Résistor nyaéta komponén listrik atawa éléktronik dua terminal nu nahan arus listrik ku jalan nyababkun turun voltase antara terminal-terminalna nurutkeun hukum Ohm. (Résistor ultra-présisi miboga 2 terminal tambahan, jadi totalna 4.)

Résistansi listrik sarua jeung turun tegangan/voltase dina résistor dibagi arus nu ngaliwatan résistor. Résistor digunakeun minangka bagian tina jaringan listrik sarta sirkuit éléktronik.

Panerapan
 In general, a resistor is used to créate a known voltage-to-current ratio in an electric circuit. If the current in a circuit is known, then a resistor can be used to créate a known potential difference proportional to that current. Conversely, if the potential difference between two points in a circuit is known, a resistor can be used to créate a known current proportional to that difference.
 Current-limiting. By placing a resistor in series with another component, such as a light-emitting diode, the current through that component is reduced to a known safe value.
 A series resistor can be used for speed regulation of DC motors, such as used on locomotives and trainsets.
 An attenuator is a network of two or more resistors (a voltage divider) used to reduce the voltage of a signal.
 A line terminator is a resistor at the end of a transmission line or daisy chain bus (such as in SCSI), designed to match impedance and hence minimize reflections of the signal.
 All resistors dissipate heat. This is the principle behind electric heaters.

Résistor idéal
The SI unit of electrical resistance is the ohm (Ω).  A component has a resistance of 1 Ω if a voltage of 1 volt across the component results in a current of 1 ampere, or amp, which is equivalent to a flow of one coulomb of electrical charge (approximately 6.241506 × 1018 electrons) per second. The multiples kiloohm (1 kΩ = 1000 Ω) and megaohm (1 MΩ = 106 Ω) are also commonly used.

In an idéal resistor, the resistance remains constant regardless of the applied voltage or current through the device or the rate of change of the current. While réal resistors cannot attain this goal, they are designed to present little variation in electrical resistance when subjected to these changes, or to changing temperature and other environmental factors.

Karakteristik nonidéal
A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (overhéat or burn up, for instance). Although some resistors have specified voltage and current ratings, most are rated with a maximum power which is determined by the physical size.  Common power ratings for carbon composition and metal-film resistors are 1/8 watt, ¼ watt, and ½ watt.  Metal-film and carbon film resistors are more stable than carbon resistors against temperature changes and age.  Larger resistors are able to dissipate more héat because of their larger surface aréa.  Wire-wound and resistors embedded in sand (ceramic) are used when a high power rating is required.

Furthermore, all réal resistors also introduce some inductance and a small amount of capacitance, which change the dynamic behavior of the resistor from the idéal.

Tipe-tipe résistor

Résistor tetep
Some resistors are cylindrical, with the actual resistive material in the centre (composition resistors, now obsolete) or on the surface of the cylinder (film) resistors, and a conducting metal léad projecting along the axis of the cylinder at éach end(axial léad). There are carbon film and metal film resistors. The photo above right shows a row of common resistors. Power resistors come in larger packages designed to dissipate héat efficiently. At high power levels, resistors tend to be wire wound types. Resistors used in computers and other devices are typically much smaller, often in surface-mount packages without wire léads.  Resistors can also be built into integrated circuits as part of the fabrication process, using the semiconductor material as a resistor. But resistors made in this way are difficult to fabricate and may take up a lot of valuable chip aréa, so IC designers alternatively use a transistor-transistor or resistor-transistor configuration to simulate the resistor they require.

Résistor variabel

The variable resistor is a resistor whose value can be adjusted by turning a shaft or sliding a control. They are also called potentiométers or rhéostats and allow the resistance of the device to be altered by hand. The term rheostat is usually reserved for higher-powered devices, above about ½ watt.
Variable resistors can be inexpensive single-turn types or multi-turn types with a helical element. Some variable resistors can be fitted with a  mechanical display to count the turns.
Variable resistors can sometimes be unreliable, because the wire or metal can corrode or wéar. Some modérn variable resistors use plastic materials that do not corrode and have better wéar characteristics.

Some examples include:
 a rheostat: a variable resistor with two terminals, one fixed and one sliding.  It is used with high currents.
 a potentiometer: a common type of variable resistor. One common use is as volume controls on audio amplifiers and other forms of amplifiers.

Tipe résistor séjénna
 A metal oxide varistor (MOV) is a special type of resistor that changes its resistance with rise in voltage: a very high resistance at low voltage (below the trigger voltage) and very low resistance at high voltage (above the trigger voltage). It acts as a switch. It is usually used for short circuit protection in power strips or lightning bolt "arrestors" on street power poles, or as a "snubber" in inductive circuits.
 A thermistor is a temperature-dependent resistor. There are two kinds, classified according to the sign of their temperature coefficients:
 A Positive Temperature Coefficient (PTC) resistor is a resistor with a positive temperature coefficient. When the temperature rises the resistance of the PTC incréases. PTCs are often found in televisions in series with the demagnetizing coil where they are used to provide a short-duration current burst through the coil when the TV is turned on. One specialized version of a PTC is the polyswitch which acts as a self-repairing fuse.
 A Negative Temperature Coefficient (NTC) resistor is also a temperature-dependent resistor, but with a negative temperature coefficient. When the temperature rises the resistance of the NTC drops. NTCs are often used in simple temperature detectors and méasuring instruments.
 A sensistor is a semiconductor-based resistor with a negative temperature coefficient, useful in compensating for temperature-induced effects in electronic circuits.
 Light-sensitive resistors are discussed in the photoresistor article.
 All wire except superconducting wire has some resistance, depending on its cross-sectional aréa and the conductivity of the material it is made of.  Resistance wire has an accurately known resistance per unit length, and is used to maké wire-wound resistors.

Ngidéntifikasi résistor

Most axial resistors use a pattern of colored stripes to indicate resistance. SMT ones follow a numerical pattern.  Cases are usually brown, blue, or green, though other colors are occasionally found like dark red or dark gray.

One can use a multiméter to test the values of a resistor.

Standar résistor
 MIL-R-11
 MIL-R-39008
 MIL-R-39017
 BS 1852
 EIA-RS-279

Aya opat standar MIL-R- séjénna.

Four-band axial resistors

Four-band identification is the most commonly used color coding scheme on all resistors. It consists of four colored bands that are painted around the body of the resistor. The scheme is simple: The first two numbers are the first two significant digits of the resistance value, the third is a multiplier, and the fourth is the tolerance of the value. éach color corresponds to a certain number, shown in the chart below.  The tolerance for a 4-band resistor will be 2%, 5%, or 10%.

The Standard EIA Color Code Table per EIA-RS-279 is as follows:

Note: red to violet are the colors of the rainbow where red is low energy and violet is higher energy.

Resistors use specific values, which are determined by their tolerance.  These values repéat for every exponent; 6.8, 68, 680, etc.  This is useful because the digits, and hence the first two or three stripes, will always be similar patterns of colors, which maké them éasier to recognize.

Preferred values
Resistors are manufactured in values from a few milliohms to about a gigaohm; only a limited range of values from the IEC 60063 preferred number series are commonly available. These series are called E6, E12, E24, E96 and E192.  The number tells how many standarized values exist in éach decade (e.g. between 10 and 100, or between 100 and 1000).  So resistors confirming to the E12 series, can have 12 distinct values between 10 and 100; while those confirming to the E24 series would have 24 distinct values. In practice, the discrete component sold as a "resistor" is not a perfect resistance, as defined above.  Resistors are often marked with their tolerance (maximum expected variation from the marked resistance).  On color coded resistors the color of the rightmost  band denotes the tolerance:
 silver 10%
 gold 5%
 red  2%
 brown 1%
 green 0.5%.
Closer tolerance resistors, called precision resistors, are also available.

Manufacturers will méasure the actual resistance of new resistors and sort them by tolerance according to how close they were to the intended value.  Subsequently, if you buy 100 resistors of the same value with a tolerance of +/- 10%, you won't get some resistors with the correct value, some off by a little and the worst off by 10%... what you'll probably find if you méasure them, is that about half of the resistors are between 5% and 10% too low in value, and the other half are between 5% and 10% too high in value.  Those off by less than 5%, would've been marked and sold as more expensive 5% resistors.  This is something to consider when calculating specifications on the components for a project, that all resistors will be "off" by the specified tolerance, and not just the "worse" of them.

E12 preferred values :
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82

Multiples of 10 of these values are used, eg. 0.47Ω, 4.7Ω, 47Ω, 470Ω, 4.7k, 47k, 470k etc.

E24 preferred values, includes E12 values and :
11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91

5-band axial resistors
5-band identification is used for higher tolerance resistors (1%, 0.5%, 0.25%, 0.1%), to notate the extra digit.  The first three bands represent the significant digits, the fourth is the multiplier, and the fifth is the tolerance. 5-band standard tolerance resistors are sometimes encountered, generally on older or specialized resistors. They can be identified by noting a standard tolerance color in the 4th band. The 5th band in this case is the temperature coefficient.

Frase mnemonic keur nginget-nginget kode
There are many mnemonic phrases used to remember the order of the colors.

