04 February 2009


The invention: A submersible vessel capable of exploring the deepest trenches of the world’s oceans. The people behind the invention: William Beebe (1877-1962), an American biologist and explorer Auguste Piccard (1884-1962), a Swiss-born Belgian physicist Jacques Piccard (1922- ), a Swiss ocean engineer Early Exploration of the Deep Sea The first human penetration of the deep ocean was made byWilliam Beebe in 1934, when he descended 923 meters into the Atlantic Ocean near Bermuda. His diving chamber was a 1.5-meter steel ball that he named Bathysphere, from the Greek word bathys (deep) and the word sphere, for its shape. He found that a sphere resists pressure in all directions equally and is not easily crushed if it is constructed of thick steel. The bathysphere weighed 2.5 metric tons. It had no buoyancy and was lowered from a surface ship on a single 2.2-centimeter cable; a broken cable would have meant certain death for the bathysphere’s passengers. Numerous deep dives by Beebe and his engineer colleague, Otis Barton, were the first uses of submersibles for science. Through two small viewing ports, they were able to observe and photograph many deep-sea creatures in their natural habitats for the first time. They also made valuable observations on the behavior of light as the submersible descended, noting that the green surface water became pale blue at 100 meters, dark blue at 200 meters, and nearly black at 300 meters. A technique called “contour diving” was particularly dangerous. In this practice, the bathysphere was slowly towed close to the seafloor. On one such dive, the bathysphere narrowly missed crashing into a coral crag, but the explorers learned a great deal about the submarine geology of Bermuda and the biology of a coral-reef community. Beebe wrote several popular and scientific books about his adventures that did much to arouse interest in the ocean. Testing the Bathyscaphe The next important phase in the exploration of the deep ocean was led by the Swiss physicist Auguste Piccard. In 1948, he launched a new type of deep-sea research craft that did not require a cable and that could return to the surface by means of its own buoyancy. He called the craft a bathyscaphe, which is Greek for “deep boat.” Piccard began work on the bathyscaphe in 1937, supported by a grant from the Belgian National Scientific Research Fund. The German occupation of Belgium early in World War II cut the project short, but Piccard continued his work after the war. The finished bathyscaphe was named FNRS 2, for the initials of the Belgian fund that had sponsored the project. The vessel was ready for testing in the fall of 1948. The first bathyscaphe, as well as later versions, consisted of two basic components: first, a heavy steel cabin to accommodate observers, which looked somewhat like an enlarged version of Beebe’s bathysphere; and second, a light container called a float, filled with gasoline, that provided lifting power because it was lighter than water. Enough iron shot was stored in silos to cause the vessel to descend. When this ballast was released, the gasoline in the float gave the bathyscaphe sufficient buoyancy to return to the surface. Piccard’s bathyscaphe had a number of ingenious devices. Jacques- Yves Cousteau, inventor of the Aqualung six years earlier, contributed a mechanical claw that was used to take samples of rocks, sediment, and bottom creatures. A seven-barreled harpoon gun, operated by water pressure, was attached to the sphere to capture specimens of giant squids or other large marine animals for study. The harpoons had electrical-shock heads to stun the “sea monsters,” and if that did not work, the harpoon could give a lethal injection of strychnine poison. Inside the sphere were various instruments for measuring the deep-sea environment, including a Geiger counter for monitoring cosmic rays. The air-purification system could support two people for up to twenty-four hours. The bathyscaphe had a radar mast to broadcast its location as soon as it surfaced. This was essential because there was no way for the crew to open the sphere from the inside.The FNRS 2 was first tested off the Cape Verde Islands with the assistance of the French navy. Although Piccard descended to only 25 meters, the dive demonstrated the potential of the bathyscaphe. On the second dive, the vessel was severely damaged by waves, and further tests were suspended. Aredesigned and rebuilt bathyscaphe, renamed FNRS 3 and operated by the French navy, descended to a depth of 4,049 meters off Dakar, Senegal, on the west coast of Africa in early 1954. In August, 1953, Auguste Piccard, with his son Jacques, launched a greatly improved bathyscaphe, the Trieste, which they named for the Italian city in which it was built. In September of the same year, the Trieste successfully dived to 3,150 meters in the Mediterranean Sea. The Piccards glimpsed, for the first time, animals living on the seafloor at that depth. In 1958, the U.S. Navy purchased the Trieste and transported it to California, where it was equipped with a new cabin designed to enable the vessel to reach the seabed of the great oceanic trenches. Several successful descents were made in the Pacific by Jacques Piccard, and on January 23, 1960, Piccard, accompanied by Lieutenant DonaldWalsh of the U.S. Navy, dived a record 10,916 meters to the bottom of the Mariana Trench near the island of Guam. Impact The oceans have always raised formidable barriers to humanity’s curiosity and understanding. In 1960, two events demonstrated the ability of humans to travel underwater for prolonged periods and to observe the extreme depths of the ocean. The nuclear submarine Triton circumnavigated the world while submerged, and Jacques Piccard and Lieutenant Donald Walsh descended nearly 11 kilometers to the bottom of the ocean’s greatest depression aboard the Trieste. After sinking for four hours and forty-eight minutes, the Trieste landed in the Challenger Deep of the Mariana Trench, the deepest known spot on the ocean floor. The explorers remained on the bottom for only twenty minutes, but they answered one of the biggest questions about the sea: Can animals live in the immense cold and pressure of the deep trenches? Observations of red shrimp and flatfishes proved that the answer was yes. The Trieste played another important role in undersea exploration when, in 1963, it located and photographed the wreckage of the nuclear submarine Thresher. The Thresher had mysteriously disappeared on a test dive off the New England coast, and the Navy had been unable to find a trace of the lost submarine using surface vessels equipped with sonar and remote-control cameras on cables. Only the Trieste could actually search the bottom. On its third dive, the bathyscaphe found a piece of the wreckage, and it eventually photographed a 3,000-meter trail of debris that led to Thresher‘s hull, at a depth of 2.5 kilometers.These exploits showed clearly that scientific submersibles could be used anywhere in the ocean. Piccard’s work thus opened the last geographic frontier on Earth.

