normal+variable

SUBJECT AREA: Broadcasting, Electronics and information technology, Photography, film and optics, Recording, Telecommunications

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De Forest was born into the family of a Congregational minister that moved to Alabama in 1879 when the father became President of a college for African-Americans; this was a position that led to the family's social ostracism by the white community. By the time he was 13 years old, De Forest was already a keen mechanical inventor, and in 1893, rejecting his father's plan for him to become a clergyman, he entered the Sheffield Scientific School of Yale University. Following his first degree, he went on to study the propagation of electromagnetic waves, gaining a PhD in physics in 1899 for his thesis on the "Reflection of Hertzian Waves from the Ends of Parallel Wires", probably the first US thesis in the field of radio.

He then joined the Western Electric Company in Chicago where he helped develop the infant technology of wireless, working his way up from a modest post in the production area to a position in the experimental laboratory. There, working alone after normal working hours, he developed a detector of electromagnetic waves based on an electrolytic device similar to that already invented by Fleming in England. Recognizing his talents, a number of financial backers enabled him to set up his own business in 1902 under the name of De Forest Wireless Telegraphy Company; he was soon demonstrating wireless telegraphy to interested parties and entering into competition with the American Marconi Company.

Despite the failure of this company because of fraud by his partners, he continued his experiments; in 1907, by adding a third electrode, a wire mesh, between the anode and cathode of the thermionic diode invented by Fleming in 1904, he was able to produce the amplifying device now known as the triode valve and achieve a sensitivity of radio-signal reception much greater than possible with the passive carborundum and electrolytic detectors hitherto available. Patented under the name Audion, this new vacuum device was soon successfully used for experimental broadcasts of music and speech in New York and Paris. The invention of the Audion has been described as the beginning of the electronic era. Although much development work was required before its full potential was realized, the Audion opened the way to progress in all areas of sound transmission, recording and reproduction. The patent was challenged by Fleming and it was not until 1943 that De Forest's claim was finally recognized.

Overcoming the near failure of his new company, the De Forest Radio Telephone Company, as well as unsuccessful charges of fraudulent promotion of the Audion, he continued to exploit the potential of his invention. By 1912 he had used transformer-coupling of several Audion stages to achieve high gain at radio frequencies, making long-distance communication a practical proposition, and had applied positive feedback from the Audion output anode to its input grid to realize a stable transmitter oscillator and modulator. These successes led to prolonged patent litigation with Edwin Armstrong and others, and he eventually sold the manufacturing rights, in retrospect often for a pittance.

During the early 1920s De Forest began a fruitful association with T.W.Case, who for around ten years had been working to perfect a moving-picture sound system. De Forest claimed to have had an interest in sound films as early as 1900, and Case now began to supply him with photoelectric cells and primitive sound cameras. He eventually devised a variable-density sound-on-film system utilizing a glow-discharge modulator, the Photion. By 1926 De Forest's Phonofilm had been successfully demonstrated in over fifty theatres and this system became the basis of Movietone. Though his ideas were on the right lines, the technology was insufficiently developed and it was left to others to produce a system acceptable to the film industry. However, De Forest had played a key role in transforming the nature of the film industry; within a space of five years the production of silent films had all but ceased.

In the following decade De Forest applied the Audion to the development of medical diathermy. Finally, after spending most of his working life as an independent inventor and entrepreneur, he worked for a time during the Second World War at the Bell Telephone Laboratories on military applications of electronics.

