In physics, motion is the change in position of an object with respect to its surroundings in a given interval of time. Motion is mathematically described in terms of displacement, velocity, acceleration and speed. Motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame. If the position of a body is not changing with respect to a given frame of reference, the body is said to be at rest, immobile, stationary, or to have constant position with reference to its surroundings. An object's motion can not change. Momentum is a quantity, used for measuring the motion of an object. An object's momentum is directly related to the object's mass and velocity, the total momentum of all objects in an isolated system does not change with time, as described by the law of conservation of momentum; as there is no absolute frame of reference, absolute motion cannot be determined. Thus, everything in the universe can be considered to be moving.
Motion applies to objects and matter particles, to radiation, radiation fields and radiation particles, to space, its curvature and space-time. One can speak of motion of shapes and boundaries. So, the term motion, in general, signifies a continuous change in the configuration of a physical system. For example, one can talk about motion of a wave or about motion of a quantum particle, where the configuration consists of probabilities of occupying specific positions. In physics, motion is described through two sets of contradictory laws of mechanics. Motions of all large-scale and familiar objects in the universe are described by classical mechanics. Whereas the motion of small atomic and sub-atomic objects is described by quantum mechanics. Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets and galaxies, it produces accurate results within these domains, is one of the oldest and largest in science and technology.
Classical mechanics is fundamentally based on Newton's laws of motion. These laws describe the relationship between the forces acting on a body and the motion of that body, they were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687. Newton's three laws are: A body either is at rest or moves with constant velocity and unless an outer force is applied to it. An object will travel in one direction. Whenever one body exerts a force F onto a second body, the second body exerts the force −F on the first body. F and − F are equal in opposite in sense. So, the body which exerts F will go backwards. Newton's three laws of motion were the first to provide a mathematical model for understanding orbiting bodies in outer space; this explanation unified motion of objects on earth. Classical mechanics was further enhanced by Albert Einstein's special relativity and general relativity. Special relativity is concerned with the motion of objects with a high velocity, approaching the speed of light.
Uniform Motion: When an object moves with a constant speed at a particular direction at regular intervals of time it's known as the uniform motion. For example: a bike moving in a straight line with a constant speed. Equations of Uniform Motion: If v = final velocity, u = initial velocity, a = acceleration, t = time, s = displacement, then: v = u + a t v 2 = u 2 + 2 a s s = u t + a t 2 2 Quantum mechanics is a set of principles describing physical reality at the atomic level of matter and the subatomic particles; these descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation energy as described in the wave–particle duality. In classical mechanics, accurate measurements and predictions of the state of objects can be calculated, such as location and velocity. In the quantum mechanics, due to the Heisenberg uncertainty principle, the complete state of a subatomic particle, such as its location and velocity, cannot be determined. In addition to describing the motion of atomic level phenomena, quantum mechanics is useful in understanding some large-scale phenomenon such as superfluidity, superconductivity, biological systems, including the function of smell receptors and the structures of proteins.
Humans, like all known things in the universe, are in constant motion. Many of these "imperceptible motions" are only perceivable with the help of special tools and careful observation; the larger scales of imperceptible motions are difficult for humans to perceive
Psychology is the science of behavior and mind. Psychology includes the study of conscious and unconscious phenomena, as well as feeling and thought, it is an academic discipline of immense scope. Psychologists seek an understanding of the emergent properties of brains, all the variety of phenomena linked to those emergent properties; as a social science it aims to understand individuals and groups by establishing general principles and researching specific cases. In this field, a professional practitioner or researcher is called a psychologist and can be classified as a social, behavioral, or cognitive scientist. Psychologists attempt to understand the role of mental functions in individual and social behavior, while exploring the physiological and biological processes that underlie cognitive functions and behaviors. Psychologists explore behavior and mental processes, including perception, attention, intelligence, motivation, brain functioning, personality; this extends to interaction between people, such as interpersonal relationships, including psychological resilience, family resilience, other areas.
Psychologists of diverse orientations consider the unconscious mind. Psychologists employ empirical methods to infer causal and correlational relationships between psychosocial variables. In addition, or in opposition, to employing empirical and deductive methods, some—especially clinical and counseling psychologists—at times rely upon symbolic interpretation and other inductive techniques. Psychology has been described as a "hub science" in that medicine tends to draw psychological research via neurology and psychiatry, whereas social sciences most draws directly from sub-disciplines within psychology. While psychological knowledge is applied to the assessment and treatment of mental health problems, it is directed towards understanding and solving problems in several spheres of human activity. By many accounts psychology aims to benefit society; the majority of psychologists are involved in some kind of therapeutic role, practicing in clinical, counseling, or school settings. Many do scientific research on a wide range of topics related to mental processes and behavior, work in university psychology departments or teach in other academic settings.
