riot-control gas: see poison gas; tear gas.

rent control, in economics and law, government regulation of rent to prevent unreasonable or excessive increases. In the United States, the federal government imposed rent control (and other price controls) during World War II, and continued it in several cities after the war because of housing shortages. It was later turned over to the control of individual states and municipalities and has since ended in most locations.

numerical control: see computer-aided manufacturing.

gun control, government limitation of the purchase and ownership of firearms. The availability of guns is controlled by nations and localities throughout the world. In the United States the "right of the people to keep and bear arms" is guaranteed by the Constitution, but has been variously interpreted through the years. Since the late 1930s federal judicial and law enforcement officials generally held that the right exists mainly in the context of the maintenance of a state militia, but in 2002 the Justice Dept., under Attorney General John Ashcroft, indicated that it interpreted the amendment as more broadly supporting the rights of individuals to possess and bear firearms. Such an interpretation was upheld by a 2008 Supreme Court decision that nonetheless did not challenge the government's right to place some limitations on the ownership and possession of firearms.

Some U.S. states and localities have enacted strict licensing and other control measures, and federal legislation (1968) prohibited the sale of rifles by mail. Gun control has continued to be widely debated, however, and has often been opposed, notably by the National Rifle Association (NRA). Increasing gun-related crimes together with citizen pressure propelled congressional passage (1993) of the "Brady bill" (named for James Brady, the press secretary seriously wounded in the 1981 assassination attempt on President Ronald Reagan) after years of controversy. It required a minimum of a five-day waiting period and background check before a handgun purchase. Parts of the bill were challenged in court and in 1997 the Supreme Court invalidated its background-check provision. The 1994 Crime Bill outlawed the manufacture, sale, and possession of military-style assault weapons, but it expired in 2004. In 1999, following a rash of shootings at U.S. schools, further gun-control legislation was passed by the Senate but was voted down by the House of Representatives. Attempts by localities (through legislation) and individuals (through lawsuits) to pursue gun control through the courts by permitting or bringing negligence suits against a gun manufacturer or dealer when a weapon it made or sold is used in a crime led many states and, in 2005, Congress to pass laws limiting such suits.

control systems, combinations of components (electrical, mechanical, thermal, or hydraulic) that act together to maintain actual system performance close to a desired set of performance specifications. Open-loop control systems (e.g., automatic toasters and alarm clocks) are those in which the output has no effect on the input. Closed-loop control systems (e.g., thermostats, engine governors, automotive cruise-control systems, and automatic tuning control circuits) are those in which the output has an effect on the input in such a way as to maintain the desired output value. A closed-loop system includes some way to measure its output to sense changes so that corrective action can be taken. The speed with which a simple closed-loop control system moves to correct its output is described by its damping ratio and natural frequency. A system with a small damping ratio is characterized by overshooting the desired output before settling down. Systems with larger damping ratios do not overshoot the desired output, but respond more slowly. See feedback.

control surface: see airfoil.

birth control, practice of contraception for the purpose of limiting reproduction.

Methods of Birth Control

Male birth control methods include withdrawal of the male before ejaculation (the oldest contraceptive technique) and use of the condom, a rubber sheath covering the penis. The condom, because of its use as a protection against sexually transmitted diseases, including AIDS, has become a frequently used birth control device.

Contraceptive methods for women include the rhythm method—abstinence around the most likely time of ovulation—and precoital insertion into the vagina of substances (creams, foams, jellies, or suppositories) containing spermicidal chemicals. The use of a diaphragm, a rubber cup-shaped device inserted before intercourse, prevents sperm from reaching the uterine cervix; it is usually used with a spermicide. Contraceptive sponges, which are impregnated with a spermicide, also are inserted into the vagina before intercourse and work primarily by acting as a barrier to the sperm. Intrauterine devices, or IUDs, are variously shaped small objects inserted by a doctor into the uterus; they apparently act by creating a uterine environment hostile either to sperm or to the fertilized egg.

