a href="/facts/Direct_current/r9MNUx84">direct current</a>, <a href="/facts/Electrical_resistance_and_conductance/hxSt0UAj">electrical resistance R and electrical conductance G</a> are defined as

G
        =
        
          
            1
            R
          
        
        =
        
          
            I
            V
          
        
      
    
    {\displaystyle G={\frac {1}{R}}={\frac {I}{V}}}

where I is the <a href="/facts/Electric_current/Dc1xdnUX">electric current</a> through the object and V is the <a href="/facts/Voltage/EN2po6N2">voltage</a> (electrical potential difference) across the object.
The unit siemens for the conductance G is defined by

[
          S
          ]
        
        =
        [
        
          Ω
          
            −
            1
          
        
        ]
        =
        [
        
          A
        
        
          /
        
        
          V
        
        ]
      
    
    {\displaystyle \mathrm {[S]} =[\Omega ^{-1}]=[\mathrm {A} /\mathrm {V} ]}

where Ω is the <a href="/facts/Ohm_(unit)/QNVrlGjw">ohm</a>, A is the <a href="/facts/Ampere/MScqkCgz">ampere</a>, and V is the <a href="/facts/Volt/KF18MXP2">volt</a>.
For a device with a conductance of one siemens, the electric current through the device will increase by one ampere for every increase of one volt of electric potential difference across the device.
The conductance of a resistor with a resistance of five ohms, for example, is (5 Ω)−1, which is equal to a conductance of 200 mS.

<h2 id="mho">Mho</h2>
For other uses, see <a href="/facts/MHO_(disambiguation)/zKAJAADA">MHO (disambiguation)</a>.
A historical equivalent for the siemens is the mho (/ˈmoʊ/). The name is derived from the word <a href="/facts/Ohm/QNVrlGjw">ohm</a> spelled backwards as the reciprocal of one ohm, at the suggestion of <a href="/facts/Sir_William_Thomson/LTIxG1jV">Sir William Thomson</a> (Lord Kelvin) in 1883.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> Its symbol is an upside-down capital <a href="/facts/Greek_alphabet/1cHwB8Dg">Greek</a> letter <a href="/facts/Omega/1CrSPtKM">omega</a>.
<a href="/facts/NIST/gr9Fnh85">NIST</a>'s Guide for the Use of the International System of Units (SI) refers to the mho as an "unaccepted special name for an SI unit", and indicates that it should be strictly avoided.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
The SI term siemens is used universally in science and often in electrical applications, while mho is still used in some electronic contexts.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a><a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
The upside-down capital omega symbol (℧), while not an official SI abbreviation, is less likely to be confused with a variable than the letter "S" when writing the symbol by hand. The usual typographical distinctions (such as italic for variables and roman for units) are difficult to maintain. Likewise, it is difficult to distinguish the symbol "S" (siemens) from the lower-case "s" (seconds), potentially causing confusion.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> So, for example, a <a href="/facts/Pentode/HuVlNVSt">pentode</a>’s <a href="/facts/Transconductance/lwh8krQa">transconductance</a> of 2.2 mS might alternatively be written as 2.2 m℧ or 2200 μ℧ (most common in the 1930s) or 2.2 mA/V.
The ohm had officially replaced the <a href="/facts/Siemens_mercury_unit/j6vKaz5r">old "siemens unit"</a>, a unit of resistance, at an international conference in 1881.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a>

<h2 id="notes-and-references">Notes and references</h2>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.bipm.org/en/si/si_brochure/">"The International System of Units"</a> (brochure). <a href="/facts/BIPM/tPEu4X0s">BIPM</a>.</li>
<li><a href="http://www.sizes.com/units/siemens.htm">"Different units named after Siemens"</a>. sizes.com.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Minutes (PDF). 14th General Conference on Weights and Measures. 1971. p. 53. <a href="http://www.bipm.org/utils/common/pdf/CGPM/CGPM14.pdf#page=53" target="_blank">http://www.bipm.org/utils/common/pdf/CGPM/CGPM14.pdf#page=53</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Minutes (PDF). 14th General Conference on Weights and Measures. 1971. p. 78. <a href="http://www.bipm.org/utils/common/pdf/CGPM/CGPM14.pdf#page=78" target="_blank">http://www.bipm.org/utils/common/pdf/CGPM/CGPM14.pdf#page=78</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"Chapter 9: Rules and Style Conventions for Spelling Unit Names". NIST Guide to the SI (Report). National Institute of Standards and Technology. 2008. Archived from the original on 2019-05-06. Retrieved 2017-12-22. <a href="https://web.archive.org/web/20190506222742/https://www.nist.gov/pml/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names" target="_blank">https://web.archive.org/web/20190506222742/https://www.nist.gov/pml/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Thomson, William (1884). "Electrical Units of Measurement". The Practical Applications of Electricity. Institution of Civil Engineers. pp. 149–174 at p 171 (Lecture given 3 May 1883). Available online. Thomson helpfully added that the proper pronunciation of "mho" could be obtained by taking a phonograph and turning it backwards. <a href="https://archive.org/details/practicalapplic00bramgoog/page/n182/mode/2up?q=mho" target="_blank">https://archive.org/details/practicalapplic00bramgoog/page/n182/mode/2up?q=mho</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">"Chapter 5: Units Outside the SI". NIST Guide to the SI (Report). National Institute of Standards and Technology. 2008. Retrieved 2017-12-22. <a href="https://www.nist.gov/pml/nist-guide-si-chapter-5-units-outside-si" target="_blank">https://www.nist.gov/pml/nist-guide-si-chapter-5-units-outside-si</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Al Dahaan, S., Al-Ansari, N., & Knutsson, S. (2016). Influence of groundwater hypothetical salts on electrical conductivity total dissolved solids. Engineering, 8(11), 823-830. <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">SINGH, S., GAUTAM, P. K., KUMAR, P., BISWAS, A., & SARKAR, T. (2021). Delineating the characteristics of saline water intrusion in the coastal aquifers of Tamil Nadu, India by analysing the Dar-Zarrouk parameters. Contributions to Geophysics & Geodesy, 51(2). <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Weiner, Eugene R. (2013). Applications of Environmental Aquatic Chemistry: A practical guide. CRC Press. p. 109. ISBN 978-1439853320. <a href="978-1439853320" target="_blank">978-1439853320</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">"Siemens (unit of electrical conductance)". www.tech-faq.com. 6 April 2019. <a href="http://www.tech-faq.com/siemens.html" target="_blank">http://www.tech-faq.com/siemens.html</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
</ol>

Siemens (unit) open-in-new

Siemens (unit)