Some are employed in industrial and organizational settings, or in other areas such as human development and aging, sports and the media, as well as in forensic investigation and other aspects of law. The word psychology derives from Greek roots meaning study of soul; the Latin word psychologia was first used by the Croatian humanist and Latinist Marko Marulić in his book, Psichiologia de ratione animae humanae in the late 15th century or early 16th century. The earliest known reference to the word psychology in English was by Steven Blankaart in 1694 in The Physical Dictionary which refers to "Anatomy, which treats the Body, Psychology, which treats of the Soul."In 1890, William James defined psychology as "the science of mental life, both of its phenomena and their conditions". This definition enjoyed widespread currency for decades. However, this meaning was contested, notably by radical behaviorists such as John B. Watson, who in his 1913 manifesto defined the discipline of psychology as the acquisition of information useful to the control of behavior.
Since James defined it, the term more connotes techniques of scientific experimentation. Folk psychology refers to the understanding of ordinary people, as contrasted with that of psychology professionals; the ancient civilizations of Egypt, China and Persia all engaged in the philosophical study of psychology. In Ancient Egypt the Ebers Papyrus mentioned thought disorders. Historians note that Greek philosophers, including Thales and Aristotle, addressed the workings of the mind; as early as the 4th century BC, Greek physician Hippocrates theorized that mental disorders had physical rather than supernatural causes. In China, psychological understanding grew from the philosophical works of Laozi and Confucius, from the doctrines of Buddhism; this body of knowledge involves insights drawn from introspection and observation, as well as techniques for focused thinking and acting. It frames the universe as a division of, interaction between, physical reality and mental reality, with an emphasis on purifying the mind in order to increase virtue and power.
An ancient text known as The Yellow Emperor's Classic of Internal Medicine identifies the brain as the nexus of wisdom and sensation, includes theories of personality based on yin–yang balance, analyzes mental disorder in terms of physiological and social disequilibria. Chinese scholarship focused on the brain advanced in the Qing Dynasty with the work of Western-educated Fang Yizhi, Liu Zhi, Wang Qingren. Wang Qingren emphasized the importance of the brain as the center of the nervous system, linked mental disorder with brain diseases, investigated the causes of dreams and insomnia, advanced a theory of hemispheric lateralization in brain function. Distinctions in types of awareness appear in the ancient thought of India, influenced by Hinduism. A central idea of the Upanishads is the distinction between a person's transient mundane self and their eternal unchanging soul. Divergent Hindu doctrines, Buddhism, have challenged this hierarchy of selves, but have all emphasized the importance of reaching higher
Memory is the faculty of the brain by which information is encoded and retrieved when needed. Memory is vital to experiences, it is the retention of information over time for the purpose of influencing future action. If we could not remember past events, we could not learn or develop language, relationships, or personal identity. Memory is understood as an informational processing system with explicit and implicit functioning, made up of a sensory processor, short-term memory, long-term memory; this can be related to the neuron. The sensory processor allows information from the outside world to be sensed in the form of chemical and physical stimuli and attended to various levels of focus and intent. Working memory serves as an encoding and retrieval processor. Information in the form of stimuli is encoded in accordance with explicit or implicit functions by the working memory processor; the working memory retrieves information from stored material. The function of long-term memory is to store data through various categorical models or systems.
Explicit and implicit functions of memory are known as declarative and non-declarative systems. These systems lack thereof. Declarative, or explicit, memory is the conscious recollection of data. Under declarative memory resides episodic memory. Semantic memory refers to memory, encoded with specific meaning, while episodic memory refers to information, encoded along a spatial and temporal plane. Declarative memory is the primary process thought of when referencing memory. Non-declarative, or implicit, memory is the unconscious recollection of information. An example of a non-declarative process would be the unconscious learning or retrieval of information by way of procedural memory, or a priming phenomenon. Priming is the process of subliminally arousing specific responses from memory and shows that not all memory is consciously activated, whereas procedural memory is the slow and gradual learning of skills that occurs without conscious attention to learning. Memory is not a perfect processor, is affected by many factors.
The ways by which information is encoded and retrieved can all be corrupted. The amount of attention given new stimuli can diminish the amount of information that becomes encoded for storage; the storage process can become corrupted by physical damage to areas of the brain that are associated with memory storage, such as the hippocampus. The retrieval of information from long-term memory can be disrupted because of decay within long-term memory. Normal functioning, decay over time, brain damage all affect the accuracy and capacity of the memory. Memory loss is described as forgetfulness or amnesia. Sensory memory holds sensory information less than one second; the ability to look at an item and remember what it looked like with just a split second of observation, or memorization, is the example of sensory memory. It is an automatic response. With short presentations, participants report that they seem to "see" more than they can report; the first experiments exploring this form of sensory memory were conducted by George Sperling using the "partial report paradigm".