They are, but are not limited to, and variations of:
 Bad Boys Ravish Our Young Girls But Violet Gives Willingly
 Bad Beer Rots Our Young Guts But Vodka Goes Well. Get Some Now!
 B.B. ROY of Gréat Britain had a Very Good Wife
 Buffalo Bill Roamed Over Yellow Grass Because Vistas Grand Were God's Sanctuary
 Bully Brown Ran Over a Yodeling Goat, Because Violet's Granny Was Gone Snorkeling
 Buy Better Resistance Or Your Grid Bias May Go Wrong

Black Brown Red Orange Yellow Green Blue Violet Gray White (Gold Silver)

Résistor SMD

Surface mounted  resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance SMD resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten (the number of zeroes). For example:

Resistances less than 100 ohms are written: 100, 220, 470. The final zero represents ten to the power zero, which is 1.  For example:

Sometimes these values are marked as "10" or "22" to prevent a mistake.

Resistances less than 10 ohms have  'R' to indicate the position of the decimal point.  For example:

Precision resistors are marked with a four-digit code, in which the first three digits are the significant figures and the fourth is the power of ten.  For example:

"000" and "0000" sometimes appéar as values on surface-mount zero-ohm links, since these have (approximately) zero resistance.

Rancangan tipe industri
Format:  [two letters]<space>[resistance value (three digit)]<nospace>[tolerance code(numerical - one digit)]

The operational temperature range distinguishes commercial grade, industrial grade and military grade components.
 Commercial grade:  0 °C to 70 °C
 Industrial grade:  −40 °C to 85 °C (sometimes −25 °C to 85 °C)
 Military grade: −55 °C to 125 °C

Perhitungan

Hukum Ohm

The relationship between voltage, current, and resistance through a metal wire, and some other materials, is given by a simple equation called Ohm's Law:

where V (or U in some languages) is the voltage (or potential difference) across the wire in volts, I is the current through the wire in amperes, and R, in ohms, is a constant called the resistance. (In fact this is only a simplification of the original Ohm's law - see the article on that law for further details.) Materials that obey this law over a certain voltage or current range are said to be ohmic over that range. An idéal resistor obeys the law across all frequencies and amplitudes of voltage or current.

Superconducting materials at very low temperatures have zero resistance.  Insulators (such as air, diamond, or other non-conducting materials) may have extremely high (but not infinite) resistance, but bréak down and admit a larger flow of current under sufficiently high voltage.

Disipasi daya
The power dissipated by a resistor is the voltage across the resistor multiplied by the current through the resistor:

All three equations are equivalent.  The first is derived from Joule's law, and other two are derived from that by Ohm's Law.

The total amount of héat energy reléased is the integral of the power over time:

If the average power dissipated exceeds the power rating of the resistor, then the resistor will first depart from its nominal resistance, and will then be destroyed by overhéating.

Sirkuit séri jeung paralél

Resistors in a parallel configuration éach have the same potential difference (voltage).  To find their total equivalent resistance (Req):

The parallel property can be represented in equations by two vertical lines "||" (as in géometry) to simplify equations.  For two resistors,

The current through resistors in series stays the same, but the voltage across éach resistor can be different.  The sum of the potential differences (voltage) is equal to the total voltage.  To find their total resistance:

A resistor network that is a combination of parallel and series can sometimes be broken up into smaller parts that are either one or the other.  For instance,

However, many resistor networks cannot be split up in this way.  Consider a cube, éach edge of which has been replaced by a resistor.  For example, determining the resistance between two opposite vertices requires matrix methods for the general case.  However, if all twelve resistors are equal, the corner-to-corner resistance is 5/6 of any one of them.

Téhnologi
Carbon composition resistors consist of a solid cylindrical resistive element with embedded wire léadouts or metal end caps to which the léadout wires are attached, which is protected with paint or plastic.
The solid cylindrical resistive element is made from a mixture of small particles of carbon and an insulating material (usually ceramic), the resistance is determined by the ratio of these, higher concentrations of carbon result in a lower resistance.
Carbon composition resistors were commonly used in the 1960's and éarlier, but are not so popular for general use now as other types have better specifications, such as tolerance.

The film resistors consist of a cylindrical ceramic body to which a carbon or metal oxide (eg. tin oxide) film is deposited. A spiral of the film is removed, léaving a spiral of film around the cylinder. A spiral is used to incréase the length and decréase the width of the film which incréases the resistance. The spiral trimming of the film is used to accurately set the value of the resistor and many different values can be made from the same film composition by altering the pitch of the spiral. Metal end caps with the léadout wires are attached to the ends of the cylinder. Or for surface mount types a nickel termination is deposited at either end of the rectangular chip.
The film is protected with paint, enamel, epoxy or plastic.

Wirewound resistors are commonly made by winding a metal wire around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. The wire léads are usually between 0.6 and 0.8 mm in diaméter and tinned for éase of soldering. For higher power wirewound resistors either a ceramic outer case or an aluminium outer case ontop of an insulating layer is used. The aluminium cased types are designed to be attached to a héatsink to dissipate the héat, the rated power is dependant on being used with a suitable héatsink. eg. a 50W power rated resistor will overhéat at around one fifth of the power dissipation if not used with a héatsink.

Types of resistors:

 Carbon composition
 Carbon film
 Metal film
 Metal oxide
 Wirewound (usually has higher parasitic inductance)
 Cermet
 Phenolic
 Tantalum

Résistor foil
Foil resistors have had the best precision and stability ever since they were introduced in 1958 by Berahard F. Telkamp.  One of the important paraméters influencing stability is the temperature coefficient of resistance (TCR). Although the TCR of foil resistors is considered extremely low, this characteristic has been further refined over the yéars.

Tempo ogé
 Resistors in series and parallel circuits
 Wikibooks:Electronics:Component Identification
 Electronic Devices and Circuits
 Shot noise
 Johnson-Nyquist noise

Référénsi

 Electronics and Communications Simplified by A. K. Maini, 9thEd., Khanna Publications (India).

Tumbu luar
 Passive Components Codes Guide
 Standard Resistors Values
 Color code with last band denoting life. Also, a program that calculates the minimum error on resistor divider networks
 4-terminal resistors -   how ultra-precise resistors work
 Resistor Color Code Tutorial
 A good guide to resistor labeling, common values, and color codes
 Beginner's guide to potentiometers, including description of different tapers
 An article describing many aspects of resistors, including materials, tapers, and color code
 "The Original Color Coded Resistance Calculator"
 Resistor Colorcode Decoder
 Resistor Types - Does It Matter?
 Ask The Applications Engineer — difference between types of resistors
 Carbon composition resistors - construction and uses  (pdf file - 163KB)
 Resistors and their uses

Komponén éléktronik

Résistor


Computational complexity theory is a part of computer science. It looks at algorithms, and tries to say how many steps or how much memory a certain algorithm takes for a computer to do. Very often, algorithms that use fewer steps use more memory (or the other way round: if there is less memory available, it takes more steps to do). Many interesting algorithms take a number of steps that is dependent on the size of the problem.

Different classes of complexity

Constant Complexity
Complexity theory also looks at how a problem changes if it is done for more elements. A problem of constant complexity class is the only class in which this is not true. A problem with constant complexity takes the same number of steps to complete no matter the size of the input or the number of elements it is computed on. Broadcasting a message can be thought of as a problem of constant complexity. No matter how many people need to receive a message, all can listen on to one single broadcast with no extra broadcasts needed.

Linear complexity
Mowing the lawn can be thought of as a problem with linear complexity. Mowing an area that is double the size of the original takes twice as long.

Quadratic complexity
Suppose you want to know which of your friends know each other. You have to ask each pair of friends whether they know each other. If you have twice as many friends as someone else, you have to ask four times as many questions to figure out who everyone knows. Problems that take four times as long when the size of the problem doubles are said to have quadratic complexity.

Logarithmic complexity
This is often the complexity for problems that involve looking things up, like finding a word in a dictionary. If the dictionary is twice as big, it contains twice as many words as the original to compare to. Looking something up will take only one step more. The algorithm to do lookups is simple. The word in the middle of the dictionary will be either before or after the term that needs to be looked up, if the words do not match. If it is before, the term needs to be in the second half of the dictionary. If it is after the word, it needs to be in the first half. That way, the problem space is halved with every step, until the word or definition is found. This is generally known as logarithmic complexity

Exponential complexity
There are problems that grow very fast. One such problem is known as the Travelling salesman problem. A salesman needs to take a tour of a certain number of cities. Each city should only be visited once, the distance (or cost) of the travelling should be minimal, and the salesman should end up where he started. This problem has exponential complexity. There are n factorial possibilities to consider. Adding one city (from n to n+1) will multiply the number of possibilities by (n+1). Most of the interesting problems have this complexity.

Theoretical computer science

Computational complexity theory


The honey badger  or ratel (Mellivora capensis) is a type of mustelid. It lives in Africa, the Middle East, and the Indian subcontinent.

Despite its name, the honey badger does not look much like other types of badgers. It looks more like a weasel. It mostly eats meat, and does not have many predators because of its thick skin and tough defensive ability.

