altarpiece Painting, relief, sculpture, screen, or decorated wall stand¬ ing on or behind an altar in a Christian church. The images depict holy personages, saints, and biblical subjects. There are two types of altar- pieces: the reredos, which rises from the floor behind the altar, and the retable, which stands on the altar itself or on a pedestal behind it. The diptych is an altarpiece consisting of two panels; a triptych, three panels; and a polyptych, four or more panels. Altarpieces vary in size; some are small and portable, some are huge and stationary, and some have mov¬ able wings that can be opened and closed. The practice of erecting sculp¬ tural altarpieces dates from the 11th century; altar paintings became common in the 14th century.
Altdorfer Valt-.dor-farV, Albrecht (b. c. 1480—d. Feb. 12, 1538, Regensburg) German painter and printmaker. He was the leading artist of the Danube school. Most of his works depict religious subjects, but he was
one of the first artists to develop landscape painting as an independent genre, specializing in sunset lighting and ruins in twilight. His drawings demonstrate these skills in black with white highlights on dark paper. The influence of Albrecht DOrer is evident in his miniature engravings and woodcuts. From 1526 until his death he was town architect of Regens¬ burg; no architectural work of his is known to have survived.
Alte Pinakothek Val-to-.pe-na-ko-'tekX German "Old Museum of
Painting" Art museum, one of several collections within the Bavarian State Picture Galleries in Munich, Germany, and one of the great muse¬ ums of the world. It specializes in European painting from the Middle Ages through the late 18th century; its core collections once belonged to several early Bavarian electors palatine. The building is a 1957 recon¬ struction of the original 19th-century gallery, destroyed in World War II. Other state museums include the Neue (“New”) Pinakothek, which is based on the private collections of Bavarian kings and features 18th— 20th-century European paintings and sculpture; the Schack Gallery col¬ lection of late Romantic German painting; and the State Gallery of Modem Art.
alternating current (AC) Flow of electric charge that reverses peri¬ odically, unlike direct current. It starts from zero, grows to a maximum, decreases to zero, reverses, reaches a maximum in the opposite direction, returns again to zero, and repeats the cycle indefinitely. The time taken to complete one cycle is called the period (see periodic motion), and the number of cycles per second is the frequency; the maximum value in either direction is the current’s amplitude. Low frequencies (50-60 cycles per second) are used for domestic and commercial power, but frequencies of around 100 million cycles per second (100 megahertz) are used in tele¬ vision and of several thousand megahertz in radar and microwave com¬ munication. A major advantage of alternating current is that the voltage can be increased and decreased by a transformer for more efficient trans¬ mission over long distances. Direct current cannot use transformers to change voltage. See also electric current.
alternation of generations In biology, alternation of a sexual phase (gametophyte) and a nonsexual phase (sporophyte) in the life cycle of an organism. The two phases, or generations, are often distinct in structure and sometimes in chromosome makeup. Alternation of generations is com¬ mon in algae, fungi (see fungus), Mosses, ferns, and seed plants. The char¬ acter and extent of the two phases vary greatly among different groups of plants and algae. During the course of evolution, the gametophyte stage has been progressively reduced. Thus in higher (vascular) plants, the sporophyte is the dominant phase; in more primitive, nonvascular plants the gametophyte is dominant. Among animals, many invertebrates (e.g., protozoans, jellyfish, flatworms) have an alternation of sexual and asexual generations.
alternative energy Any of various renewable power sources to use in place of fossil fuels and uranium. Fusion devices (see nuclear fusion) are believed by some to be the best long-term option, because their primary energy source would be deuterium, abundant in ordinary water. Other tech¬ nologies include solar energy, wind power, tidal power, wave power, hydro¬ electric power, and geothermal energy. The amount of energy in such renewable and virtually pollution-free sources is large in relation to world energy needs, yet at present only a small portion of it can be converted to electric power at reasonable cost.
alternative medicine or complementary medicine Any of a
broad range of healing approaches not used in conventional Western medi¬ cine. Many are holistic (see holistic medicine); many also emphasize pre¬ vention and education. Alternative therapies include acupuncture, aromatherapy, Ayurveda medicine, Chinese medicine, chiropractic, herbal medicine, homeopathy, massage, meditation, naturopathy, therapeutic touch, and Yoga. Though considered alternative in the West, such medicine is the main source of health care for up to 80% of people in less-developed countries. Some alternative-medicine practices are useless or harmful; others are effective and may offer treatments in areas where conventional approaches have not succeeded (e.g., chronic disorders).
alternator Source of direct electric current in modern vehicles for igni¬ tion, lights, fans, and other uses. The electric power is generated by an alternator mechanically coupled to the engine, with a rotor field coil sup¬ plied with current through slip rings, and a stator with a three-phase winding. A rectifier converts the power from alternating to direct form. A regulator ensures that the output voltage is properly matched to the bat-
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54 I Altgeld ► Alvarado
tery voltage as engine speed varies. An inductor alternator is a special kind of synchronous generator in which both the field and the output winding are on the stator.
bearing assembly
Exploded view of an automotive alternator. The engine's turning crankshaft, con¬ nected to the alternator's pulley by a belt, turns the magnetic rotor inside the sta¬ tionary stator assembly, generating an alternating current. The diode assembly rectifies the alternating current, producing direct current, which is used to meet the demands of the vehicle's electrical system, including recharging the battery.
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Altgeld, John Peter (b. Dec. 30, 1847, Niederselters, Prussia—d. March 12, 1902, Joliet, Ill., U.S.) German-bom U.S. politician, governor of Illinois (1893-97). He emigrated from Germany as a child. In the 1870s he moved to Chicago, where he accumulated a small fortune in real estate and became active in Democratic Party politics. In 1892 he won the gov¬ ernorship as a reformist candidate. In 1893, at the urging of Clarence Dar- row and labour leaders, he granted clemency to three men convicted of complicity in the Haymarket Riot. The controversial pardon provoked an outcry from conservatives and contributed to his defeat for reelection in 1896, though Altgeld’s decision later gained wide approval in judicial circles.
altimeter \al-'tim-3t-3r\ Instrument that measures the altitude of the land surface or of any object, such as an airplane. The mechanical pres¬ sure altimeter measures atmospheric pressure relative to sea level through a series of bellows, gears, and springs, which move pointers on a dial. Radio altimeters measure the distance of an aircraft above the ground rather than above sea level by indicating the time a pulse of radio energy takes to travel from the aircraft to the ground and back; they are used in automatic navigation and blind-landing systems.
Altiplano \,al-ti-'pla-n6\ Spanish "High Plateau" Region, south¬ eastern Peru and western Bolivia. Comprising a series of intermontane basins, it originates northwest of Lake Titicaca in southern Peru and extends southeast to southwestern Bolivia. The region’s wildlife originally included alpacas and llamas, both now bred for wool. The cities of Puno and Juliaca (Peru) and La Paz (Bolivia) are located in the Lake Titicaca basin, an area that has been populated since ancient times.