amplifier Device that responds to a small input signal (voltage, current, or power) and delivers a larger output signal with the same waveform features. Amplifiers are used in radio and television receivers, high- fidelity audio equipment, and computers. Amplification can be provided by electromechanical devices (e.g., transformers and generators) and vacuum tubes, but most electronic systems now employ solid-state micro- circuits. One amplifier is usually insufficient, so its output is fed into a second, whose output is fed to a third, and so on, until the output level is satisfactory.
amplitude modulation See AM
amputation Removal of any part of the body, usually surgical removal of part or all of a limb. Congenital amputation means lack of a limb at birth (see agenesis). Surgical amputation may be a lifesaving measure to prevent excessive blood loss from injury or to check the spread of infection, gan¬ grene, or malignant soft-tissue or bone tumours. Reconstructive surgery, prompt treatment with blood and plasma, and rehabilitation have made amputation rarer than in the past. Prostheses reduce handicaps for ampu¬ tees, whose surgery may have been designed with a prosthesis in mind.
‘Amr ibn al-'As Vam-or-.ib-on-ol-'asV (d. 663, Al-Fustat, Egypt) Arab conqueror of Egypt. After accepting Islam (c. 630), he led a force to Oman, where he converted the region’s rulers. He was a leader in the Muslim force that conquered southwestern Palestine in the 630s but gained fame when, on his own initiative, he set out to conquer Egypt, succeeding (642) after a two-year campaign. A good administrator and politician, late in his career he aided the governor of Syria, Mu'awiyah I, against c Al 1, Islam’s fourth caliph. He was rewarded with the governor¬ ship of Egypt at the beginning of the Umayyad dynasty (661).
Amritsar X.om-'rit-sorV Massacre of (1919) Incident in which Brit¬ ish troops fired on a crowd of Indian protesters. In 1919 the British gov¬ ernment of India enacted the Rowlatt Acts, extending its World War I emergency powers to combat subversive activities. On April 13 a large crowd gathered at Amritsar in the Punjab to protest the measures; troops opened fire, killing about 379 and wounding some 1,200. The massacre permanently scarred relations between India and Britain and was the pre¬ lude to Mohandas K. Gandhi’s noncooperation movement of 1920-22.
Amsterdam City (pop., 2005 est.: city, 739,295; metro, area, 1,017,332), western Netherlands. It lies at the head of the IJsselmeer. It is the nominal capital of The Netherlands, whose seat of government is at The Hague. Originally a fishing village, it received its charter as a town in 1306. It joined the Hanseatic League in 1369 and grew steadily in the 14th and 15th centuries. After the decline of Antwerp at the end of the 16th
century, Amsterdam became the source of growing Dutch commercial and naval power. It was the centre for the Dutch East India and West India companies and became the leading trade metropolis of Europe. It became part of the Kingdom of Holland, which entered the Kingdom of the Neth¬ erlands in 1815. After suffering a partial decline in the 18th century, its prosperity increased when it became connected by canal to the North Sea in the late 19th century. It was occupied by Germany in World War II. After the war Amsterdam became known as a place of tolerance and lib¬ eralism. The city is now a major European port and a hub for interna¬ tional finance and trade.
Amtrak formally National Railroad Passenger Corp. Feder¬ ally supported corporation that operates nearly all intercity passenger trains in the U.S. It was established by Congress in 1970 in the face of private railroads’ heavy financial losses. Routes were cut back severely, with service maintained only in highly populated areas and between the largest cities. Amtrak pays the railroads to run passenger trains and com¬ pensates them for the use of tracks and terminals. Amtrak bears all admin¬ istrative costs and manages scheduling, route planning, and ticket sales. Despite income from tickets and mail-carrying services, Amtrak requires federal support to cover its operating costs. See also Conrail; Railway Express Agency.
Amu Darya Va-mii-'dar-yoN ancient Oxus River River, Central Asia. It is one of the longest rivers in Central Asia, 1,578 mi (2,540 km) long measured from the remotest sources of the Panj River; its other head- stream is the Vakhsh. It flows west-northwest to its mouth on the Aral Sea. It forms part of Afghanistan’s borders with Tajikistan, Uzbekistan, and Turkmenistan and part of Uzbekistan’s border with Turkmenistan.
