The ohm (symbol: Ω) is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time and of a convenient size for practical work as early as 1861. The definition of the ohm was revised several times. Today, the definition of the ohm is expressed from the quantum Hall effect.
A laboratory one-ohm standard resistor, circa 1917.
|Unit system||SI derived unit|
|Unit of||Electrical resistance|
|Named after||Georg Ohm|
|In SI base units:||kg⋅m2⋅s−3⋅A−2|
The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the conductor not being the seat of any electromotive force.
In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters. These are called linear resistors. In other cases resistance varies (e.g., thermistors).
A vowel of the prefixed units kiloohm and megaohm is commonly omitted, producing kilohm and megohm.
In alternating current circuits, electrical impedance is also measured in ohms.
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).
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. Resistance was often expressed as a multiple of the resistance of a standard length of telegraph wires; different agencies used different bases for a standard, so units were not readily interchangeable. Electrical units so defined were not a coherent system with the units for energy, mass, length, and time, requiring conversion factors to be used in calculations relating energy or power to resistance.
Two different methods of establishing a system of electrical units can be chosen. 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. Alternatively, the electrical units can be related to the mechanical units by defining, for example, a unit of current that gives a specified force between two wires, or a unit of charge that gives a unit of force between two unit charges. This latter method ensures coherence with the units of energy. Defining a unit for resistance that is coherent with units of energy and time in effect also requires defining units for potential and current. It is desirable that one unit of electrical potential will force one unit of electric current through one unit of electrical resistance, doing one unit of work in one unit of time, otherwise all electrical calculations will require conversion factors.
Since so-called "absolute" units of charge and current are expressed as combinations of units of mass, length, and time, dimensional analysis of the relations between potential, current, and resistance show that resistance is expressed in units of length per time — a velocity. Some early definitions of a unit of resistance, for example, defined a unit resistance as one quadrant of the Earth per second.
The absolute-units 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. However, the centimeter-gram-second, CGS, units turned out to have impractical sizes for practical measurements.
Various artifact standards were proposed as the definition of the unit of resistance. In 1860 Werner Siemens (1816–1892) published a suggestion for a reproducible resistance standard in Poggendorffs Annalen der Physik und Chemie. He proposed a column of pure mercury, of one square millimeter cross section, one metre long: Siemens mercury unit. However, this unit was not coherent with other units. 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. Not all users of units had the resources to carry out metrology experiments to the required precision, so working standards notionally based on the physical definition were required.
In 1861, Latimer Clark (1822–1898) and Sir Charles Bright (1832–1888) presented a paper at the British Association for the Advancement of Science meeting  suggesting that standards for electrical units be established and suggesting names for these units derived from eminent philosophers, 'Ohma', 'Farad' and 'Volt'. The BAAS in 1861 appointed a committee including Maxwell and Thomson to report upon standards of electrical resistance. Their objectives were to devise a unit that was of convenient size, part of a complete system for electrical measurements, coherent with the units for energy, stable, reproducible and based on the French metrical system. In the third report of the committee, 1864, the resistance unit is referred to as "B.A. unit, or Ohmad". By 1867 the unit is referred to as simply ohm.
The B.A. ohm was intended to be 109 CGS units but owing to an error in calculations the definition was 1.3% too small. The error was significant for preparation of working standards.
On September 21, 1881 the Congrès internationale des électriciens (international conference of electricians) defined a practical unit of ohm for the resistance, based on CGS units, using a mercury column 1 sq. mm. in cross-section, approximately 104.9 cm in length at 0 °C, similar to the apparatus suggested by Siemens.
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. A. unit (equivalent to 104.7 cm), the Siemens unit (100 cm by definition), and the CGS unit. Although called "legal", this standard was not adopted by any national legislation. The "international" ohm was recommended by unanimous resolution at the International Electrical Congress 1893 in Chicago. The unit was based upon the ohm equal to 109 units of resistance of the C.G.S. system of electromagnetic units. 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. This definition became the basis for the legal definition of the ohm in several countries. 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 standard was maintained until the 1948 General Conference on Weights and Measures, at which the ohm was redefined in absolute terms instead of as an artifact standard.