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KF / JW

   1) A Unitary Search Process for All the Phenomena of Memory

   To what extent can we lump together what goes on when you try to recall: (1) your name; (2) how you kick a football; and (3) the present location of your car keys? If we use introspective evidence as a guide, the first seems an immediate automatic response. The second may require constructive internal replay prior to our being able to produce a verbal description. The third... quite likely involves complex operational responses under the control of some general strategy system. Is any unitary search process, with a single set of characteristics and inputoutput relations, likely to cover all these cases? (Reitman, 1970, p. 485)

   2) Semantic Memory Is a Mental Thesaurus

   [Semantic memory] Is a mental thesaurus, organized knowledge a person possesses about words and other verbal symbols, their meanings and referents, about relations among them, and about rules, formulas, and algorithms for the manipulation of these symbols, concepts, and relations. Semantic memory does not register perceptible properties of inputs, but rather cognitive referents of input signals. (Tulving, 1972, p. 386)

   3) The Development of Mnemonic Codes

   The mnemonic code, far from being fixed and unchangeable, is structured and restructured along with general development. Such a restructuring of the code takes place in close dependence on the schemes of intelligence. The clearest indication of this is the observation of different types of memory organisation in accordance with the age level of a child so that a longer interval of retention without any new presentation, far from causing a deterioration of memory, may actually improve it. (Piaget & Inhelder, 1973, p. 36)

   4) The Logic of Some Memory Theorization Is of Dubious Worth in the History of Psychology

   If a cue was effective in memory retrieval, then one could infer it was encoded; if a cue was not effective, then it was not encoded. The logic of this theorization is "heads I win, tails you lose" and is of dubious worth in the history of psychology. We might ask how long scientists will puzzle over questions with no answers. (Solso, 1974, p. 28)

   5) The Constituent Elements of Memory Theory

   We have iconic, echoic, active, working, acoustic, articulatory, primary, secondary, episodic, semantic, short-term, intermediate-term, and longterm memories, and these memories contain tags, traces, images, attributes, markers, concepts, cognitive maps, natural-language mediators, kernel sentences, relational rules, nodes, associations, propositions, higher-order memory units, and features. (Eysenck, 1977, p. 4)

   6) The Problem with the Memory Metaphor

   The problem with the memory metaphor is that storage and retrieval of traces only deals [ sic] with old, previously articulated information. Memory traces can perhaps provide a basis for dealing with the "sameness" of the present experience with previous experiences, but the memory metaphor has no mechanisms for dealing with novel information. (Bransford, McCarrell, Franks & Nitsch, 1977, p. 434)

   7) The Results of a Hundred Years of the Psychological Study of Memory Are Somewhat Discouraging

   The results of a hundred years of the psychological study of memory are somewhat discouraging. We have established firm empirical generalisations, but most of them are so obvious that every ten-year-old knows them anyway. We have made discoveries, but they are only marginally about memory; in many cases we don't know what to do with them, and wear them out with endless experimental variations. We have an intellectually impressive group of theories, but history offers little confidence that they will provide any meaningful insight into natural behavior. (Neisser, 1978, pp. 12-13)

   8) The Structure and Function of a Schema in Memory

   A schema, then is a data structure for representing the generic concepts stored in memory. There are schemata representing our knowledge about all concepts; those underlying objects, situations, events, sequences of events, actions and sequences of actions. A schema contains, as part of its specification, the network of interrelations that is believed to normally hold among the constituents of the concept in question. A schema theory embodies a prototype theory of meaning. That is, inasmuch as a schema underlying a concept stored in memory corresponds to the mean ing of that concept, meanings are encoded in terms of the typical or normal situations or events that instantiate that concept. (Rumelhart, 1980, p. 34)

   9) Competence and Performance in Theories of Memory

   Memory appears to be constrained by a structure, a "syntax," perhaps at quite a low level, but it is free to be variable, deviant, even erratic at a higher level....

   Like the information system of language, memory can be explained in part by the abstract rules which underlie it, but only in part. The rules provide a basic competence, but they do not fully determine performance. (Campbell, 1982, pp. 228, 229)

    10) Metaphors of Memory

   When people think about the mind, they often liken it to a physical space, with memories and ideas as objects contained within that space. Thus, we speak of ideas being in the dark corners or dim recesses of our minds, and of holding ideas in mind. Ideas may be in the front or back of our minds, or they may be difficult to grasp. With respect to the processes involved in memory, we talk about storing memories, of searching or looking for lost memories, and sometimes of finding them. An examination of common parlance, therefore, suggests that there is general adherence to what might be called the spatial metaphor. The basic assumptions of this metaphor are that memories are treated as objects stored in specific locations within the mind, and the retrieval process involves a search through the mind in order to find specific memories....