BASIC programming language

The invention: An interactive computer system and simple programming language that made it easier for nontechnical people to use computers. The people behind the invention: John G. Kemeny (1926-1992), the chairman of Dartmouth’s mathematics department Thomas E. Kurtz (1928- ), the director of the Kiewit Computation Center at Dartmouth Bill Gates (1955- ), a cofounder and later chairman of the board and chief operating officer of the Microsoft Corporation The Evolution of Programming The first digital computers were developed duringWorldWar II (1939-1945) to speed the complex calculations required for ballistics, cryptography, and other military applications. Computer technology developed rapidly, and the 1950’s and 1960’s saw computer systems installed throughout the world. These systems were very large and expensive, requiring many highly trained people for their operation. The calculations performed by the first computers were determined solely by their electrical circuits. In the 1940’s, The American mathematician John von Neumann and others pioneered the idea of computers storing their instructions in a program, so that changes in calculations could be made without rewiring their circuits. The programs were written in machine language, long lists of zeros and ones corresponding to on and off conditions of circuits. During the 1950’s, “assemblers” were introduced that used short names for common sequences of instructions and were, in turn, transformed into the zeros and ones intelligible to the computer. The late 1950’s saw the introduction of high-level languages, notably Formula Translation (FORTRAN), CommonBusinessOriented Language (COBOL), and Algorithmic Language (ALGOL), which used English words to communicate instructions to the computer. Unfortunately, these high-level languages were complicated; they required some knowledge of the computer equipment and were designed to be used by scientists, engineers, and other technical experts. Developing BASIC John G. Kemeny was chairman of the department of mathematics at Dartmouth College in Hanover, New Hampshire. In 1962, Thomas E. Kurtz, Dartmouth’s computing director, approached Kemeny with the idea of implementing a computer system at Dartmouth College. Both men were dedicated to the idea that liberal arts students should be able to make use of computers. Although the English commands of FORTRAN and ALGOL were a tremendous improvement over the cryptic instructions of assembly language, they were both too complicated for beginners. Kemeny convinced Kurtz that they needed a completely new language, simple enough for beginners to learn quickly, yet flexible enough for many different kinds of applications. The language they developed was known as the “Beginner’s Allpurpose Symbolic Instruction Code,” or BASIC. The original language consisted of fourteen different statements. Each line of a BASIC program was preceded by a number. Line numbers were referenced by control flow statements, such as, “IF X = 9 THEN GOTO 200.” Line numbers were also used as an editing reference. If line 30 of a program contained an error, the programmer could make the necessary correction merely by retyping line 30. Programming in BASIC was first taught at Dartmouth in the fall of 1964. Students were ready to begin writing programs after two hours of classroom lectures. By June of 1968, more than 80 percent of the undergraduates at Dartmouth could write a BASIC program. Most of them were not science majors and used their programs in conjunction with other nontechnical courses. Kemeny and Kurtz, and later others under their supervision, wrote more powerful versions of BASIC that included support for graphics on video terminals and structured programming. The creators of BASIC, however, always tried to maintain their original design goal of keeping BASIC simple enough for beginners. Consequences Kemeny and Kurtz encouraged the widespread adoption of BASIC by allowing other institutions to use their computer system and by placing BASIC in the public domain. Over time, they shaped BASIC into a powerful language with numerous features added in response to the needs of its users. What Kemeny and Kurtz had not foreseen was the advent of the microprocessor chip in the early 1970’s, which revolutionized computer technology. By 1975, microcomputer kits were being sold to hobbyists for well under a thousand dollars. The earliest of these was the Altair. That same year, prelaw studentWilliam H. Gates (1955- ) was persuaded by a friend, Paul Allen, to drop out of Harvard University and help create a version of BASIC that would run on the Altair. Gates and Allen formed a company, Microsoft Corporation, to sell their BASIC interpreter, which was designed to fit into the tiny memory of the Altair. It was about as simple as the original Dartmouth BASIC but had to depend heavily on the computer hardware. Most computers purchased for home use still include a version of Microsoft Corporation’s BASIC. See also BINAC computer; COBOL computer language; FORTRAN programming language; SAINT; Supercomputer.