By the end of the 19th century, units were well understood and consistent. Definitions would change with little effect on commercial uses of the units. Advances in metrology allowed definitions to be formulated with a high degree of precision and repeatability.
|Unit||Definition||Value in B.A. ohms||Remarks|
|Absolute foot/second × 107||using imperial units||0.3048||considered obsolete even in 1884|
|Thomson's unit||using imperial units||0.3202||100 million feet/second, considered obsolete even in 1884|
|Jacobi copper unit||A specified copper wire 25 feet long weighing 345 grains||0.6367||Used in 1850s|
|Weber's absolute unit × 107||Based on the metre and the second||0.9191|
|Siemens mercury unit||1860. A column of pure mercury||0.9537||100 cm and 1 mm2 cross section at 0 °C|
|British Association (B.A.) "ohm"||1863||1.000||Standard coils deposited at Kew Observatory in 1863|
|Digney, Breguet, Swiss||9.266–10.420||Iron wire 1 km long and 4 square mm cross section|
|Matthiessen||13.59||One mile of 1/16 inch diameter pure annealed copper wire at 15.5 °C|
|Varley||25.61||One mile of special 1/16 inch diameter copper wire|
|German mile||57.44||A German mile (8,238 yard) of iron wire 1/6th inch diameter|
|Abohm||10−9||Electromagnetic absolute unit in centimeter–gram–second units|
|Statohm||8.987551787 × 1011||Electrostatic absolute unit in centimeter–gram–second units|
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.
The symbol Ω was suggested, because of the similar sound of ohm and omega, by William Henry Preece in 1867. In documents printed before WWII 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. Where the font is not supported, a W is displayed instead ("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.
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 the British standard BS 1852 code. It is used in many instances where the value has a decimal place. For example, 5.6 Ω is listed as 5R6. 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 backwards compatibility and the Greek uppercase omega character U+03A9 Ω GREEK CAPITAL LETTER OMEGA (HTML
Ω) is preferred. In DOS and Windows, the alt code ALT 234 may produce the Ω symbol. In Mac OS, ⌥ Opt+Z does the same.
6-Hydroxymelatonin (6-OHM) is a naturally occurring, endogenous, major active metabolite of melatonin. Similar to melatonin, 6-OHM is a full agonist of the MT1 and MT2 receptors. It is also an antioxidant and neuroprotective, and is even more potent in this regard relative to melatonin.BNC connector
The BNC (Bayonet Neill–Concelman) connector is a miniature quick connect/disconnect radio frequency connector used for coaxial cable. The interface specifications for the BNC and many other connectors are referenced in MIL-STD-348. It features two bayonet lugs on the female connector; mating is fully achieved with a quarter turn of the coupling nut. BNC connectors are used with miniature-to-subminiature coaxial cable in radio, television, and other radio-frequency electronic equipment, test instruments, and video signals. The BNC was commonly used for early computer networks, including ARCnet, the IBM PC Network, and the 10BASE2 variant of Ethernet. BNC connectors are made to match the characteristic impedance of cable at either 50 ohms or 75 ohms. They are usually applied for frequencies below 4 GHz and voltages below 500 volts.Similar connectors using the bayonet connection principle exist, and a threaded connector is also available. United States military standard MIL-PRF-39012 entitled Connectors, Coaxial, Radio Frequency, General Specification for (formerly MIL-C-39012) covers the general requirements and tests for radio frequency connectors used with flexible cables and certain other types of coaxial transmission lines in military, aerospace, and spaceflight applications.Chris Poland
Chris Poland (born December 1, 1957) is an American musician, best known for being the former guitarist for American thrash metal band Megadeth from 1984 to 1987 and again for some session work in 2004. During this time, he recorded the albums Killing Is My Business... And Business Is Good! (1985), Peace Sells... But Who's Buying? (1986) and The System Has Failed (2004). He is currently the guitarist of the instrumental rock/jazz fusion band OHM, and is known for having appeared on several projects and albums from a variety of different genres. Recently, he has been associated with the progressive metal band Redemption, with whom he has recorded two albums and made a live guest appearance.Comrie (crater)
Comrie is a lunar impact crater. It is located on the rugged far side of the Moon relative to the Earth, beyond the western limb. Nearby craters of note include Ohm to the south-southwest, Shternberg to the southwest, and Parenago to the northeast.