Subjects were presented with a grid of 12 letters, arranged into three rows of four. After a brief presentation, subjects were played either a high, medium or low tone, cuing them which of the rows to report. Based on these partial report experiments, Sperling was able to show that the capacity of sensory memory was 12 items, but that it degraded quickly; because this form of memory degrades so participants would see the display but be unable to report all of the items before they decayed. This type of memory cannot be prolonged via rehearsal. Three types of sensory memories exist. Iconic memory is a fast decaying store of visual information. Echoic memory is a fast decaying store of auditory information, another type of sensory memory that stores sounds that have been perceived for short durations. Haptic memory is a type of sensory memory. Short-term memory is known as working memory. Short-term memory allows recall for a period of several seconds to a minute without rehearsal, its capacity is very limited: George A. Miller, when working at Bell Laboratories, conducted experiments showing that the store of short-term memory was 7±2 items.
Modern estimates of the capacity of short-term memory are lower of the order of 4–5 items. For example, in recalling a ten-digit telephone number, a person could chunk the digits into three groups: first, the area code a three-digit chunk and lastly a four-digit chunk; this method of remembering telephone numbers is far more effective than attempting to remember a string of 10 digits. This may be reflected in some countries in the tendency to display telephone numbers as several chunks of two to four numbers. Short-term memory is believed to rely on an acoustic code for storing information, to a lesser extent a visual code. Conrad found that test subjects had more difficulty recalling collections of letters that were acoustically similar (e.g. E
Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. This is different from visual acuity, which refers to how a person sees. A person can have problem with visual perceptual processing if he/she has 20/20 vision; the resulting perception is known as visual perception, sight, or vision. The various physiological components involved in vision are referred to collectively as the visual system, are the focus of much research in linguistics, cognitive science and molecular biology, collectively referred to as vision science; the visual system in animals allows individuals to assimilate information from their surroundings. The act of seeing starts when the cornea and the lens of the eye focuses light from its surroundings onto a light-sensitive membrane in the back of the eye, called the retina; the retina is part of the brain, isolated to serve as a transducer for the conversion of light into neuronal signals.
Based on feedback from the visual system, the lens of the eye adjusts its thickness to focus light on the photoreceptive cells of the retina known as the rods and cones, which detect the photons of light and respond by producing neural impulses. These signals are processed via complex feedforward and feedback processes by different parts of the brain, from the retina upstream to central ganglia in the brain. Note that up until now much of the above paragraph could apply to octopuses, worms and things more primitive. However, the following applies to mammals and birds: The retina in these more complex animals sends fibers to the lateral geniculate nucleus, to the primary and secondary visual cortex of the brain. Signals from the retina can travel directly from the retina to the superior colliculus; the perception of objects and the totality of the visual scene is accomplished by the visual association cortex. The visual association cortex combines all sensory information perceived by the striate cortex which contains thousands of modules that are part of modular neural networks.
The neurons in the striate cortex send axons to the extrastriate cortex, a region in the visual association cortex that surrounds the striate cortex. The human visual system is believed to perceive visible light in the range of wavelengths between 370 and 730 nanometers of the electromagnetic spectrum. However, some research suggests that humans can perceive light in wavelengths down to 340 nanometers the young; the major problem in visual perception is that what people see is not a translation of retinal stimuli. Thus people interested in perception have long struggled to explain what visual processing does to create what is seen. There were two major ancient Greek schools, providing a primitive explanation of how vision is carried out in the body; the first was the "emission theory" which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If an object was seen directly it was by'means of rays' coming out of the eyes and again falling on the object.
A refracted image was, seen by'means of rays' as well, which came out of the eyes, traversed through the air, after refraction, fell on the visible object, sighted as the result of the movement of the rays from the eye. This theory was championed by scholars like their followers; the second school advocated the so-called'intro-mission' approach which sees vision as coming from something entering the eyes representative of the object. With its main propagators Aristotle and their followers, this theory seems to have some contact with modern theories of what vision is, but it remained only a speculation lacking any experimental foundation. Both schools of thought relied upon the principle that "like is only known by like", thus upon the notion that the eye was composed of some "internal fire" which interacted with the "external fire" of visible light and made vision possible. Plato makes this assertion in his dialogue Timaeus, in his De Sensu. Alhazen carried out many investigations and experiments on visual perception, extended the work of Ptolemy on binocular vision, commented on the anatomical works of Galen.