Autochrome plate

The invention: The first commercially successful process in which a single exposure in a regular camera produced a color image. The people behind the invention: Louis Lumière (1864-1948), a French inventor and scientist Auguste Lumière (1862-1954), an inventor, physician, physicist, chemist, and botanist Alphonse Seyewetz, a skilled scientist and assistant of the Lumière brothers Adding Color In 1882, Antoine Lumière, painter, pioneer photographer, and father of Auguste and Louis, founded a factory to manufacture photographic gelatin dry-plates. After the Lumière brothers took over the factory’s management, they expanded production to include roll film and printing papers in 1887 and also carried out joint research that led to fundamental discoveries and improvements in photographic development and other aspects of photographic chemistry. While recording and reproducing the actual colors of a subject was not possible at the time of photography’s inception (about 1822), the first practical photographic process, the daguerreotype, was able to render both striking detail and good tonal quality. Thus, the desire to produce full-color images, or some approximation to realistic color, occupied the minds of many photographers and inventors, including Louis and Auguste Lumière, throughout the nineteenth century. As researchers set out to reproduce the colors of nature, the first process that met with any practical success was based on the additive color theory expounded by the Scottish physicist James Clerk Maxwell in 1861. He believed that any color can be created by adding together red, green, and blue light in certain proportions. Maxwell, in his experiments, had taken three negatives through screens or filters of these additive primary colors. He then took slides made from these negatives and projected the slides through the same filters onto a screen so that their images were superimposed. As a result, he found that it was possible to reproduce the exact colors as well as the form of an object. Unfortunately, since colors could not be printed in their tonal relationships on paper before the end of the nineteenth century,Maxwell’s experiment was unsuccessful. Although Frederick E. Ives of Philadelphia, in 1892, optically united three transparencies so that they could be viewed in proper alignment by looking through a peephole, viewing the transparencies was still not as simple as looking at a black-and-white photograph. The Autochrome Plate The first practical method of making a single photograph that could be viewed without any apparatus was devised by John Joly of Dublin in 1893. Instead of taking three separate pictures through three colored filters, he took one negative through one filter minutely checkered with microscopic areas colored red, green, and blue. The filter and the plate were exactly the same size and were placed in contact with each other in the camera. After the plate was developed, a transparency was made, and the filter was permanently attached to it. The black-and-white areas of the picture allowed more or less light to shine through the filters; if viewed froma proper distance, the colored lights blended to form the various colors of nature. In sum, the potential principles of additive color and other methods and their potential applications in photography had been discovered and even experimentally demonstrated by 1880. Yet a practical process of color photography utilizing these principles could not be produced until a truly panchromatic emulsion was available, since making a color print required being able to record the primary colors of the light cast by the subject. Louis and Auguste Lumière, along with their research associate Alphonse Seyewetz, succeeded in creating a single-plate process based on this method in 1903. It was introduced commercially as the autochrome plate in 1907 and was soon in use throughout the world. This process is one of many that take advantage of the limited resolving power of the eye. Grains or dots too small to be recognized as separate units are accepted in their entirety and, to the sense of vision, appear as tones and continuous color.Impact While the autochrome plate remained one of the most popular color processes until the 1930’s, soon this process was superseded by subtractive color processes. Leopold Mannes and Leopold Godowsky, both musicians and amateur photographic researchers who eventually joined forces with Eastman Kodak research scientists, did the most to perfect the Lumière brothers’ advances in making color photography practical. Their collaboration led to the introduction in 1935 of Kodachrome, a subtractive process in which a single sheet of film is coated with three layers of emulsion, each sensitive to one primary color. A single exposure produces a color image. Color photography is now commonplace. The amateur market is enormous, and the snapshot is almost always taken in color. Commercial and publishing markets use color extensively. Even photography as an art form, which was done in black and white through most of its history, has turned increasingly to color.

