ohm

[After Georg Simon OHM.]

Symbol Ω. The derived SI unit of electrical resistance, being the resistance between two points on a conductor when a constant potential difference of one volt, applied between these points, produces a current of one ampere in the conductor. The former international ohm (sometimes called the 'mercury ohm') was defined in terms of the resistance of a column of mercury. The unit is named after Georg Ohm (1787–1854).

The unit of electrical resistance, named after the nineteenth-century German physicist Georg Ohm.

The unit of measurement of electrical resistance in a material. One ohm is the resistance in a circuit when one volt maintains a current of one amp. The symbol for ohm is the Greek letter omega. See impedance.

Ohm's Law

The equation "R=V/I" is the more streamlined version of the one developed by German physicist Georg Simon Ohm in 1827. Ohm's law is used to calculate the resistance in materials such as metal, which maintain a linear relationship between voltage and current. In addition, Ohm's formulas, which are derived from Ohm's Law, are used to calculate voltage and current if the other two measurements are known.

     Ohm's Law

     Resistance = voltage divided by current
     R = V / I   or   R = E / I


     Ohm's Formulas

     Voltage = current times resistance
     V = I * R   or   E = I * R

     Current = voltage divided by resistance
     I = V / R   or   I = E / R


         V or E = voltage (E=energy)
         I = current in amps (I=intensity)
         R = resistance in ohms


     Electric Power

     Power in watts = voltage times current
     P = V * I

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[Etymology: G. S. Ohm; Germany 1787-1854] electric resistance. Symbol officially Ω sometimes, to avoid an alien graphic, R. The ohms of resistance between two points of a conductor carrying a steady current equals the ratio of potential difference in volts across these points to the amperes of current flowing in the conductor (the conductor not being the seat of any electromotive force). Ω = V·A^-1 = W·A^-2.

SI, Metric-m.k.s 1948 (= m²·kg·s^-3·A^-2 in base terms). The following are among the coherent derived units:

• Ω·m²·m^-1(=Ω·m) for electric resistivity;
• ohms per square for surface resistivity - the resistance, between perfect conducting strips, of any square on a surface being independent of size (the mutliplicative effect of any change in distance between the wires exactly offsetting the reciprocal multiplication of the width of the conducting surface).

The reciprocal of the ohm in the SI is the siemens, in older m.k.s. the mho.

Metric-c.g.s. See abohm; statohm. See also practical unit.

History

The ohm, initially the ohmad, was the original unit defined specifically for electrical usage, in 1838, with 1 ft of #11 copper wire being the standard. The burgeoning telegraph, the pioneer of telecommunications, was critically affected by the resistance of its wires, which set the interval at which repeater stations had to be provided. Resistance measurements based on familiar wires were natural, but laboratory specifications and more theoretical approaches were a necessary parallel. The abohm (the ohm of the absolute e.m.u. system) was one form, but a very small unit. This gave the practical ohm, agreed in 1881 at the first International Electrical Conference
[Nature Vol. 24, 512 (1881)] as being 10⁹ abΩ.

That Conference agreed that the ohm should be represented by a column of mercury of cross-section 1 mm² and at 0°C, the length to be determined, making it a base unit instead of a derived unit; an accuracy of one part in 2 000 was expected. The initial determination for the established ohm gave a length of 106 cm for what has often been called the mercury ohm or Hg ohm. The specification was subsequently shown to have made the ohm smaller than intended by about 0.3%, prompting the IEC of 1891 to change the specification to 106.3 cm of mercury, elaborated in 1893 to 106.300 cm of mercury with a mass of 14.452 1 g, at the temperature of melting ice, i.e. precise now to six significant figures, plus defining the more important conditions.
[Nature Vol. 78, 678-81 (1908)] In 1908 the IEC, still beset with discrepancies between the e.m.u.-based units and the experimental specifications, adopted the unadorned qualifier ‘international’ for the latter, giving the international ohm, lacking reference to being absolute or practical (though it was the latter). Because of experimental vagaries, the value for conversions is normally referred to as the mean international ohm, Ω_M = 1.000 49~ Ω.
[Nature Vol. 163, 427-8 (1949)] The US international ohm = 1.000 495~ Ω.

