resistance, in biology: see immunity.

resistance, in psychiatry: see psychoanalysis.

resistance, property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. Optimum resistance is provided by a conductor that is long, small in cross section, and of a material that conducts poorly. Resistance is basically the same for alternating and direct current circuits (see impedance). However, an alternating current of high frequency tends to travel near the surface of a conductor. Since such a current uses less of the available cross section of the conductor than a direct current, it meets with more resistance than a direct current. In circuit analysis an ideal resistor, i.e., a circuit component whose only property is resistance, is called a resistance. The phenomenon of resistance arises from the interactions of electrons with ions in the conductor. The unit of resistance is the ohm. See superconductivity; Ohm's law; conduction.

passive resistance a method of nonviolent protest against laws or policies in order to force a change or secure concessions; it is also known as nonviolent resistance and is the main tactic of civil disobedience. Passive resistance typically involves such activities as mass demonstrations, refusal to obey or carry out a law or to pay taxes, the occupation of buildings or the blockade of roads, labor strikes, economic boycotts, and similar activities.

Possibly originating with the Quakers, it was adopted by Africans, Indians, and U.S. civil-rights and anti-Vietnam War protesters. Among its most articulate advocates have been Gandhi, who maintained that action needs to be accompanied by love and a willingness to search for the truth, and Martin Luther King, Jr., who called for "tough-mindedness and tenderheartedness." Two of the most massive examples of passive resistance were the Solidarity movement in Poland (1980-81) and the Velvet Revolution in Czechoslovakia (1989). Opponents of passive resistance as a means of forcing a change in policy have criticized it for potentially fostering a general disrespect for law that could result in anarchy.

nonviolent resistance: see passive resistence.

drug resistance, condition in which infecting bacteria can resist the destructive effects of drugs such as antibiotics and sulfa drugs. Drug resistance has become a serious public health problem, since many disease-causing bacteria are no longer susceptible to previously effective drug therapies. For nearly 50 years after the first antibiotic, penicillin, became available in the 1940s, people took antibiotics' effectiveness against bacterial infections for granted. By the 1960s some doctors were predicting the end of infectious diseases. By the 1990s, however, a "post-antibiotic era," characterized by the proliferation of untreatable bacterial strains, was considered inevitable, and the rate at which bacteria were becoming resistant to antibiotics was catching up with the rate at which new antibiotics were being produced.

The number of drug-resistant bacterial strains has increased in part because of the indiscriminate use of antibiotics, which have sometimes been overprescribed. Such misuse speeds the process by destroying bacteria that would compete with resistant strains. In addition, patients sometimes stop treatment when they start to feel better, leaving a residual population of bacteria that is likely to be more resistant to drug treatment. Another source of resistance is the routine use of antibiotics in animal feed to enhance growth, a practice that has led to resistant strains of Escherichia coli and Salmonella that have been passed on to consumers. The presence of drugs in the water supply, due at least in part to human and animal excretion and the disposal of unused drugs, is also believed to contribute to drug resistance in bacteria.

Resistance is due to random genetic mutations in the bacterial cell that alter its sensitivity to a single drug or to chemically similar drugs through a variety of mechanisms. Many bacteria can transfer their resistance to other bacteria of the same or different species. Resistance has occurred in common infectious bacteria such as pneumococcus (a cause of pneumonia, meningitis, and childhood ear infections) and enterococcus (a cause of wound infections). It has also occurred in such diseases as malaria and tuberculosis. Concerns are increasing as strains develop resistance to multiple drugs, including even the most powerful antibiotics (e.g., vancomycin). Although drug companies are again concentrating on antibiotic research, no new products are expected until the turn of the century, and many infectious-disease experts are urging that doctors consider the public health risk before prescribing antibiotics and that the government regulate the use of antibiotics in agriculture.

or passive resistance

Refusal to obey government demands or commands and nonresistance to consequent arrest and punishment. It is used especially as a nonviolent and usually collective means of forcing government concessions and has been a major tactic of nationalist movements in Africa and India, of the U.S. civil rights movement, and of labour and antiwar movements in many countries. Civil disobedience is a symbolic or ritualistic violation of the law, rather than a rejection of the system as a whole. The civil disobedient, finding legitimate avenues of change blocked or nonexistent, sees himself as obligated by a higher, extralegal principle to break some specific law. By submitting to punishment, the civil disobedient hopes to set a moral example that will provoke the majority or the government into effecting meaningful political, social, or economic change. The philosophical roots of civil disobedience lie deep in Western thought. Cicero, Thomas Aquinas, and John Locke, among others, appealed to systems of natural law that take precedence over the laws created by communities or states (positive law). More modern advocates and practitioners of civil disobedience include Henry David Thoreau, Mohandas K. Gandhi, and Martin Luther King, Jr.

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or hovercraft

Vehicle supported above the surface of land or water by an air cushion, produced by downwardly directed fans, enclosed within a flexible skirt beneath the hull. The concept was first proposed by John Thornycroft in the 1870s, but a working model was not produced until 1955, when Christopher Cockerell solved the problem of keeping the air cushion from escaping from under the vehicle, and formed Hovercraft Ltd. to manufacture prototypes. Problems with skirt design and engine maintenance have restricted the vehicle's commercial application; today hovercraft are used mainly as ferries.