The birth control pill, an oral contraceptive, involves a hormonal method in which estrogen and progestins (progesteronelike substances) are taken cyclically for 21 or 84 days, followed by 7 days of inactive or no pills. The elevated levels of hormones in the blood suppress production of the pituitary hormones (luteinizing hormone and follicle-stimulating hormone) that would ordinarily cause ovulation. An oral contraceptive formulation that utilizes no inactive pills and is taken every day (and completely suppresses menstruation) also exists. Estrogen and progestins may also be delivered through the weekly use of a contraceptive skin patch or the monthly use of a vaginal ring (a flexible plastic ring inserted in the vagina); both slowly release the hormones they contain.

Sterilization of the female, often but not always performed during a Cesarean section or shortly after childbirth, consists of cutting or tying both Fallopian tubes, the vessels that carry the egg cells from the ovaries to the uterus. In male sterilization (vasectomy) the vas deferens, the tubes that carry sperm from the testes to the penis, are interrupted. Sterilization, in most cases irreversible, involves no loss of libido or capacity for sex.

No contraceptive yet devised is at once simple, acceptable, safe, effective, and reversible. Some, such as the diaphragm, condom, and chemical and rhythm methods, require high motivation by users; the pill, which must be taken daily, sometimes induces undesirable side effects, such as nausea, headache, weight gain, and increased tendency to develop blood clots. The IUDs, although requiring no personal effort or motivation, are often not tolerated or are expelled, and they sometimes, particularly if poorly designed, cause uterine infection, septic abortion, and other problems.

If birth control fails (or is not used), doctors may prescribe several large doses of certain oral contraceptives as "morning after" pills; the high level of hormones can inhibit the establishment of pregnancy even if fertilization has taken place. Mifepristone, or RU-486, the so-called abortion pill, is effective within seven weeks after conception and requires close medical supervision. It was first approved in Europe and was tested in the mid-1990s in United States, where it was approved in 2000. Another experimental technique is immunization against human chorionic gonadotropin (HCG), a hormone secreted by a developing fertilized egg that stimulates production of progesterone by the ovary; the effect of the anti-HCG antibody would be to inactivate HCG and thereby induce menstruation even if fertilization occurred.

See also abortion; menstruation; reproductive system.

History of the Birth Control Movement

Although contraceptive techniques had been known in ancient Egypt, Greece, and Rome, the modern movement for birth control began in Great Britain, where the writings of Thomas Robert Malthus stirred interest in the problem of overpopulation. By the 1870s a wide variety of birth control devices were available in English and American pharmacies, including rubber condoms and diaphragms, chemical suppositories, vaginal sponges, and medicated tampons. Easy public access to contraceptive devices in the United States aroused the ire of Anthony Comstock and others, who lobbied Congress until it passed (1873) a bill prohibiting the distribution of these devices across state lines or through the mail. Moreover, in England in 1877, Annie Besant and Charles Bradlaugh were tried for selling The Fruits of Philosophy, a pamphlet on contraceptive methods, written in 1832 by an American, Charles Knowlton. After their famous trial, the Malthusian League was founded. Meanwhile, a variety of contraceptive devices remained available to a large public, usually advertised in veiled but unmistakable language.

In 1878 the first birth control clinic was founded in Amsterdam by Aletta Jacobs. The first U.S. birth control clinic, opened (1916) by Margaret Sanger in Brooklyn, N.Y., was closed by the police; she received a 30-day jail sentence. She later permanently established a clinic in New York City in 1923. In Great Britain the Malthusian League, aided by Marie Stopes, established a birth control clinic in London in 1921.

Sanger also helped organize (1917) the National Birth Control League in the United States; in 1921 it became the American Birth Control League, and in 1942 the Planned Parenthood Federation of America. Meanwhile, in 1918 an American judge ruled that contraceptive devices were legal as instruments for the prevention of disease, and the federal law prohibiting dissemination of contraceptive information through the mails was modified in 1936. Throughout the 1940s and 50s, birth control advocates were engaged in numerous legal suits. In 1965 the U.S. Supreme Court struck down the one remaining state law (in Connecticut) prohibiting the use of contraceptives.