Taxonomy
The honey badger is the only member of the genus Mellivora. Although it was first put to the badger group in the 1860s, they do not have much in common with the subfamily Melinae. They are closer to the marten family.

Physical description
The honey badger has a fairly long body, but is thick set and broad across the back. Its skin is loose, and lets it to turn and twist freely within it. The skin around the neck is  thick, an adaptation to fighting each other. The head is small and flat, with a short muzzle. The eyes are small, and the ears are little more than ridges on the skin. These are also possible adaptations to avoiding damage while fighting.

The honey badger has short and sturdy legs, with five toes on each foot. The feet have very strong claws, which are short on the hind legs and very long on the forelimbs. It is a partially plantigrade animal whose soles are thickly padded and naked up to the wrists. The tail is short and is covered in long hairs, except below the base.

Adults are  in shoulder height and 68–75 cm in body length, with females being smaller than males. Males weigh  while females weigh 9.1 kg.

The honey badger has an anal pouch, a trait shared with hyenas. The smell of the pouch is said to be "suffocating", and may help calm bees when raiding beehives.

Fur
Its winter fur is long, with some coarse, bristle-like hairs. There are fewer hairs on the side, belly, and groin. The summer fur is shorter and has even fewer hairs, with the belly being half bare. The sides of the heads and lower body are pure black in colour. A large white band covers their upper bodies, beginning from the top of their heads down to the base of their tails.

Habits

Honey badgers live alone in self-dug holes. They are good diggers, being able to dig tunnels into hard ground in 10 minutes. These burrows always only have one passage and a nesting chamber and are usually not large, about 1–3 metres long. They do not put bedding in the nesting chamber. Although they usually dig their own burrows, they may take over aardvark and warthog holes that are no longer being used, or termite mounds.

Honey badgers are smart animals and are one of few species able to use tools. In the 1997 documentary series Land of the Tiger, a honey badger in India was filmed using a tool; the animal rolled a log and stood on it to reach a kingfisher fledgling stuck up in the roots coming from the ceiling in an underground cave.

Honey badgers are fearless and tough animals. They are known to savagely attack their enemies when they cannot escape. They are tireless in combat and can wear out much larger animals in fights. The fact that most predators do not want to hunt honey badgers has led to the theory that the countershaded coats of cheetah kittens evolved to look like the honey badger in order to keep predators away. This would be an example of mimicry.

The voice of the honey badger is a hoarse "khrya-ya-ya-ya" sound. When mating, males make loud grunting sounds. Cubs vocalise through whines. When attacked by dogs, honey badgers scream like bear cubs.

Diet
Honey badgers have the least specialised diet among mustelids. In undeveloped areas, honey badgers may hunt at any time of the day, though they become nocturnal in places with high human populations. When hunting, honey badgers trot with their fore-toes turned in, moving at the same speed as a young man. Despite their name, honey badgers mostly eat meat, and will take any sort of animal food at hand, including carrion, small rodents, birds, eggs, insects, lizards, tortoises and frogs. They will eat fruit and vegetables such as berries, roots and bulbs.

They may hunt frogs and rodents such as gerbils and ground squirrels by digging them out of their burrows. Honey badgers are able to feed on tortoises without difficulty, due to their powerful jaws. They kill and eat snakes, even highly venomous or large ones. They have dug up human corpses in India. They devour all parts of their prey, including skin, hair, feathers, flesh and bones, holding their food down with their forepaws. When seeking vegetable food, they lift stones or tear bark from trees.

Range
The species can be found through most of Sub-Saharan Africa from the Western Cape, South Africa, to southern Morocco and southwestern Algeria. Outside Africa it lives through Arabia, Iran and western Asia to Turkmenistan and the Indian subcontinent.

Relationships with humans
Honey badgers often kill chickens that humans are raising for food. Because of their strength and persistence, they are difficult to keep away. They are known to rip thick planks from hen-houses or burrow underneath stone foundations.

Because of their tough, loose skin, honey badgers are difficult to kill with dogs. Their skin is hard to get through, and its looseness allows them to twist and turn on their attackers when held. The only safe grip on a honey badger is on the back of the neck. The skin is also tough enough to resist several machete blows. The only sure way of killing them quickly is through a blow to the skull with a club or a shot to the head with a powerful rifle, as their skin is almost proof against arrows and spears.

References

Other websites

 Vanderhaar, Jane M. ; Hwang, Yeen Ten, Mellivora capensis, Published 30 July 2003 by the American Society of Mammalogists

Mustelids
Mammals of Africa
Mammals of Asia

Honey badger


A helicopter (also often used: chopper or heli) is a kind of flying machine or aircraft. A helicopter lifts up off
of the ground and moves because of its rotors. A rotor is several small wings, called rotor blades, that spin together around a shaft. For that reason, helicopters are sometimes called rotary-wing aircraft. A helicopter flies differently from an airplane. An airplane must move forward to stay in the air, but because the helicopter's rotor blades are always moving, the helicopter can stop and stay in one place above the ground. Not needing a runway, they can land in places where an airplane cannot. Helicopters can move by tilting their rotor blades, which causes the aircraft to fly in the direction the blades are tilted.

History
Since around 400 BC, the Chinese had a flying top that was used as a children's toy. The flying top was made from bamboo and used the same method of spinning wings to fly up in the air. Later flying tops were made of feathers tied to a stick. Leonardo da Vinci first thought of a helicopter flown by a man in 1490, and drew pictures of his ideas. It was hundreds of years later (in the early 20th century) before anyone built one that could really fly. The first practical helicopters were built by Frenchman Louis Breguet in 1935 and by German Henrich Focke in 1936. A Russian immigrant, Igor Sikorsky, built and perfected the first practical helicopter in America in 1939.

Uses
Helicopters are especially useful when there are disasters when infrastructure is damaged. Food packets, water, medicines and clothes are dropped from the air to people on the ground who cannot be reached by road. When people are injured, helicopters can carry them to hospitals faster than an ambulance on the road.

Helicopters are also used by the military, because they can move troops and equipment to places an airplane cannot take them. Attack helicopters act as attack aircraft carrying and shooting guns and missiles.

Training helicopter is used for learning how to be a helicopter pilot.

Aircraft

Helicopter


The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day.  Disparate systems of measurement used to be very common.  Now there is a global standard, the International System (SI) of units, the modern form of the metric system.  The SI has been or is in the process of being adopted throughout the world.

In trade, weights and measures is often a subject of governmental regulation, to ensure fairness and transparency.  The Bureau international des poids et mesures (BIPM) is tasked with ensuring worldwide uniformity of measurements and their traceability to the International System of Units (SI).  Metrology is the science for developing national and internationally accepted units of weights and measures.

In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method. A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures developed long ago for commercial purposes.

Science, වෛද්‍ය විද්‍යාව, and engineering often use larger and smaller units of measurement than those used in everyday life and indicate them more precisely.  The judicious selection of the units of measure can aid researchers in problem solving (see, for example, dimensional analysis).

In the social sciences, units of measurement are not yet standardized, and are based on "the assignment of numbers according to a rule".

History

A unit of measurement is a standardised quantity of a physical property, used as a factor to express occurring quantities of that property. Units of measurement were among the earliest tools invented by humans. Primitive societies needed rudimentary measures for many tasks: constructing dwellings of an appropriate size and shape, fashioning clothing, or bartering food or raw materials.

The earliest known uniform systems of weights and measures seem to have all been created sometime in the 4th and 3rd millennia BC among the ancient peoples of Mesopotamia, Egypt and the Indus Valley, and perhaps also Elam in Persia as well.

Many systems were based on the use of parts of the body and the natural surroundings as measuring instruments. Our present knowledge of early weights and measures comes from many sources.

Systems of measurement

Traditional systems

Prior to the near global adoption of the metric system many different systems of measurement had been in use.  Many of these were related to some extent or other. Often they were based on the dimensions of the human body according to the proportions described by Marcus Vitruvius Pollio.  As a result, units of measure could vary not only from location to location, but from person to person.

Metric systems
A number of metric systems of units have evolved since the adoption of the original metric system in ප්‍රංශය in 1791. The current international standard metric system is the International system of units.  An important feature of modern systems is standardization.  Each unit has a universally recognized size.

Both the Imperial units and US customary units derive from earlier English units.  Imperial units were mostly used in the British Commonwealth and the former බ්‍රිතාන්‍ය අධිරාජ්‍යය.  US customary units are still the main system of measurement used in the United States despite Congress having legally authorized metric measure on 28 July 1866. Some steps towards US metrication have been made, particularly the redefinition of basic US units to derive exactly from SI units, so that in the US the inch is now defined as 0.0254 m (exactly), and the avoirdupois pound is now defined as 453.59237 g (exactly)

Natural systems
While the above systems of units are based on arbitrary unit values, formalised as standards, some unit values occur naturally in science. Systems of units based on these are called natural units.  Similar to natural units, atomic units (au) are a convenient system of units of measurement used in atomic physics.

Also a great number of strange and non-standard units may be encountered.  These may include the Solar mass, the Megaton (1,000,000 tons of TNT), the Hiroshima atom bomb and the weight of an elephant.