Atomic-powered ship

The invention: The world’s first atomic-powered merchant ship demonstrated a peaceful use of atomic power. The people behind the invention: Otto Hahn (1879-1968), a German chemist Enrico Fermi (1901-1954), an Italian American physicist Dwight D. Eisenhower (1890-1969), president of the United States, 1953-1961 Splitting the Atom In 1938, Otto Hahn, working at the Kaiser Wilhelm Institute for Chemistry, discovered that bombarding uranium atoms with neutrons causes them to split into two smaller, lighter atoms. A large amount of energy is released during this process, which is called “fission.” When one kilogram of uranium is fissioned, it releases the same amount of energy as does the burning of 3,000 metric tons of coal. The fission process also releases new neutrons. Enrico Fermi suggested that these new neutrons could be used to split more uranium atoms and produce a chain reaction. Fermi and his assistants produced the first human-made chain reaction at the University of Chicago on December 2, 1942. Although the first use of this new energy source was the atomic bombs that were used to defeat Japan in World War II, it was later realized that a carefully controlled chain reaction could produce useful energy. The submarine Nautilus, launched in 1954, used the energy released from fission to make steam to drive its turbines. U.S. President Dwight David Eisenhower proposed his “Atoms for Peace” program in December, 1953. On April 25, 1955, President Eisenhower announced that the “Atoms for Peace” program would be expanded to include the design and construction of an atomicpowered merchant ship, and he signed the legislation authorizing the construction of the ship in 1956.Savannah’s Design and Construction A contract to design an atomic-powered merchant ship was awarded to George G. Sharp, Inc., on April 4, 1957. The ship was to carry approximately one hundred passengers (later reduced to sixty to reduce the ship’s cost) and 10,886 metric tons of cargo while making a speed of 21 knots, about 39 kilometers per hour. The ship was to be 181 meters long and 23.7 meters wide. The reactor was to provide steam for a 20,000-horsepower turbine that would drive the ship’s propeller. Most of the ship’s machinery was similar to that of existing ships; the major difference was that steam came from a reactor instead of a coal- or oil-burning boiler. New York Shipbuilding Corporation of Camden, New Jersey, won the contract to build the ship on November 16, 1957. States Marine Lines was selected in July, 1958, to operate the ship. It was christened Savannah and launched on July 21, 1959. The name Savannah was chosen to honor the first ship to use steam power while crossing an ocean. This earlier Savannah was launched in New York City in 1818. Ships are normally launched long before their construction is complete, and the new Savannah was no exception. It was finally turned over to States Marine Lines on May 1, 1962. After extensive testing by its operators and delays caused by labor union disputes, it began its maiden voyage from Yorktown, Virginia, to Savannah, Georgia, on August 20, 1962. The original budget for design and construction was $35 million, but by this time, the actual cost was about $80 million. Savannah‘s nuclear reactor was fueled with about 7,000 kilograms (15,400 pounds) of uranium. Uranium consists of two forms, or “isotopes.” These are uranium 235, which can fission, and uranium 238, which cannot. Naturally occurring uranium is less than 1 percent uranium 235, but the uranium in Savannah‘s reactor had been enriched to contain nearly 5 percent of this isotope. Thus, there was less than 362 kilograms of usable uranium in the reactor. The ship was able to travel about 800,000 kilometers on this initial fuel load. Three and a half million kilograms of water per hour flowed through the reactor under a pressure of 5,413 kilograms per square centimeter. It entered the reactor at 298.8 degrees Celsius and left at 317.7 degrees Celsius. Water leaving the reactor passed through a heat exchanger called a “steam generator.” In the steam generator, reactor water flowed through many small tubes. Heat passed through the walls of these tubes and boiled water outside them. About 113,000 kilograms of steam per hour were produced in this way at a pressure of 1,434 kilograms per square centimeter and a temperature of 240.5 degrees Celsius. Labor union disputes dogged Savannah‘s early operations, and it did not start its first trans-Atlantic crossing until June 8, 1964. Savannah was never a money maker. Even in the 1960’s, the trend was toward much bigger ships. It was announced that the ship would be retired in August, 1967, but that did not happen. It was finally put out of service in 1971. Later, Savannah was placed on permanent display at Charleston, South Carolina. Consequences Following the United States’ lead, Germany and Japan built atomic-powered merchant ships. The Soviet Union is believed to have built several atomic-powered icebreakers. Germany’s Otto Hahn, named for the scientist who first split the atom, began service in 1968, and Japan’s Mutsuai was under construction as Savannah retired. Numerous studies conducted in the early 1970’s claimed to prove that large atomic-powered merchant ships were more profitable than oil-fired ships of the same size. Several conferences devoted to this subject were held, but no new ships were built. Although the U.S. Navy has continued to use reactors to power submarines, aircraft carriers, and cruisers, atomic power has not been widely used for merchant-ship propulsion. Labor union problems such as those that haunted Savannah, high insurance costs, and high construction costs are probably the reasons. Public opinion, after the reactor accidents at Three Mile Island (in 1979) and Chernobyl (in 1986) is also a factor.