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Control of temperature, humidity, purity, and motion of air in an enclosed space, independent of outside conditions. In a self-contained air-conditioning unit, air is heated in a boiler unit or cooled by being blown across a refrigerant-filled coil and then distributed to a controlled indoor environment. Central air-conditioning in a large building generally consists of a main plant located on the roof or mechanical floor and intermittently spaced air-handling units, or fans that deliver air through ducts to zones within the building. The air then returns to the central air-conditioning machinery through spaces called plenums to be recooled (or reheated) and recirculated. Alternate systems of cooling use chilled water, with water cooled by a refrigerant at a central location and circulated by pumps to units with fans that circulate air locally.

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Military operations conducted by airplanes, helicopters, or other aircraft against aircraft or targets on the ground and in the water. Air warfare did not become important until World War I (1914–18). The British, French, German, Russian, and Italian armed forces had flying units, including biplanes armed with machine guns for “dogfights” with enemy fighter aircraft. Zeppelins and larger airplanes carried out bombing raids. The 1920s and '30s saw the development of the monoplane, the all-metal fuselage, and the aircraft carrier. During World War II (1939–45), the Battle of Britain was the first fought exclusively in the air, the Battle of the Coral Sea was the first between carrier-based aircraft, and the atomic bombings of Hiroshima and Nagasaki were the first use of nuclear-armed bombers. In the jet age, air power has continued to be used in strategic bombing of an enemy's home territory (as in the Vietnam War, 1965–74), destroying enemy air forces (as in the Arab-Israeli wars), attacking and defending carrier-based naval fleets (as in the Falkland Islands War, 1982), and supporting ground forces (as in the Persian Gulf War, 1990–91).

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In meteorology, a large body of air having nearly uniform conditions of temperature and humidity at any given altitude. Such a mass has distinct boundaries and may extend hundreds or thousands of miles horizontally and sometimes as high as the top of the troposphere. An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface long enough to acquire its temperature and moisture properties. The Earth's major air masses all originate in polar or subtropical latitudes. The middle latitudes constitute essentially a zone of modification, interaction, and mixing of the polar and tropical air masses.

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Military organization that has the primary responsibility for conducting air warfare. The air force must gain control of the air, support ground forces (e.g., by attacking enemy ground forces), and accomplish strategic-bombing objectives. Its basic weapons platforms are fighters, bombers, attack aircraft, and early warning and control aircraft. Since the mid-20th century, some air forces have also been responsible for land-based nuclear missiles as well as nuclear-armed bombers. The army and naval branches of a state's armed forces may also operate aircraft.

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Either of two kinds of braking systems. The first, used by trains, trucks, and buses, operates by a piston driven by compressed air from reservoirs connected to brake cylinders (see piston and cylinder). When air pressure in the brake pipe is reduced, air is automatically admitted into the brake cylinder. The first practical air brake for railroads was invented in the 1860s by George Westinghouse. The second type, used by aircraft and race cars, consists of a flap or surface that can be mechanically projected into the airstream to increase the resistance of the vehicle to air and lower its speed.

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Mixture of gases constituting the earth's atmosphere. Some gases occur in steady concentrations. The most important are molecular nitrogen (N₂), 78percnt by volume, and molecular oxygen (O₂), 21percnt. Small amounts of argon (Ar; 1.9percnt), neon (Ne), helium (He), methane (CH₄), krypton (Kr), hydrogen (H₂), nitrous oxide (N₂O), and xenon (Xe) are also present in almost constant proportions. Other gases occur in variable concentrations: water vapour (H₂O), ozone (O₃), carbon dioxide (CO₂), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Air also contains trace amounts of ammonia and hydrogen sulfide. The variable constituents are important for maintaining life. Water vapour is the source for all forms of precipitation and is an important absorber and emitter of infrared radiation. Carbon dioxide is necessary for photosynthesis and is also an important absorber and emitter of infrared radiation. Ozone in the stratosphere (see ozone layer) is an effective absorber of ultraviolet radiation from the Sun but at ground-level is a corrosive pollutant and a major constituent of smog.

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Institution for the training of commissioned officers for the U.S. Air Force, located in Colorado Springs, Colorado. Created by an act of Congress in 1954, it opened in 1955. Graduates receive a bachelor's degree and a second lieutenant's commission. Most physically qualified graduates go on to Air Force pilot-training schools. Candidates may come from the ranks of the U.S. Army or Air Force, may be children of deceased veterans of the armed forces, or may be nominated by U.S. senators or representatives or by the president or vice president. All applicants must take a competitive entrance examination.

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or Underground

Clandestine groups opposed to Nazi rule in German-occupied Europe in World War II. The groups included civilians who worked secretly against the occupation and armed bands of partisans or guerrilla fighters. Resistance activities ranged from assisting the escape of Jews and Allied airmen shot down over enemy territory to committing sabotage, ambushing German patrols, and sending intelligence information to the Allies. Resistance groups were not always unified; in some countries, rival groups divided along communist and noncommunist lines. However, in France the clandestine National Council of the Resistance coordinated all French groups, which gave support to the Normandy Campaign and participated in the August 1944 uprising that helped liberate Paris. Resistance groups in other northern European countries also undertook military actions to help the Allied forces in 1944–45.

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