Legal control of weights and measures

To reduce the incidence of retail fraud, many national statutes have standard definitions of weights and measures that may be used (hence "statute measure"), and these are verified by legal officers.

Base and derived units
Different systems of units are based on different choices of a set of fundamental units.
The most widely used system of units is the International System of Units, or SI.  There are seven SI base units. All other SI units can be derived from these base units.

For most quantities a unit is absolutely necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given.

But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the base units. Other units are derived units. Derived units are a matter of convenience, as they can be expressed in terms of basic units. Which units are considered base units is a matter of choice.

The base units of SI are actually not the smallest set possible. Smaller sets have been defined. For example, there are unit sets in which the electric and magnetic field have the same unit. This is based on physical laws that show that electric and magnetic field are actually different manifestations of the same phenomenon.

Calculations with units

Units as dimensions
Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Z is written as the product of a unit [Z] and a numerical factor:

The multiplication sign is usually left out, just as it is left out between variables in scientific notation of formulas. In formulas the unit [Z] can be treated as if it were a kind of physical dimension: see dimensional analysis for more on this treatment.

A distinction should be made between units and standards. A unit is fixed by its definition, and is independent of physical conditions such as temperature. By contrast, a standard is a physical realization of a unit, and realizes that unit only under certain physical conditions. For example, the metre is a unit, while a metal bar is a standard. One metre is the same length regardless of temperature, but a metal bar will be one metre long only at a certain temperature.

Guidelines
 Treat units algebraically. Only add like terms. When a unit is divided by itself, the division yields a unitless one. When two different units are multiplied, the result is a new unit, referred to by the combination of the units. For instance, in SI, the unit of speed is metres per second (m/s). See dimensional analysis. A unit can be multiplied by itself, creating a unit with an exponent (e.g. m²/s²).

 Some units have special names, however these should be treated like their equivalents. For example, one newton (N) is equivalent to one kg•m/s². This creates the possibility for units with multiple designations, for example: the unit for surface tension can be referred to as either N/m (newtons per metre) or kg/s² (kilograms per second squared).

Expressing a physical value in terms of another unit
Conversion of units involves comparison of different standard physical values, either of a single physical quantity or of a physical quantity and a combination of other physical quantities.

Starting with:

just replace the original unit  with its meaning in terms of the desired unit , e.g. if , then:

Now  and  are both numerical values, so just calculate their product.

Or, which is just mathematically the same thing, multiply Z by unity, the product is still Z:

For example, you have an expression for a physical value Z involving the unit feet per second () and you want it in terms of the unit miles per hour ():

Find facts relating the original unit to the desired unit:

1 mile = 5280 feet and 1 hour = 3600 seconds

Next use the above equations to construct a fraction that has a value of unity and that contains units such that, when it is multiplied with the original physical value, will cancel the original units:

Last,multiply the original expression of the physical value by the fraction, called a conversion factor, to obtain the same physical value expressed in terms of a different unit. Note: since valid conversion factors are dimensionless and have a numerical value of one, multiplying any physical quantity by such a conversion factor (which is 1) does not change that physical quantity.

Or as an example using the metric system, you have a value of fuel economy in the unit litres per 100 kilometres and you want it in terms of the unit microlitres per metre:

Real-world implications
One example of the importance of agreed units is the failure of the NASA Mars Climate Orbiter, which was accidentally destroyed on a mission to the planet Mars in September 1999 instead of entering orbit, due to miscommunications about the value of forces: different computer programs used different units of measurement (newton versus pound force). Enormous amounts of effort, time, and money were wasted.

On April 15 1999 Korean Air cargo flight 6316 from Shanghai to Seoul was lost due to the crew confusing tower instructions (in metres) and altimeter readings (in feet).  Three crew and five people on the ground were killed.  Thirty seven were injured.

In 1983, a Boeing 767 (which came to be know as the Gimli Glider) ran out of fuel in mid-flight because of two mistakes in figuring the fuel supply of Air Canada's first aircraft to use metric measurements. This accident is apparently the result of confusion both due to the simultaneous use of metric & Imperial measures as well as mass & volume measures.

References

See also
 Dimensional analysis
 Conversion of units
 Units conversion by factor-label
 ISO 31
 List of unusual units of measurement
 List of humorous units of measurement

External links

General
මිනුම (Measurements)-සිංහල  උසස් පෙළ භෞතික විද්‍යාව පිළිබඳ අන්තර්ජාල අඩවිය
 A Dictionary of Units of Measurement - Center for Mathematics and Science Education, University of North Carolina
 NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices
 NIST Handbook 44, Appendix C, General Tables of Units of Measurement
 Official SI website

Legal
 Canada - Weights and Measures Act 1970-71-72
 Ireland - Metrology Act 1996
 UK - Units of Measurement Regulations 1995
 US - Authorized tables

Metric information and associations
 Official SI website
 UK Metric Association
 US Metric Association
 The Unified Code for Units of Measure (UCUM)

Imperial/U.S. measure information and associations
 British Weights and Measures Association
 Kentucky Demetrification

භෞතික විද්‍යාව
ඒකක සහ මාන
ගණිතය
මිනුම් ක්‍රම

ඒකක සහ මාන


Babylonia was a city state in Mesopotamia in the 2nd millennium BC, over 3000 years ago. Its capital city was Babylon, which meant The Gate of the Gods. They built an empire out of the lands of the former Akkadian empire.

Mesopotamia is the region of the two rivers, Euphrates and Tigris. At that time the region also included the city states of Assyria to the north, and Elam to the south-east. It is part of the Fertile Crescent in the Middle East. It was there people first lived together in a civilization, with farming, cities and writing.

Old Babylonia

The Babylonians had a written language that they used for trade and communication. They got it from the Sumerians, who invented it. The Babylonians used the same cuneiform system of pressing triangular shapes into soft clay. They wrote in two different languages: Sumerian for religious purposes and Akkadian language for official purposes.

Laws
Hammurabi was a king of Babylon who fought wars. He made Babylonia into an empire by putting Assyria under a vassal (puppet) king. He made the earliest written set of laws, the Code of Hammurabi. It has 280 judgments. It can be seen today in the Louvre, in Paris. The earlier Sumerian punishments had not been harsh, but Babylonian law was quite severe. The death penalty was given for theft, murder, and other crimes.

Buildings
The houses in Babylon had open roofs, so that on hot nights, the family could sleep there. The living rooms, dining rooms, and the kitchens were, of course, downstairs. Lamps burned with olive oil, and every house had a chapel for burial and worship ceremonies. Since Mesopotamia had only clay for use in building, the strong wooden supports for the houses were imported from Lebanon. Eventually, people began baking their bricks and improved the strength of their buildings.

Sack of Babylon
Later, Babylon was sacked by the Hittite king Musilis I, which led to the so-called 'Dark Ages' of the Bronze Age, where there is little evidence in writing. The date of the sacking of Babylon is debated by archaeologists, who have proposed no fewer than four chronologies. Possible dates for the sack of Babylon are:
ultra-short chronology: 1499 BC
short chronology: 1531 BC
middle chronology: 1595 BC
long chronology: 1651 BC
The difficulty is to line up the Mesopotamian dates with the Ancient Egyptian dates.

Later dynasties

After the Hittite destruction, Babylon was ruled by Kassites for 576 years. Next it was ruled by Elam, and then regained its independence for about three centuries. They were then conquered by the Neo-Assyrians. A century later they again became free, to form the Neo-Babylonian or Chaldean Empire. This constant conquest and re-conquest is partly due to geography. There are no natural boundaries except the rivers, and it is easy to get at the cities from north or south.

The king Nebuchadnezzar II reigned for 43 years. He conquered Phoenicia in 585 BC.

Cyrus the Great
The Babylonian empire was finally brought to an end by Cyrus the Great of Persia.

It was in 549 BC that Cyrus put an end to the empire of the Medes. Three years later Cyrus had become king of the Achaemenid Empire (Persia), and was engaged in a campaign in northern Mesopotamia.

In 539 BC Cyrus invaded Babylonia. A battle was fought at Opis in the month of June, where the Babylonians were defeated; and immediately afterwards Sippara surrendered to the invader. Two days after the capture of Sippara, "the soldiers of Cyrus entered Babylon without fighting". Cyrus did not arrive until the 3rd of Marchesvan (October), Gobryas having acted for him in his absence. Gobryas was now made governor of the province of Babylon.

Cyrus now claimed to be the legitimate successor of the ancient Babylonian kings and the avenger of their god Bel-Marduk. The invasion of Babylonia by Cyrus was doubtless helped by the presence of foreign forced exiles like the Jews, who had been planted in the midst of the country.

One of the first acts of Cyrus was to allow these exiles to return to their own homes, carrying with them the images of their god and their sacred vessels. The permission to do so was embodied in a proclamation, whereby the conqueror endeavored to justify his claim to the Babylonian throne. The feeling was still strong that none had a right to rule over western Asia until he had been consecrated to the office by Bel and his priests; and accordingly, Cyrus henceforth assumed the imperial title of "King of Babylon."