The federal government began to take a more active part in the birth control movement in 1967, when 6% of the funds allotted to the Child Health Act was set aside for family planning; in 1970, the Family Planning Services and Population Act established separate funds for birth control. Birth control and sex education in schools continue to be emotional issues in the United States, where adolescent sexual activity and pregnancy rates are high and bring with them increased risks of sexually transmitted diseases and complications of pregnancy, as well as societal and personal costs.

Birth control on the international level is led by the International Planned Parenthood Federation, founded in 1952, with members in 134 countries by 1995. Sweden was one of the first countries to provide government assistance for birth control, which it did as early as the 1930s. Two of the more successful birth control programs have been in Japan, where the birthrate has been dramatically reduced, and—more controversially—in China, where the government has a "one family, one child" policy and local authorities have typically intimated women pregnant into aborting a second pregnancy. Several of the so-called underpopulated nations, however, have a stated policy of encouraging an increased birthrate, e.g., Argentina, and concern over declining populations has increased in recent years in certain Western European countries and Russia. Among religious bodies, the Roman Catholic Church has provided the main opposition to the birth control movement; popes Paul VI and John Paul II reaffirmed this stance in encyclicals.

automatic tuning control (ATC), method or device to keep a radio or television receiver automatically tuned to a desired frequency or channel. Assuming that the receiver is at least approximately tuned to the desired frequency, a circuit in the receiver develops an error voltage proportional to the degree to which the receiver is mistuned. This error voltage is then fed back to the tuning circuit in such a way that the tuning error is reduced. In most frequency modulation (FM) detectors an error voltage of this type is easily available; in television receivers extra circuits may be used to develop it. In an FM receiver, automatic frequency control (AFC) may make it difficult to receive a weak signal located near in frequency to a strong one. In tuning circuits that use frequency synthesis, in which digital circuitry, in conjunction with a crystal oscillator, produces precise signals, automatic tuning control is not necessary.

automatic frequency control: see automatic tuning control.

air traffic control, the system by which airplanes are safely routed into and out of major airports. Air traffic control in the United States is centered in a number of regional control centers that route airplanes along established airways to airport traffic control centers. There Instrument Landing Systems and Microwave Landing Systems enable planes to land safely in almost any weather conditions. Air traffic controllers, who are responsible for maintaining safe distances between planes, are employees of the U.S. Dept. of Transportation. Air traffic control is made possible by special transponders installed in every commercial and many private aircraft, which automatically transmit information on a plane's altitude and speed to the ground controller. The distortion that often affects voice transmissions can be eliminated by the use of cockpit datalinks; collision avoidance systems provide further safety margins. Knowing the course, speed, and altitude of every plane in the sector, the controller can contact each in turn to give landing or course instructions. Modern air traffic control has contributed to making air travel far safer than highway travel, and on a passenger-mile basis safer even than rail travel.

Centers for Disease Control and Prevention (CDC), agency of the U.S. Public Health Service since 1973, with headquarters in Atlanta; it was established in 1946 as the Communicable Disease Center. The CDC is the federal agency responsible for administering national programs for the prevention and control of communicable and vector-borne diseases and for developing and implementing programs for dealing with environmental health problems. It also directs quarantine activities and conducts epidemiological research, and it provides consultation on an international basis for the control of preventable diseases. The 11 centers, institutes, and offices of the agency include the centers for chronic disease prevention and health promotion, environmental health, health statistics, infectious diseases, injury prevention and control, immunizations, and occupational safety and health.

Setting of government guidelines to limit increases in wages and prices. It is one of the most extreme approaches to incomes policy. By controlling wages and prices, governments hope to control inflation and prevent extremes in the business cycle. Countries with highly centralized methods of setting wages tend to have the greatest degree of public or collective regulation of wage and price levels. For example, wage settlements in The Netherlands must be approved by the government, and price increases are investigated by the Ministry of Economic Affairs. Other countries, including the U.S., have also made efforts at restraining wage and price increases, usually seeking the voluntary cooperation of management and labour. In the U.S., wage-price controls were instituted by Pres. Franklin D. Roosevelt during World War II and by Pres. Richard M. Nixon in the early 1970s, when high inflation combined with rising unemployment to create instability.