   However, while the spatial metaphor has shown extraordinary longevity, there have been some interesting changes over time in the precise form of analogy used. In particular, technological advances have influenced theoretical conceptualisations.... The original Greek analogies were based on wax tablets and aviaries; these were superseded by analogies involving switchboards, gramophones, tape recorders, libraries, conveyor belts, and underground maps. Most recently, the workings of human memory have been compared to computer functioning... and it has been suggested that the various memory stores found in computers have their counterparts in the human memory system. (Eysenck, 1984, pp. 79-80)

    11) Comparison of Primary Memory and Secondary Memory

   Primary memory [as proposed by William James] relates to information that remains in consciousness after it has been perceived, and thus forms part of the psychological present, whereas secondary memory contains information about events that have left consciousness, and are therefore part of the psychological past. (Eysenck, 1984, p. 86)

    12) Semantic Memory and Episodic Memory

   Once psychologists began to study long-term memory per se, they realized it may be divided into two main categories.... Semantic memories have to do with our general knowledge about the working of the world. We know what cars do, what stoves do, what the laws of gravity are, and so on. Episodic memories are largely events that took place at a time and place in our personal history. Remembering specific events about our own actions, about our family, and about our individual past falls into this category. With amnesia or in aging, what dims... is our personal episodic memories, save for those that are especially dear or painful to us. Our knowledge of how the world works remains pretty much intact. (Gazzaniga, 1988, p. 42)

    13) The Relation of Memory to Thinking

   The nature of memory... provides a natural starting point for an analysis of thinking. Memory is the repository of many of the beliefs and representations that enter into thinking, and the retrievability of these representations can limit the quality of our thought. (Smith, 1990, p. 1)

 

явление электрической дуги
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[Интент]

Параллельные тексты EN-RU

Electric arc phenomenon

The electric arc is a phenomenon which takes place as a consequence of a discharge which occurs when the voltage between two points exceeds the insulating strength limit of the interposed gas; then, in the presence of suitable conditions, a plasma is generated which carries the electric current till the opening of the protective device on the supply side.

Gases, which are good insulating means under normal conditions, may become current conductors in consequence of a change in their chemical-physical properties due to a temperature rise or to other external factors.

To understand how an electrical arc originates, reference can be made to what happens when a circuit opens or closes.

During the opening phase of an electric circuit the contacts of the protective device start to separate thus offering to the current a gradually decreasing section; therefore the current meets growing resistance with a consequent rise in the temperature.

As soon as the contacts start to separate, the voltage applied to the circuit exceeds the dielectric strength of the air, causing its perforation through a discharge.

The high temperature causes the ionization of the surrounding air which keeps the current circulating in the form of electrical arc. Besides thermal ionization, there is also an electron emission from the cathode due to the thermionic effect; the ions formed in the gas due to the very high temperature are accelerated by the electric field, strike the cathode, release energy in the collision thus causing a localized heating which generates electron emission.

The electrical arc lasts till the voltage at its ends supplies the energy sufficient to compensate for the quantity of heat dissipated and to maintain the suitable conditions of temperature. If the arc is elongated and cooled, the conditions necessary for its maintenance lack and it extinguishes.

Analogously, an arc can originate also as a consequence of a short-circuit between phases. A short-circuit is a low impedance connection between two conductors at different voltages.

The conducting element which constitutes the low impedance connection (e.g. a metallic tool forgotten on the busbars inside the enclosure, a wrong wiring or a body of an animal entered inside the enclosure), subject to the difference of potential is passed through by a current of generally high value, depending on the characteristics of the circuit.