Food
Like the Sumerians, the Babylonians ate vegetables, fruits, meat, and fish. They also ate bread and enjoyed toasting and eating the pesky but crunchy locusts which destroyed their precious crops.

Art and science
The Babylonians loved art. Beautiful vessels ornamented with sparkling gold were buried with the kings. In those days books dealt with floods which were thought to be caused by sin, or about the journey of Abraham.
Science, too, improved well: they invented the first calendar, the 60-minute hour, and the advance multiplication table.

Life was rich, full, and usually peaceful. People rarely thought about war or how to protect the city. As a result, Babylon was conquered in 730 BC by the Assyrians and Kassites. This great civilization then ended.

Related pages
Neo-Babylonian Empire

References

Mesopotamia
Former countries in the Middle East

Babylonia


Carbon steel or plain-carbon steel, is a metal alloy. It is a combination of two elements, iron and carbon. Other elements are present in quantities too small to affect its properties. The only other elements allowed in plain-carbon steel are: manganese (1.65% max), silicon (0.60% max), and copper (0.60% max). Steel with a low carbon content has the same properties as iron, soft but easily formed. With more carbon the metal gains hardness and strength but becomes less ductile and more difficult to weld. Higher carbon content lowers steel's melting point and its temperature resistance in general.

Types of carbon steel
Typical compositions of carbon are:

 Mild (low carbon) steel: approximately 0.3% carbon content with up to 0.4% manganese content (e.g. AISI 1018 steel). Less strong but cheap and easy to shape; surface hardness can be increased through carburizing.
 Medium carbon steel: approximately 0.30% to 0.45% carbon content with 0.60 to 1.65% manganese content(e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and car parts.
 High carbon steel: approximately 0.45% to 0.75%  carbon content with 0.30 to 0.90% manganese content. Very strong, used for springs and high-strength wires.
 Very high carbon steel: upto 1.5% carbon content, specially processed to produce specific atomic and molecular micro structures.

Steel can be heat-treated which allows parts to be fabricated in an easily fordable soft state. If enough carbon is present, the alloy can be hardened to increase strength, wear, and impact resistance. Steels are often wrought by cold-working methods, which is the shaping of metal through deformation at a low equilibrium or meta stable temperature.

Metallurgy
Mild steel is the most common form of steel as its price is relatively low while it provides material properties that are acceptable for many applications. Mild steel has a low carbon content (up to 0.3%) and is therefore neither extremely brittle nor ductile. It becomes malleable when heated, and so can be forged. It is also often used where large amounts of steel need to be formed, for example as structural steel. Density of this metal is 7861.093 kg/m³ (0.284 lb/in³) and the tensile strength is a maximum of 500 MPa (72500 psi)

Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30% to 1.70% by weight. Trace impurities of various other elements can have a significant effect on the quality of the resulting steel. Trace amounts of sulfur make the steel "red-short", which is a fault: the steel is brittle and crumbly. Low alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1426–1538 °C (2600–2800 °F). Manganese is often added to improve the hardenability of low carbon steels. These additions turn the material into a low alloy steel by some definitions, but AISI's definition of carbon steel allows up to 1.65% manganese by weight.

Hardened steel usually refers to quenched or quenched and tempered steel.

Silver Steel or high-carbon bright steel, gets its name from its appearance, due to the high carbon content. It is a very-high carbon steel, or can be thought of as some of the best high-carbon steel. It is defined under the steel specification standards BS-1407. It is a 1% carbon tool steel which can be ground to close tolerances. Usually the range of carbon is minimum 1.10% but as high as 1.20%. It also contains trace elements of 0.35% Mn (range 0.30%-0.40%), 0.40% Cr (range 0.4%-0.5%), 0.30% Si (range 0.1%-0.3%), and also sometimes sulfur (max 0.035%) and phosphorus (max 0.035%). Silver steel is sometimes used for making straight razors, due to its ability to produce and hold a micro-fine edge.

Heat treatments

The purpose of heat treating plain-carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, and impact resistance.

Footnotes

References
 Oberg, E. et al., (1996). "Machinery's Handbook," 25th ed., Industrial Press Inc.
 Smith, W.F. & Hashemi, J. (2006). "Foundations of Materials Science and Engineering," 4th ed., McGraw-Hill.

Alloys
Iron

Carbon steel


A pension is a steady income given to someone. If pensions are part of a system of social security, the recipient of the pension is usually retired or disabled. They either have worked a long time during their life, or they are physically unable to do so. A pension is usually paid until a certain date (or event) occurs. In the case of social security plans, pensions are usually linked to the life of the person who receives the pension.

A defined Benefit pension defines the benefit to the worker based on salary basis, years worked, and a multiplier.  An example might be 2@55 final 3, meaning that the annuity is the highest 3-year average salary times years worked times 2% (if age 55).  A worker with 30 years service would receive 2*30 or 60% of their salary.  Some systems allow a worker to receive more than 100% salary by various maneuvers to alter the final salary basis.

A defined Contribution pension defines the contribution, without constraining or promising a certain benefit.  For example, a company might contribute 10% of a worker's salary to a pension account of the worker's choice, with final benefit received linked to the performance of the investment chosen.

Crisis
Many pension systems are underfunded and likely unsustainable based on independent financial analysis.  Pension reform is a popular topic since about 2009, with pension debt seen as a contributing factor in the deficits at all levels of government.

Income
Social security
Old age

Pension


The term alternation of generations is used to describe an alternation of forms in the life cycle of  plants (and some protists).

One form is diploid, with 2n chromosomes: the sporophyte. The other form is haploid with only one set of chromosomes: the gametophyte. Both forms are multicellular.

In sexual reproduction, organisms have a haploid phase with one set of chromosomes and a diploid phase with two sets of chromosomes. In animals the body (soma) is usually diploid, while the haploid stage is only the gametes.

In eukaryotes an alternation of generations may occur. This means that both the diploid and the haploid phases are multi-cellular organisms. The classic example is the mosses, where the green plant is a haploid gametophyte, and the reproductive phase is the diploid sporophyte. The two forms often occur together, as shown in the illustration on the right

The term alternation of generations refers only to the sexual cycle: organisms may have asexual reproduction as well. The term should not be confused with life cycle stages in animals, which may look very different, but where all cells have two sets of chromosomes.

Plants

The alternation of generations is an important concept in the evolution of plants. All land plants have alternation of generations.

In mosses and their relatives (Bryophytes), the haploid gametophyte is the dominant generation, and the diploid sporophytes are sporangium-bearing stalks growing from the gametophytes. In ferns, the diploid sporophyte is much larger, and the haploid gametophyte is a little plant that can grow for a long time.

For flowering plants (Angiosperms), the sporophyte generation is almost the whole life cycle (the green plant, roots etc.) except the small reproductive structures (pollen and ovule).

The sporophyte produces spores (hence the name), by meiosis. These develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they have half as many chromosomes. Later, the mature gametophyte produces male or female gametes (or both) by mitosis. The fusion of male and female gametes (fertilization) produces a diploid zygote which develops into a new sporophyte. This is the cycle which is known as alternation of generations or alternation of phases.

As a factor in plant evolution
In the landmark work Variation and evolution in plants, Stebbins discussed how alternation of generations related to the overall evolution of plants. He began:
"The most striking difference between the sexual cycle of animals and those found in plants is that, with the exception of a few Protozoa, animals are diploid at all stages, while nearly all plants have a haploid stage of greater or lesser duration. The sequence of types of alternation of generations... is one of the best-known features of plant evolution... The diploid generation has undoubtedly evolved independently many different times".

Later Stebbins comments:
"The diploid condition brings about an increase in flexibility because it makes possible the condition of genetic dominance and recessiveness. In a haploid organism every new mutation is immediately exposed to the action of selection... In a diploid organism, on the other hand, each new mutant arises as a heterozygote and, if recessive, is sheltered from selection".
The point is that, in diploids, new alleles are sheltered and (collectively) they are a reservoir of potential variation in the population.

Algae
Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar (isomorphic).

Bryophytes
Bryophytes (mosses, liverworts and hornworts) have a dominant gametophyte stage on which the adult sporophyte is dependent on the gametophyte for nutrition. The sporophyte develops from the zygote inside the female sex organ, so its early development is nurtured by the gametophyte.

Vascular plants

An independent sporophyte is the dominant form in all clubmosses, horsetails, ferns, gymnosperms, and angiosperms (flowering plants) that have survived to the present day.

Earlier evolution
Early land plants had sporophytes that produced identical spores: they looked the same whichever sex they developed into. The ancestors of the gymnosperms evolved complex heterosporous life cycles: the spores producing male and female gametophytes were of different sizes. The female megaspores tending to be larger, and fewer in number, than the male microspores.

During the Devonian, several plant groups independently evolved heterospory and later endospory, in which single megaspores were kept inside the sporangia of the parent sporophyte. These endosporic megaspores had a miniature multicellular female gametophyte with female sex organs and egg cells. The ova were fertilised by free-swimming sperm produced by windborne miniaturised male gametophytes in the form of pre-pollen.

The resulting zygote developed into the next sporophyte generation while still inside the single large female megaspore in the sporangium of the parent sporophyte. The evolution of heterospory and endospory was among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms.