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Systematic effort to destroy an individual's former loyalties and beliefs and to substitute loyalty to a new ideology or power. It has been used by religious cults as well as by radical political groups. The techniques of brainwashing usually involve isolation from former associates and sources of information; an exacting regimen calling for absolute obedience and humility; strong social pressures and rewards for cooperation; physical and psychological punishments for noncooperation, including social ostracism and criticism, deprivation of food, sleep, and social contacts, bondage, and torture; and constant reinforcement. Its effects are sometimes reversed through deprogramming, which combines confrontation and intensive psychotherapy.

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Governmental restrictions on private transactions in foreign money or claims on foreign money. Residents are required to sell foreign money coming into their possession to a central bank or specialized government agency at exchange rates set by the government. The chief function of most systems of exchange control is to maintain a favourable balance of payments. Seealso foreign exchange.

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Field of applied mathematics relevant to the control of certain physical processes and systems. It became a field in its own right in the late 1950s and early '60s. After World War II, problems arising in engineering and economics were recognized as variants of problems in differential equations and in the calculus of variations, though they were not covered by existing theories. Special modifications of classical techniques and theories were devised to solve individual problems, until it was recognized that these seemingly diverse problems all had the same mathematical structure, and control theory emerged. Seealso control system.

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Means by which a set of variable quantities is held constant or caused to vary in a prescribed way. Control systems are intimately related to the concept of automation but have an ancient history. Roman engineers maintained water levels in aqueducts by means of floating valves that opened and closed at appropriate levels. James Watt's flyball governor (1769) regulated steam flow to a steam engine to maintain constant engine speed despite a changing load. In World War II, control-system theory was applied to anti-aircraft batteries and fire-control systems. The introduction of analog and digital computers opened the way for much greater complexity in automatic control theory. Seealso Jacquard loom, pneumatic device, servomechanism.

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Voluntary limiting of human reproduction, using such means as contraception, sexual abstinence, surgical sterilization, and induced abortion. The term was coined in 1914–15 by Margaret Sanger. Medically, birth control is often advised when childbirth might endanger the mother’s health or substantial risk exists of bearing a severely disabled child. Socially and economically, limitation of reproduction frequently reflects a desire to maintain or improve family living standards. Most religious leaders now generally agree that some form of fertility regulation is desirable, though the means are strongly debated. Seealso family planning.

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Limitation of the development, testing, production, deployment, proliferation, or use of weapons through international agreements. Arms control did not arise in international diplomacy until the first Hague Convention (1899). The Washington Conference (1921–22) and the Kellogg-Briand Pact (1928) were broken without much fear of sanction. U.S.-Soviet treaties to control nuclear weapons during the Cold War were taken more seriously. In 1968 the two superpowers and Britain sponsored the Nuclear Non-proliferation Treaty (signed also by 59 other countries), which committed signatory countries not to promote the spread, or proliferation, of nuclear weapons to countries that did not already possess them. Seealso Nuclear Test-Ban Treaty; Strategic Arms Limitation Talks; Strategic Arms Reduction Talks.

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Inhibition or activation of an enzyme by a small regulatory molecule that interacts with the enzyme at a site (allosteric site) other than the active site (at which catalytic activity occurs). The interaction changes the shape of the enzyme, thus affecting the active site of the usual complex between the enzyme and its substrate (the substance on which the enzyme acts). As a result, the enzyme's ability to catalyze a reaction (see catalysis) is either inhibited or enhanced. If the regulatory molecule inhibits an enzyme in the pathway of its own synthesis, the control is said to be feedback inhibition. Allosteric control enables the cell to regulate needed substances rapidly.

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