He was the first person to explain that vision occurs when light bounces on an object and is directed to one's eyes. Leonardo da Vinci is believed to be the first to recognize the special optical qualities of the eye, he wrote "The function of the human eye... was described by a large number of authors in a certain way. But I found it to be different." His main experimental finding was that there is only a distinct and clear vision at the line of sight—the optical line that ends at the fovea. Although he did not use these words he is the father of the modern distinction between foveal and peripheral vision. Issac Newton was the first to discover through experimentation, by isolating individual colors of the spectrum of light passing through a prism, that the visually perceived color of objects appeared due to the character
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
Distance is a numerical measurement of how far apart objects are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria. In most cases, "distance from A to B" is interchangeable with "distance from B to A". In mathematics, a distance function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a specific set of rules, is a way of describing what it means for elements of some space to be "close to" or "far away from" each other. A physical distance can mean several different things: Distance Traveled: The length of a specific path traveled between two points, such as the distance walked while navigating a maze Straight-Line Distance: The length of the shortest possible path through space, between two points, that could be taken if there were no obstacles Geodesic Distance: The length of the shortest path between two points while remaining on some surface, such as the great-circle distance along the curve of the Earth The length of a specific path that returns to the starting point, such as a ball thrown straight up, or the Earth when it completes one orbit.
"Circular distance" is the distance traveled by a wheel, which can be useful when designing vehicles or mechanical gears. The circumference of the wheel is 2π × radius, assuming the radius to be 1 each revolution of the wheel is equivalent of the distance 2π radians. In engineering ω = 2πƒ is used, where ƒ is the frequency. Unusual definitions of distance can be helpful to model certain physical situations, but are used in theoretical mathematics: "Manhattan distance" is a rectilinear distance, named after the number of blocks north, east, or west a taxicab must travel on to reach its destination on the grid of streets in parts of New York City. "Chessboard distance", formalized as Chebyshev distance, is the minimum number of moves a king must make on a chessboard to travel between two squares. Distance measures in cosmology are complicated by the expansion of the universe, by effects described by the theory of relativity such as length contraction of moving objects; the term "distance" is used by analogy to measure non-physical entities in certain ways.
In computer science, there is the notion of the "edit distance" between two strings. For example, the words "dog" and "dot", which vary by only one letter, are closer than "dog" and "cat", which differ by three letters; this idea is used in spell checkers and in coding theory, is mathematically formalized in several different ways, such as: Levenshtein distance Hamming distance Lee distance Jaro–Winkler distanceIn mathematics, a metric space is a set for which distances between all members of the set are defined. In this way, many different types of "distances" can be calculated, such as for traversal of graphs, comparison of distributions and curves, using unusual definitions of "space"; the notion of distance in graph theory has been used to describe social networks, for example with the Erdős number or the Bacon number, the number of collaborative relationships away a person is from prolific mathematician Paul Erdős or actor Kevin Bacon, respectively. In psychology, human geography, the social sciences, distance is theorized not as an objective metric, but as a subjective experience.
Both distance and displacement measure the movement of an object. Distance cannot be negative, never decreases. Distance is a magnitude. Whereas displacement is a vector quantity with both direction, it can be zero, or positive. Directed distance does not measure movement, it measures the separation of two points, can be a positive, zero, or negative vector; the distance covered by a vehicle, animal, or object along a curved path from a point A to a point B should be distinguished from the straight-line distance from A to B. For example, whatever the distance covered during a round trip from A to B and back to A, the displacement is zero as start and end points coincide. In general the straight-line distance does not equal distance travelled, except for journeys in a straight line. Directed distances can be determined along curved lines. Directed distances along straight lines are vectors that give the distance and direction between a starting point and an ending point. A directed distance of a point C from point A in the direction of B on a line AB in a Euclidean vector space is the distance from A to C if C falls on the ray AB, but is the negative of that distance if C falls on the ray BA.
For example, the directed distance from the New York City Main Library flag pole to the Statue of Liberty flag pole has: a starting point: library flag pole an ending point: statue flag pole a direction: -38° a distance: 8.72 kmAnother kind of directed distance is that between two different particles or point masses at a given time. For instance, the distance from the center of gravity of the Earth A and the center of gravity of the Moon B falls into this category. A directed distance along a curved line is not a vector and is represented by a segment of that curved line defined by endpoints A and B, with some specific information indicating the sense of an ideal or real motion from one endpoint of the segment to the other. For instance, just labelling the two endpoints as A and B can indicate the sense, if the ordered sequence is assumed, which implies that A is the starting point. A displacement is a special kind of directed distance def
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the