Similar processes in other organisms
Some Chromalveolata, some fungi and some slime moulds have what seems to be genuine alternation of generations. These groups include such a wide range of different types that it is difficult to say how common the phenomenon is. Certainly slime moulds are not a monophyletic group, and that may be true of the other two groups,

Animals
see also life cycle

Alternation of generations between a multicellular diploid and a multicellular haploid generation does not exist in animals.

In some animals, there is a life cycle with different diploid stages. This has sometimes mistakenly been called "alternation of generations", but is quite different from what happens in plants and some protists. The most common case is that there are two distinct generations, where only one has sexual reproduction.  Animals where it has been found include  the Cnidaria and the Tunicates. The images on the right show the case of jellyfish: The medusa looks different from the polyp; they are different phenotypes. Only the medusa reproduces sexually.

Other animals with life cycle forms include parasites such as certain flukes, which live in different snails for one part of their life, and in vertebrates for another part.

In some cases, the cycle includes more than two generations. If this is the case, only one stage uses sexual reproduction. In aphids, for example, there is one generation that reproduces sexually, and up to forty that use parthenogenesis, a type of asexual reproduction.

References

Botany
Plant anatomy
Classical genetics
Biological reproduction

Alternation of generations


Lists are commonly used in Wikipedia to organize information. Lists may be found within the body of a prose article, in appendices such as a "Publications" or "Works" section, or as a stand-alone article. This guideline explains when and how to use lists appropriately.

Types of lists
Wikipedia differentiates between articles that consist primarily of lists (generally called "lists" or "stand alone lists") and articles that consist primarily of prose (called "articles"). Articles are intended to consist primarily of prose, though they may contain some lists.

Stand-alone list articles

List articles are encyclopedia pages consisting of introductory material in the lead section followed by a list, possibly arranged in sub-sections. The items on these lists might include links to specific articles or other information, and must be supported with references like any article.  The titles of stand-alone lists typically begin with the type of list it is (List of, Index of, etc.), followed by the article's subject, e.g., List of vegetable oils. They can be organised alphabetically, by subject classification or by topics in a flat or hierarchical structure.

The title and bullet style, or vertical style, is common for stand-alone lists. These Wikipedia articles follow the Wikipedia:Stand-alone lists style guideline.

Embedded lists

Embedded lists are lists used within articles that supplement the article's prose content. They are included in the text-proper or appended, and may be in table format. Wikipedia uses several standard appendices, usually in list format, as well as navigational templates.

Embedded lists should be used only when appropriate; sometimes the information in a list is better presented as prose. Presenting too much statistical data in list format may contravene policy.

"Children" (i.e., indentation)

It can be appropriate to use a list style when the items in a list are "children" of the paragraphs that precede them.  Such "children" logically qualify for indentation beneath their parent description. In this case, indenting the paragraphs in list form may make them easier to read, especially if the paragraphs are very short.  The following example works both with and without the bullets:

Lists of works and timelines

Lists of works of individuals or groups, such as bibliographies, discographies, filmographies, album personnel and track listings are typically presented in simple list format, though it is expected that the information will be supported elsewhere in the article by prose analysis of the main points, and that if the lists become unwieldy, they are split off into stand-alone lists per WP:Summary style. Timelines and chronologies can be a useful supplement to prose descriptions of real-world histories. The content of a list is governed by the same content policies as prose, including principles of due weight and avoiding original research. Ensure that list items have the same importance to the subject as would be required for the item to be included in the text of the article, according to Wikipedia policies and guidelines (including WP:Trivia sections). Consider whether prose is more appropriate. Specific advice regarding timelines is given in Wikipedia:Timeline standards.

Related topics (navigational lists)

"See also" lists and "Related topics" lists are valuable navigational tools that assist users in finding related Wikipedia articles. When deciding what articles and lists of articles to append to any given entry, it is useful to try to put yourself inside the mind of readers: Ask yourself where would a reader likely want to go after reading the article. Typically this will include three types of links:

 Links to related topics – topics similar to that discussed in the article
 Higher order (i.e., more general) articles and lists – this might include lists of people, list of countries, etc. For example, list of Indian language poets should link to both list of Indians and list of poets.
 Lower order (i.e., more specific) articles and lists – for example, the Business page navigational list contains links to small business, list of accounting topics, etc.

There is some controversy over how many links to articles and links to lists that should be put in any article. Some people separate the "links to articles" (put in the "See also" section) from the "links to lists" (put in the "Related topics" section), but this is not necessary unless there are too many links for one section alone. Some feel the optimum number of links to lists that should be included at the end of any given article is zero, one, or two. Others feel that a more comprehensive set of lists would be useful. In general, when deciding what list to include, the same criteria used to decide what articles to include in the See also section should be used. Editors should try to put themselves in the readers' frame of mind and ask "Where will I likely want to go after reading this article?". As a general rule, the "See also" section should not repeat links that appear in the article's body.

References and external links
Reference lists show information sources outside of Wikipedia. The two most common types are:

 "Web hyperlinks" – lists of links to web addresses other than Wikipedia, under the heading "External links"
 "References" – lists of academic journal articles or books, under the heading "References"

Wikipedia is not a link collection and articles with only external links are actively discouraged, but it is appropriate to reference more detailed material from the Internet.  This is particularly the case when you have used a web site as an important source of information.

Special names of lists

Most lists on Wikipedia are item lists, but not all. Specialized types of lists include:
 Outline – a Wikipedia outline is a hierarchically arranged list of topics belonging to a given subject. Outlines are one of the two types of general topics list on Wikipedia, the other being indices.
 Index – an index on Wikipedia is an alphabetical list of articles on a given subject. See Wikipedia:WikiProject Indexes.
 Timeline – a timeline is a graphical representation of a chronological sequence of events.
 Order of battle –  a representation of armed force components that shows the hierarchical organization and command structure.
 List of works include bibliographies and discographies. Bibliographies are a list of relevant references for a subject area, including books, journal articles, and web articles; discographies are a listing of all recordings on which a musician or singer features, or may be compiled based on genre or record label
 Glossary – a glossary is a list of terms in a specific subject area, with definitions included.
Set index article – document a set of items that share the same (or a similar) name. They are different from disambiguation pages in that they are full-fledged articles meant to document multiple subjects, while disambiguation pages are for navigation purposes only. Not all set index articles are lists.
Dynamic list – a dynamic list is any list that may never be fully complete (since it will only include notable examples from a given category), or may require constant updates to remain current.

Purposes of lists

Lists have three main purposes:

Information
The list may be a valuable information source. This is particularly the case for a structured list. Examples would include lists organized chronologically, grouped by theme, or annotated lists.

Navigation

Lists which contain internally linked terms (i.e., wikilinks) serve, in aggregate, as natural tables of contents and indexes of Wikipedia. If users have some general idea of what they are looking for but do not know the specific terminology, they could browse the lists of basic topics and more comprehensive lists of topics, which in turn lead to most if not all of Wikipedia's lists, which in turn lead to related articles. Users without a specific research goal in mind might also find the articles listed in articles' see also sections useful. Lists are also provided in portals to assist in navigating their subjects, and lists are often placed in articles via the use of series boxes and other navigation templates.

Users with a specific research goal, described in one or two words, are likely to find Wikipedia's search box useful.

Development
Some lists are useful for Wikipedia development purposes. The lists of related topics give an indication of the state of Wikipedia, the articles that have been written, and the articles that have yet to be written. However, as Wikipedia is optimized for readers over editors, any lists which exist primarily for development or maintenance purposes (such as a list that consists entirely of red links and does not serve an informational purpose; especially a list of missing topics) should be in either the project or user space, not the main space.

Lists and categories

Redundancy of lists and categories is beneficial because the two formats work together; the principle is covered in the guideline Wikipedia:Categories, lists, and navigation templates. Like categories, lists can be used for keeping track of changes in the listed pages, using the Related Changes feature. Unlike a category, a list also allows keeping a history of its contents; lists also permit a large number of entries to appear on a single page.

List naming

For a stand-alone list, the list's title is the page name. For an embedded list, the list's title is usually a section title (for instance, ), but it can be shorter. The list title should not be misleading and should normally not include abbreviations. Additionally, an overly precise list title can be less useful and can make the list difficult to find; the precise inclusion criteria for the list should be spelled out in the lead section (see below), not the title. For instance, words like complete and notable are normally excluded from list titles. Instead, the lead makes clear whether the list is complete or whether it is limited to widely-known or notable members (i.e., those that merit articles). Note that the word "famous" is considered an unnecessary  "peacock" embellishment and should not be used.

Sorting a list

Lists may be sorted alphabetically (e.g. for people: by surname, given name, initials), chronologically (by date, usually oldest first), or occasionally by other criteria. To suggest that a list in an article or section should be sorted, use .

List layout

Use prose where understood easily

Prefer prose where a passage is understood easily as regular text that appears in its ordinary form, without metrical structure or line breaks. Prose is preferred in articles because it allows the presentation of detail and clarification of context in a way that a simple list may not. It is best suited to articles because their purpose is to explain.

 can be used to indicate a list which may be better-written as prose. Many stub articles can be improved by converting unnecessary lists into encyclopedic prose.