The flow of the high fault current causes the overheating of the cables or of the circuit busbars, up to the melting of the conductors of lower section; as soon as the conductor melts, analogous conditions to those present during the circuit opening arise. At that point an arc starts which lasts either till the protective devices intervene or till the conditions necessary for its stability subsist.

The electric arc is characterized by an intense ionization of the gaseous means, by reduced drops of the anodic and cathodic voltage (10 V and 40 V respectively), by high or very high current density in the middle of the column (of the order of 102-103 up to 107 A/cm2), by very high temperatures (thousands of °C) always in the middle of the current column and – in low voltage - by a distance between the ends variable from some microns to some centimeters.

[ABB]

Явление электрической дуги

Электрическая дуга между двумя электродами в газе представляет собой физическое явление, возникающее в тот момент, когда напряжения между двумя электродами превышает значение электрической прочности изоляции данного газа.
При наличии подходящих условий образуется плазма, по которой протекает электрический ток. Ток будет протекать до тех пор, пока на стороне электропитания не сработает защитное устройство.

Газы, являющиеся хорошим изолятором, при нормальных условиях, могут стать проводником в результате изменения их физико-химических свойств, которые могут произойти вследствие увеличения температуры или в результате воздействия каких-либо иных внешних факторов.

Для того чтобы понять механизм возникновения электрической дуги, следует рассмотреть, что происходит при размыкании или замыкании электрической цепи.

При размыкании электрической цепи контакты защитного устройства начинают расходиться, в результате чего постепенно уменьшается сечение контактной поверхности, через которую протекает ток.
Сопротивление электрической цепи возрастает, что приводит к увеличению температуры.

Как только контакты начнут отходить один от другого, приложенное напряжение превысит электрическую прочность воздуха, что вызовет электрический пробой.

Высокая температура приведет к ионизации воздуха, которая обеспечит протекание электрического тока по проводнику, представляющему собой электрическую дугу. Кроме термической ионизации молекул воздуха происходит также эмиссия электронов с катода, вызванная термоэлектронным эффектом. Образующиеся под воздействием очень высокой температуры ионы ускоряются в электрическом поле и бомбардируют катод. Высвобождающаяся, в результате столкновения энергия, вызывает локальный нагрев, который, в свою очередь, приводит к эмиссии электронов.

Электрическая дуга длится до тех пор, пока напряжение на ее концах обеспечивает поступление энергии, достаточной для компенсации выделяющегося тепла и для сохранения условий поддержания высокой температуры. Если дуга вытягивается и охлаждается, то условия, необходимые для ее поддержания, исчезают и дуга гаснет.

Аналогичным образом возникает дуга в результате короткого замыкания электрической цепи. Короткое замыкание представляет собой низкоомное соединение двух проводников, находящихся под разными потенциалами.

Проводящий элемент с малым сопротивлением, например, металлический инструмент, забытый на шинах внутри комплектного устройства, ошибка в электромонтаже или тело животного, случайно попавшего в комплектное устройство, может соединить элементы, находящиеся под разными потенциалами, в результате чего через низкоомное соединение потечет электрический ток, значение которого определяется параметрами образовавшейся короткозамкнутой цепи.

Протекание большого тока короткого замыкания вызывает перегрев кабелей или шин, который может привести к расплавлению проводников с меньшим сечением. Как только проводник расплавится, возникает ситуация, аналогичная размыканию электрической цепи. Т. е. в момент размыкания возникает дуга, которая длится либо до срабатывания защитного устройства, либо до тех пор, пока существуют условия, обеспечивающие её стабильность.

Электрическая дуга характеризуется интенсивной ионизацией газов, что приводит к падению анодного и катодного напряжений (на 10 и 40 В соответственно), высокой или очень высокой плотностью тока в середине плазменного шнура (от 102-103 до 107 А/см2), очень высокой температурой (сотни градусов Цельсия) всегда в середине плазменного шнура и низкому падению напряжения при расстоянии между концами дуги от нескольких микрон до нескольких сантиметров.

[Перевод Интент]

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