This feature forms the central member of a formation of three connected craters. A slightly smaller crater is attached to the northern end, and the two share a straight rim. This crater also lies across the northern part of a larger, heavily worn feature to the south, and little of Comrie's rim survives along its southern extent. The remaining rim is worn and eroded.
Within the crater interior, a small crater lies near the northwest rim, a smaller crater just to the southwest of the midpoint, and a still smaller crater along the surviving southwest rim. There is a low central ridge at the midpoint. The eastern half of the interior floor is somewhat irregular, but contains only a few tiny craterlets. Streaks from the ray system of Ohm lie across the interior of Comrie, particularly in the western half.Georg Ohm
Georg Simon Ohm (; German: [ˈɡeːɔɐ̯k ˈʔoːm]; 16 March 1789 – 6 July 1854) was a German physicist and mathematician. As a school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta. Using equipment of his own creation, Ohm found that there is a direct proportionality between the potential difference (voltage) applied across a conductor and the resultant electric current. This relationship is known as Ohm's law.Giovanni Giorgi
Giovanni Giorgi (27 November 1871 – 19 August 1950) was an Italian physicist and electrical engineer who proposed the Giorgi system of measurement, the precursor to the International System of Units (SI).Gokarna, Karnataka
Gokarna is a small temple town on the western coast of India in the Kumta taluk of Uttara Kannada district of the state of Karnataka. The main temple and deity is Lord Shiva, who is also known as Mahabaleshwara. This temple houses what is believed to be original image of Lord Shiva's linga (Atmalinga). There are many other temples all over this small town. Ankola and Kumta on NH66 are the main towns where as Bhatkal and Karwar are the main small cities near Gokarna where almost all trains have halts and are connected to major cities like Bombay, Bangalore, Hyderabad, Madras and Mangalore with private buses.
Gokarna is known as one of the seven important Hindu pilgrimage centers. It is on what was once an unspoiled beach near the estuary of the river Aghanashini. Recently, due to the influx of tourists, the character of the town has changed; it is no longer just a centre of pilgrimage, though large numbers of devotees visit to offer prayers and worship to Lord Shiva.
Due to its laid-back, unspoiled and rustic nature many younger western tourists started visiting Gokarna about a decade ago. The beaches around Gokarna were hardly used by the locals until these mainly western tourists started coming. Enterprising locals started stores and restaurants, and now the resorts also cater to wealthier tourists.Multimeter
A multimeter or a multitester, also known as a VOM (volt-ohm-milliammeter), is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter can measure voltage, current, and resistance. Analog multimeters uses a microammeter with a moving pointer to display readings. Digital multimeters (DMM, DVOM) have a numeric display, and may also show a graphical bar representing the measured value. Digital multimeters are now far more common due to their cost and precision, but analog multimeters are still preferable in some cases, for example when monitoring a rapidly varying value.
A multimeter can be a hand-held device useful for basic fault finding and field service work, or a bench instrument which can measure to a very high degree of accuracy.
Multimeters are available in a wide range of features and prices. Cheap multimeters can cost less than US$10, while laboratory-grade models with certified calibration can cost more than US$5,000.Nazriya Nazim
Nazriya Nazim (born 20 December 1994) is an Indian film actress and producer, who predominantly works in Malayalam films.
She started her career as an anchor on Malayalam television channel Asianet before pursuing a career as an actress. She made her debut as a child artist with Palunku (2006) and then as a lead actress in the 2013 Malayalam film Maad Dad. She has gone on to star in successful films such as Raja Rani (2013), Ohm Shanthi Oshaana (2014) and Naiyaandi and Bangalore Days (2014). Following her marriage to actor Fahadh Faasil, she took a break from acting. She made a comeback in 2018 with Anjali Menon’s movie Koode.She now started producing films and Fahadh Faasil's varathan was one of themNewton metre
The newton metre (also newton-metre, symbol N m or N⋅m) is a unit of torque (also called moment) in the SI system. One newton metre is equal to the torque resulting from a force of one newton applied perpendicularly to the end of a moment arm that is one metre long.