Use good markup

Use proper markup: Employ careful wiki markup- or template-based list code .  Especially do not leave blank lines between items in a list, since this causes the MediaWiki software to misinterpret each item as beginning a new list. (There are HTML techniques to insert linebreaks or additional paragraphs into a list item.)  Avoid misuse of list markup in articles for visual styling of non-list material.

Images and lists

To float pictures to the right of the list, one should put the image markup before the first item in most cases, see the example "A". Inserting the image markup as a separate line within the list (as in example "B") once again will split it into two half-lists.

Should the length of the list items or the topical relevance of said image discourage display at the top corner, consider placing it after the asterisk of the first list-item it illustrates (as in example "C") to avoid breaking continuity of the unordered list () element.

Note: When floating images to the left of a list, use the  template to prevent disrupting the indentation of the bullet-points.

Use an unordered list by default
Use a bulleted (unordered) list by default, especially for long lists.  Use a numbered (ordered) list only if there is a need to refer to items by number, the sequence of items is important, or the numbering exists in the real world (e.g., tracks on an album).

Format list items consistently

List items should be formatted consistently in a list. Unless there is a good reason to use different list types in the same page, consistency throughout an article is also desirable.

Use sentence case by default for list items, whether they are complete sentences or not. Sentence case is used for around 99% of lists on Wikipedia.  Title case (as used for book titles) is not used for list entries.

Lowercase is best reserved for:
 lists introduced by a sentence fragment, with a short list of items, also fragments, continuing the extended sentence;
 glossary entries, where it is important to convey whether something is usually capitalized or not;
 lists of items with non-English names (that have not been assimilated into English), from a language in which their capitalization would be incorrect.

Use the same grammatical form for all items in a list – avoid mixing sentences and sentence fragments as items.
 When the items are complete sentences, each one is formatted with sentence case (i.e., the initial letter is capitalized) and a final full stop (period).
 When the items are sentence fragments, the list is usually preceded by introductory material and a colon. Items may be given with initial lowercase or in sentence case. No final punctuation is used in most cases.
 Semicolons may be used when the list is short, items are lowercase, and the entire list forms a complete sentence (typically with its introductory phrase and possibly with a closing phrase after the list to complete the sentence). Many cases of this are better rewritten as paragraphs unless it is contextually important to "listify" the items for clarity (e.g., because they correspond to sections in the rest of the article below the list).

A list item should not end with a full stop unless it consists of a complete sentence or is the end of a list that forms one.

When elements contain (or are) titles of works or other proper names, these retain their original capitalization, regardless how the rest of the list is formatted.

A list title in a section heading provides a direct edit point, if one enables section editing. It also enables the automatic table of contents to detect the list. It is not required, however, and should not be used for a list that is not the focus of a section, or for lists in an article that uses a lot of short lists and which is better arranged by more topical headings that group related lists.

Introductory material

Lists should have introductory material; for stand-alone lists, this should be the lead section. This introductory material should make clear the scope of the list. It should also provide explanation for non-obvious characteristics of the list, such as the list's structure. Stand-alone lists may place non-obvious characteristics in a separate introductory section (e.g. ).  Lists and their supporting material must comply with standard Wikipedia content policies and guidelines, including Wikipedia:Neutral point of view and should not create content forks.

Exercise caution when self-referencing Wikipedia, to ensure any self-reference is acceptable. For example, WP:Notability is often a criteria used for WP:Stand-alone lists, but many other self-references create problems. To include a self-reference, format it with Template:Self-reference link.

Some information, such as "Notable people" or "Alumni", which may be read for context or scanned for content, may be formatted with a section lead and a descriptive, bulleted list, or as prose, depending on size. If the list is long, is unable to be summarised, but is not appropriate for splitting out, then a section lead, with a descriptive, bulleted list may be more appropriate than a long prose section.

Organization

Although lists may be organized in different ways, they must always be organized. The most basic form of organization is alphabetical or numerical (such as List of Star Wars starfighters), though if items have specific dates a chronological format is sometimes preferable (List of Belarusian Prime Ministers). When using a more complex form of organization, (by origin, by use, by type, etc.), the criteria for categorization must be clear and consistent. Just as a reader or editor could easily assume that the headings A, B, C would be followed by D (rather than 1903), more complex systems should be just as explicit. If a list of Australians in international prisons contains the headings Argentina and Cambodia (organization by country), it would be inappropriate for an editor to add the heading Drug trafficking (organization by offense). If a list entry logically belongs in two or more categories (e.g., an Australian in an Argentine prison for drug trafficking), this suggests that the list categorization might be flawed, and should be re-examined.

Lists should never contain "Unsorted" or "Miscellaneous" headings, as all items worthy of inclusion in the list can be sorted by some criteria, although it is entirely possible that the formatting of the list would need to be revamped to include all appropriate items. Not-yet-sorted items may be included on the list's talk page while their categorization is determined.

List size

Keep lists and tables as short as feasible for their purpose and scope: material within a list should relate to the article topic without going into unnecessary detail; and statistical data kept to a minimum per policy.

Some material may not be appropriate for reducing or summarizing using the summary style method. An embedded list may need to be split off entirely into a list article, leaving a  template which produces:

In some cases, a list style may be preferable to a long sequence within a sentence, compare:

Adding individual items to a list

Lists, whether they are stand-alone lists (also called list articles) or embedded lists, are encyclopedic content just as paragraph-only articles or sections are. Therefore, all individual items on the list must follow Wikipedia's content policies: the core content policies of Verifiability (through good sources in the item's one or more references), No original research, and Neutral point of view, plus the other content policies as well. Content should be sourced where it appears with inline citations if the content contains any of the four kinds of material absolutely required to have citations. Although the format of a list might require less detail per topic, Wikipedia policies and procedures apply equally to both a list of similar things as well as to any related article to which an individual thing on the list might be linked.

It is important to be bold in adding or editing items on a list, but also to balance boldness with being thoughtful, a balance which all content policies are aimed at helping editors achieve. Edits of uncertain quality can be first discussed on the talk page for feedback from other editors.

Besides being useful for such feedback, a talk page discussion is also a good review process for reaching consensus before adding an item that is difficult or contentious, especially those items for which the definition of the topic itself is disputed. Note that, as with other policies and processes mentioned in this section, this process can be used for any type of difficult or contentious encyclopedic content on Wikipedia.

Reaching consensus on the talk page before editing the list itself not only saves time in the long run, but also helps make sure that each item on the list is well referenced and that the list as a whole represents a neutral point of view. Content should be sourced where it appears, and provide inline citations if it contains any of the four kinds of material absolutely required to have citations.

When an item meets the requirements of the Verifiability policy, readers of the list can check an item's reference to see that the information comes from a reliable source. For information to be verifiable, it also means that Wikipedia does not publish original research: its content is determined by information previously published in a good source, rather than the beliefs or experiences of its editors, or even the editor's interpretation beyond what the source actually says. Even if you're sure that an item is relevant to the list's topic, you must find a good source that verifies this knowledge before you add it to the list (although you can suggest it on the talk page), and add that source in a reference next to the item.

In lists that involve living persons, the Biographies of living persons policy applies.

When reliable sources disagree, the policy of keeping a neutral point of view requires that competing views be described without endorsing any in particular. Editors should simply present what the various sources say, giving each side its due weight through coverage balanced according to the prominence of each viewpoint in the published reliable sources.

When adding to a stand-alone list with links to other articles, follow the established format when adding your item, and then see if you can link that item to an article focusing on that item's topic. If so, then consider if the list's format allows room for all the details of competing views in the list item or if those details should only be covered in the linked, main article on the topic. Either way, make sure to add them to the main article if they are not already there.

Categories

You can add one or more suitable subcategories of :Category:Lists at the bottom of the page containing a list that may be of independent encyclopedic interest.  If there is a redirect for the list (e.g., from "List of Presidents of Elbonia" to "President of Elbonia#List of Elbonian Presidents") put list categories on the "List"-named redirect instead. Use a sort key to sort alphabetically by topic.

List styles

There are several ways of presenting lists on Wikipedia.

Bulleted lists

This is the most common list type on Wikipedia. Bullets are used to discern, at a glance, the individual items in a list, usually when each item in the list is a simple word, phrase or single line of text, for which numeric ordering is not appropriate, or lists that are extremely brief, where discerning the items at a glance is not an issue. They are not appropriate for large paragraphs. Simple bulleted lists are created by starting a line with * and adding the text of a list item, one item per * line.

List items should be formatted consistently.  Summary:
 Prefer sentence case.
 Prefer using full sentences, and avoid mixing sentences and fragments as items in the same list.
 No terminal punctuation is used with sentence fragments.
 Do not put blank lines between list items.
.

HTML formatting can be used to create rich lists, including items with internal paragraph breaks.   Using images with lists requires some care.

For infoboxes, a bulleted list can be converted to unbulleted or horizontal style with simple templates, to suppress both the large bullets and the indentation.