Amundsen Va-mun-sonV, Roald (Engelbregt Gravning) (b. July 16, 1872, Borge, near Oslo, Nor.— disappeared June 18, 1928?, Arctic Ocean) Norwegian explorer, leader of the first group to reach the South Pole. In 1897 he took part in a Bel¬ gian expedition that was the first to winter in the Antarctic. In 1903-05 he was the first to navigate the Northwest Passage. He planned an expedition to the North Pole, but, after learning that Robert E. Peary had reached that goal, he set off for the South Pole in 1910. He prepared his trip carefully and in October 1911 set out with four men, 52 dogs, and four sledges. He reached the South Pole in December 1911, one month before Robert Falcon Scon’s ill-fated attempt. He returned to Norway and established a successful shipping business. In 1926 he and Umberto Nobile (1885-1978) passed over the North Pole in a dirigible. Amundsen disappeared in 1928 while flying to rescue Nobile from a dirigible crash.
Amundsen Va-mon-sonV Gulf Southeastern extension of the Beaufort Sea of the Arctic Ocean. Extending about 250 mi (400 km), the gulf is bordered by Victoria Island on the east and separates Banks Island (north) from the Canadian mainland (south). In 1850 the gulf was entered from the west by the British explorer Robert McClure. It is named for Roald Amundsen.
Amur \a-'mur\ River Chinese Heilong Vha-Tung\ Jiang or Hei-
lung Chiang River, northeastern Asia. The Amur proper begins at the confluence of the Shilka and Argun rivers and is 1,755 mi (2,824 km) long. It flows east-southeast along the Russian-Chinese border to Kha¬ barovsk, Siberia, then northeast across Russian territory to empty into the Tatar Strait. Among its tributaries are the Zeya, Bureya, and Ussuri rivers. From the 18th century, Russians have settled to the north of the river and Chinese to the south, a situation which at times has provoked border clashes.
amygdule \9- , mig- l dyiil\ Secondary deposit of minerals found in a rounded, elongated, or almond-shaped cavity in volcanic rock. The cavi¬ ties (vesicles) were created by the expansion of gas bubbles or steam within lava. Because gas bubbles tend to rise through the lava, amyg-
Roald Amundsen, 1923.
UPI/BETTMANN
© 2006 Encyclopaedia Britannica, Inc.
amyotrophic lateral sclerosis ► analog computer I 65
dules are most common near the tops of flows. Many minerals have been found as amygdules, including some spectacular museum specimens of ZEOLITES.
amyotrophic lateral sclerosis (ALS) \ 1 a- I mI-3-'tr6-fik...skb- , ro- sos\ or Lou Gehrig disease Degenerative nervous-system disorder causing muscle wasting and paralysis. The disease usually occurs after age 40, more often in men. Most victims die within two to five years from respiratory muscle atrophy. ALS affects motor neurons; the muscles they control become weak and atrophied, with debility usually beginning in the hands and creeping slowly up to the shoulders. The lower limbs become weak and spastic. Variants include progressive muscular atrophy and progressive bulbar palsy. In 1993 the defective gene that accounts for 5-10% of cases was discovered; it produces an ineffective version of an enzyme that neutralizes free radicals, which destroy motor neurons.
An Hyang See An Yu
An Lushan Van-'lii-'shanV Rebellion Rebellion beginning in 755 in China led by An Lushan (703-757), a general of non-Chinese origin. An Lushan rose through the ranks of the Tang-dynasty army in the 740s, becoming a military governor and a favourite of the emperor, Xuanzong. In 755 he turned his troops on the eastern capital city, Luoyang, and after taking it he proclaimed himself emperor. Six months later his forces took Chang 7 an, the western capital. He was murdered in 757, and the rebel¬ lion was put down in 763. The Tang government was much weakened, however, and the second half of the Tang dynasty and the subsequent Five Dynasties period were troubled by chronic warlordism.