It is also used less commonly as a unit of work, or energy, in which case it is equivalent to the more common and standard SI unit of energy, the joule. In this usage the metre term represents the distance travelled or displacement in the direction of the force, and not the perpendicular distance from a fulcrum as it does when used to express torque. This usage is generally discouraged, since it can lead to confusion as to whether a given quantity expressed in newton metres is a torque or a quantity of energy. However, since torque represents energy transferred or expended per angle of revolution, one newton metre of torque is equivalent to one joule per radian.
Newton metres and joules are dimensionally equivalent in the sense that they have the same expression in SI base units:
Again, N⋅m and J are distinguished in order to avoid misunderstandings where a torque is mistaken for an energy or vice versa. Similar examples of dimensionally equivalent units include Pa versus J/m3, Bq versus Hz, and ohm versus ohm per square.Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:
where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current. Ohm's law is an empirical relation which accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However some materials do not obey Ohm's law, these are called non-ohmic.
The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. Ohm explained his experimental results by a slightly more complex equation than the modern form above (see History).
In physics, the term Ohm's law is also used to refer to various generalizations of the law; for example the vector form of the law used in electromagnetics and material science:
where J is the current density at a given location in a resistive material, E is the electric field at that location, and σ (sigma) is a material-dependent parameter called the conductivity. This reformulation of Ohm's law is due to Gustav Kirchhoff.Ohm, California
Ohm (also, Spaulding) is a former settlement in Nevada County, California. It was the site of a power plant built in 1913.A post office operated at Ohm in 1913. Workers building the power plant lived in nearby Camp Spaulding.Ohm Krüger
Ohm Krüger (English: Uncle Krüger) is a 1941 German biographical film directed by Hans Steinhoff and starring Emil Jannings, Lucie Höflich and Werner Hinz. It was one of a series of propaganda films produced in Nazi Germany attacking the British. The film depicts the life of the South African politician Paul Kruger and his eventual defeat by the British during the Boer War.
It was the first film to be awarded the 'Film of the Nation' award. It was re-released in 1944.Ohm Shanthi Oshaana
Ohm Shanthi Oshaana is a 2014 Indian Malayalam-language coming-of-age romantic comedy film co-written and directed by Jude Anthany Joseph. The story was conceived by Midhun Manuel Thomas and he wrote the screenplay along with the director. It stars Nazriya Nazim, Nivin Pauly, Renji Panicker, Aju Varghese, and Vineeth Sreenivasan. Ohm Shanthi Oshaana was produced by Ananya Films and features music by Shaan Rahman. The film was remade in Bengali as Ke Tumi Nandini starring Bonny Sengupta and Rupsha Mukherjee.
Ohm Shanthi Oshaana was released on 7 February 2014 with positive reviews from critics and became one of the biggest hits of 2014. And won three Kerala State Film Awards including Best Popular Film, Best Actress for Nazriya Nazim and Best Editor for Lijo Paul.Ohmmeter
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (also a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms (Ω).Peter Vaughan
Peter Vaughan (born Peter Ewart Ohm; 4 April 1923 – 6 December 2016) was an English character actor, known for many supporting roles in British film and television productions. He also worked extensively on the stage.
He was best known for his role as Grouty in the sitcom Porridge (despite appearing in only three episodes and the 1979 film) and also had a recurring role alongside Robert Lindsay in Citizen Smith, written by John Sullivan. He also had parts as Tom Franklin in Chancer (1990–91), playing the father of Anthony Hopkins's character in The Remains of the Day, and as Maester Aemon in HBO's Game of Thrones (2011–15), his final role.Radio-Activity
Radio-Activity (German title: Radio-Aktivität) is the fifth studio album by German electronic band Kraftwerk, released in October 1975. The band's first entirely electronic album, it is a concept album organized around the theme of radio communication. To cater to the band's international audience, all releases of the album were bilingual with lyrics in both English and German, the only localised difference being the album and track titles.Siemens (unit)
The siemens (symbol: S) is the derived unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is redundantly equal to the reciprocal of one ohm, and is also referred to as the mho. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.
The unit is named after Ernst Werner von Siemens. In English, the same form siemens is used both for the singular and plural.
|Derived units |
with special names
|Other accepted units|