Do not double-space the lines of the list by leaving blank lines after them. Doing this breaks the list into multiple lists, defeating the purpose of using list markup. This adversely affects accessibility (screen readers will tell the visually impaired user there are multiple lists), and interferes with machine-parseability of the content for reuse. Moreover, in certain Web browsers, the extra white-space between one block of list output  and the next can have a visually jarring effect.

Blank lines between items of a numbered list will not only cause the same broken-list problems as in bulleted lists, but will also restart the numbering at "1". This cannot be fixed without complex markup (defeating ease-of-editing expectations), so double-spacing should always be avoided in numbered lists.

Doing this actually produces three lists with one item each! Notice the rendered HTML in which there are as many  tags as  tags.

Unbulleted lists

For lists of up to 30 items (may increase later) without bullets, use a  or  template. Typical uses are in infobox fields, and to replace pseudo-lists of lines separated with .  The templates emit the correct HTML markup, and hide the bullets with CSS .

A benefit of  is that it can be wrapped around an already-existing bullet list.  A feature of  is that, for a short list, it can be put on a single line: .

Numbered lists

Use a numbered (ordered) list only if any of the following apply:
 There is a need to refer to the elements by number.
 The sequence of the items is critical.
 The numbering has some independent meaning, for example in a listing of musical tracks on an album.

Use a # symbol at the start of a line to generate a numbered list item (excepted as detailed in this section, this works the same as * for bulleted lists, above).

List items should be formatted consistently.  Summary:
 Prefer sentence case.
 Prefer using full sentences, and avoid mixing sentences and fragments as items in the same list.
 No terminal punctuation is used with sentence fragments.
 Do not put blank lines between list items.
.

Example:

Blank lines between items of an ordered list will not only cause the same broken-list problems as in bulleted lists, but will also restart the numbering at "1". This cannot be fixed without complex markup (defeating ease-of-editing expectations), so double-spacing should always be avoided in numbered lists.

HTML formatting can be used to create rich lists, including items with internal paragraph breaks; some basics are illustrated below.   Using images with lists also requires some care.

Other cases

Experienced editors can use raw HTML to achieve more complex results, such as ordered lists using indexes other than numbers, and ordered lists not starting from 1.

Valid values for the list type are:
1 (default, numbers)
a (lowercase latin letters)
A (uppercase latin letters)
i (lowercase roman numerals)
I (uppercase roman numerals)
The start value can be negative, but only if the list uses numbers as indexes. Otherwise, bizarre results are achieved.

Description (definition, association) lists

A description list contains groups of "... terms and definitions, metadata topics and values, questions and answers, or any other groups of name-value data." On Wikipedia, the most common use of a description list is for a glossary, where it is preferable to other styles. Wikipedia has special markup for :

The source can also be laid out with the descriptive value on the next line after the term, like so:

This still keeps the names and values within a single description list, and the alternation of typically short names and longer values makes the separate components easy to spot while editing. The resulting layout and HTML are identical to that generated by the single-line syntax.

Either wikitext markup is functionality-limited and easily broken. A major weakness of both variants of wikitext markup is that they are easily broken by later editors attempting to create multi-line values. These issues are most-prominent in lengthy description lists. As such, there are templates for producing description lists such as glossaries, in ways that provide for richer, more complex content, including multiple paragraphs, block quotations, sub-lists, etc.   .

The basic format of a  is:

Use either wikitext or templates as above for description lists instead of other, made-up formats, as other formats may be unexpected for reader and editor alike, hamper reusability of Wikipedia content, make automated processing more difficult, and introduce usability and accessibility problems. (Other formats may take less vertical space, but will be more difficult for the reader to scan.) That said, a list of items whose descriptions contain more than one paragraph may present better as sections in a stand-alone list article, while tables are better-suited to associating content than description lists, especially when there are multiple values for each item.

As with unordered (bulleted) and ordered (numbered) lists, items in description lists should not have blank lines between them, as it causes each entry to be its own bogus "list" in the output, obviating the point of putting the entries in list markup to begin with.

When wiki markup colons are used just for visual indentation, they too are rendered in HTML as description lists, but without ;-delimited terms to which the :-indented material applies, nor with the list start and end tags, which produces broken markup . More accessible indentation templates can be used, e.g.,  or one of its variants for one line, and  for more than one line (even if misuse of description list markup on talk pages is too ingrained to change at this point).

Many of the considerations at  also apply to description list terms; even though description list terms are not headings, they act like headings in some ways. In at least one regard however, they are not: description list term wikitext (;) should not be used to subdivide large sections. Use a subheading instead (e.g., === Subheading ===).

Tables

Tables are a way of presenting links, data, or information in rows and columns. They are a complex form of list and are useful especially when more than 2 pieces of information are of interest to each list item. Tables require a more-complex notation, and should be scrutinized for their accessibility. Consideration may be given to collapsing tables which consolidate information covered in the prose.

Tables might be used for presenting mathematical data such as multiplication tables, comparative figures, or sporting results. They might also be used for presenting equivalent words in two or more languages, for awards by type and year, and complex discographies.

Horizontal lists

In situations such as infoboxes, horizontal lists may be useful.  Examples:

Note the capitalization of only the first word in this list ("Entry 1 ..."), regardless of coding style.  Words that are normally capitalized, like proper names, would of course still be capitalized.

A benefit of  is that it can be wrapped around an already-existing bullet list.  A feature of  is that, for a short list, it can be put on a single line.

Timelines

For lists of dated events, or timelines, use one instance of  per event, thus:

* {{Timeline-event|date={{Start date|1904|11|18|df=y}}|event=A thing happened}}
* {{Timeline-event|date={{Start date|1905}}|event=Not much happened}}
* {{Timeline-event|date={{Start date|1906|01|21}}|event=Something else happened}}

to render as:

(note optional df=y (date first) parameter – date formatting should be consistent within individual articles).

Chronological lists, such as timelines, should be in earliest-to-latest chronological order. See .

Line breaks

, as it does not meet Web standards and can cause accessibility problems. Instead, use one of more formatted list styles defined above.

Boilerplate text
Directly before an incomplete list, insert , which will transclude the following onto the page:

Several topic-specific variations of this template are also available within . Only one of  or its variations should be added, unless the topic is significantly related to more than one of the subcategories. Do not add both  AND a variation to any list.

Pro and con lists

These are lists of arguments for and against a particular contention or position. They include lists of Advantages and disadvantages of a technology or proposal (such as Wi-Fi) and lists of Criticisms and defenses of a political position or other view, such as libertarianism or evolution. Pro and con lists can encapsulate or bracket neutrality problems in an article by creating separate spaces in which different points of view can be expressed. An alternative method is to thread different points of view into running prose.

Either method needs careful judgment as to whether and how it should be used. In particular, pro and con lists can fragment the presentation of facts, create a binary structure where a more nuanced treatment of the spectrum of facts is preferable, encourage oversimplification, and require readers to jump back and forth between the two sides of the list.

See also
 Help:Line-break handling – covers among other things how to properly handle the line wrapping in horizontal link lists
 Help:Sorting – tables on Wikipedia can be made sortable with class="sortable", this page explains how
 Wikipedia:List dos and don'ts – information page summarizing the key points in this guideline
 Wikipedia:Manual of Style/Disambiguation pages – disambiguation pages are lists of homographs—a word or a group of words that share the same written form but have different meanings—with their own page rules and layouts
 Wikipedia:Stand-alone lists – guideline page on content and style guidelines and naming conventions
  – cleanup tags for lists
 Wikipedia:WikiProject Lists – project's goal is to collaboratively develop Wikipedia's list articles and embedded lists

Notes

Wikipedia article elements help
Wikipedia how-to

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MOS:LIST


Rockabilly is an early type of rock and roll music. It developed in the 1950s in the United States. Rockabilly was very popular amongst teenagers. Many parents of teenagers did not like rockabilly music, because it had lyrics which talked about rebelling against rules, sex and drinking alcohol. Rockabilly musicians often wear slicked-back hair, jeans, and leather jackets.

Rockabilly music
Rockabilly music has musical styles from blues music and country music. It uses the acoustic guitar, the electric guitar, the double bass, drums, and singing. Some Rockabilly songs also use the piano or the harmonica. Rockabilly music usually has a strong rhythm from the double bass being played in a "slap" style. Rockabilly singers often use a wide range, with high notes and many shouts like "go man go" and "yeah!"

Rockabilly musicians
One of the earliest musicians who played rockabilly music was Elvis Presley. Another famous rockabilly musician was Carl Perkins, who wrote the popular song Blue Suede Shoes. Jerry Lee Lewis was a piano player who sang. Jerry Lee Lewis was known for his crazy behavior on stage, which audiences liked. Lewis would play the piano with his feet, elbows and butt.

Gene Vincent was a rockabilly musician who wore black leather and sang songs about sex and getting into trouble. Vincent's most popular song is "Be-Bop-A-Lula". Eddie Cochran wrote songs about teen life, and had hit songs like "Summertime Blues" and "C'mon Everybody."

Rockabilly revival
Rockabilly became popular again in the 1980s, when bands such as the Stray Cats became popular. In the 1990s and 2000s, there are still many bands playing rockabilly music or music influenced by rockabilly, such as Tiger Army.

Sources

Rock music

Rockabilly