By the 1930s it was easier to reproduce the absolute-based ohm than the mercury ohm, allowing definition of the common ohm from abohm, as done in 1881. World War II delayed implementation until 1948, when it became simply a derived unit. The modern SI ohm, sometimes called the absolute ohm and identically that of the Metric-m.k.s.A. system, has reverted to the original value (of the practical ohm). Discovery of the Josephson effect, then of the quantum Hall effect, applying at very low temperatures with superconductors, together with subsequent development of the moving-coil balance and related work with the volt, improved accuracies about a thousand-fold for the ampere, volt, ohm, etc.
[Taylor B. N. Metrologia Vol. 21, 37-9 (1985)] Revised conventional values for the constants involved in laboratory realizations using these effects, adopted internationally beginning in 1990, resulted in reducing the resistance value of many laboratory standards by about 0.000 15%.
[Kibble B., Hartland A. New Scientist Vol. 1715, 48-51 (1990)]

The unit of electrical resistance of a conductor such that a constant current of 1 ampere in it produces a decrease in voltage across it of 1 volt.

To convert from ohms (international) to:

ohm (absolute), multiply by 1.0005.
megohms, multiply by 1E-06.
microhms, multiply by 1E+06.

This article is about the SI (Omega) derived unit. For other meanings, see Ohm (disambiguation).

A multimeter can be used to measure resistance in ohms. It can also be used to measure capacitance, voltage, current, and other electrical characteristics.

Several resistors. Their resistance, in ohms, is marked using a color code.

The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Simon Ohm.

Contents 1 Definition 2 Conversions 3 Power as a function of resistance 4 Use of the Ω symbol in electronic documents 5 References and notes 6 See also 7 External links

Definition

The ohm is defined as the resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.^[1]

In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters; one speaks of linear resistors. In other cases resistance varies (e.g., thermistors).

Commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, and megohm.^[2]

Conversions

• The SI unit of electrical conductance is the siemens, also known as the mho (ohm spelled backwards, symbol is ℧); it is the reciprocal of resistance in ohms.

Power as a function of resistance

The power dissipated by a linear resistor may be calculated from its resistance, and voltage or current. The formula is a combination of Ohm's law and Joule's laws:

where P is the power in watts, R the resistance in ohms, V the voltage across the resistor, and I the current through it.

This formula is applicable to devices whose resistance varies with current.

Use of the Ω symbol in electronic documents

Care should be taken when preparing documents (including HTML documents) which make use of the symbol Ω. Some document editing software will attempt to use the symbol typeface to render the character. Where the font is not supported, a W is displayed instead. As this represents the SI unit of power, not resistance, this can lead to confusion.

Unicode encodes an ohm symbol distinct from Greek omega among Letterlike Symbols.

References and notes

1. ^ BIPM SI Brochure: Appendix 1, p. 144
2. ^ The NIST Guide to the SI: 9.3 Spelling unit names with prefixes reports that IEEE/ASTM SI 10-2002 IEEE/ASTM Standard for Use of the International System of Units (SI): The Modern Metric System states that there are three cases in which the final vowel of an SI prefix is commonly omitted: megohm, kilohm, and hectare. « In all other cases in which the unit name begins with a vowel, both the final vowel of the prefix and the vowel of the unit name are retained and both are pronounced. » This usage is peculiar to English.

See also

• Abohm
• Ohm's law
• Resistor
• Resistivity

External links

• Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuernberg
• Official SI brochure
• NIST Special Publication 811
• History of the ohm at sizes.com

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - ohm

Nederlands (Dutch)
eenheid van weerstand

Français (French)
n. - (Élec) ohm

Deutsch (German)
n. - Ohm

Ελληνική (Greek)
n. - (ηλεκτρ.) (μονάδα) Ομ

Italiano (Italian)
Ohm

Português (Portuguese)
n. - ohm (m)

Русский (Russian)
ом

Español (Spanish)
n. - ohm, ohmio

Svenska (Swedish)
n. - ohm(fys)

中文（简体）(Chinese (Simplified))
欧姆

中文（繁體）(Chinese (Traditional))
n. - 歐姆

한국어 (Korean)
n. - 전기저항의 단위 옴

日本語 (Japanese)
n. - オーム

العربيه (Arabic)
‏(الاسم) وحدة قوة المقاومه الكهربائيه‏

עברית (Hebrew)
n. - ‮יחידת התנגדות להולכת זרם, אום (בחשמל)‬

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