Atomic clock

The invention: A clock using the ammonia molecule as its oscillator that surpasses mechanical clocks in long-term stability, precision, and accuracy. The person behind the invention: Harold Lyons (1913-1984), an American physicist Time Measurement The accurate measurement of basic quantities, such as length, electrical charge, and temperature, is the foundation of science. The results of such measurements dictate whether a scientific theory is valid or must be modified or even rejected. Many experimental quantities change over time, but time cannot be measured directly. It must be measured by the occurrence of an oscillation or rotation, such as the twenty-four-hour rotation of the earth. For centuries, the rising of the Sun was sufficient as a timekeeper, but the need for more precision and accuracy increased as human knowledge grew. Progress in science can be measured by how accurately time has been measured at any given point. In 1713, the British government, after the disastrous sinking of a British fleet in 1707 because of a miscalculation of longitude, offered a reward of 20,000 pounds for the invention of a ship’s chronometer (a very accurate clock). Latitude is determined by the altitude of the Sun above the southern horizon at noon local time, but the determination of longitude requires an accurate clock set at Greenwich, England, time. The difference between the ship’s clock and the local sun time gives the ship’s longitude. This permits the accurate charting of new lands, such as those that were being explored in the eighteenth century. John Harrison, an English instrument maker, eventually built a chronometer that was accurate within one minute after five months at sea. He received his reward from Parliament in 1765. Atomic Clocks Provide Greater Stability A clock contains four parts: energy to keep the clock operating, an oscillator, an oscillation counter, and a display. A grandfather clock has weights that fall slowly, providing energy that powers the clock’s gears. The pendulum, a weight on the end of a rod, swings back and forth (oscillates) with a regular beat. The length of the rod determines the pendulum’s period of oscillation. The pendulum is attached to gears that count the oscillations and drive the display hands. There are limits to a mechanical clock’s accuracy and stability. The length of the rod changes as the temperature changes, so the period of oscillation changes. Friction in the gears changes as they wear out. Making the clock smaller increases its accuracy, precision, and stability. Accuracy is how close the clock is to telling the actual time. Stability indicates how the accuracy changes over time, while precision is the number of accurate decimal places in the display. A grandfather clock, for example, might be accurate to ten seconds per day and precise to a second, while having a stability of minutes per week. Applying an electrical signal to a quartz crystal will make the crystal oscillate at its natural vibration frequency, which depends on its size, its shape, and the way in which it was cut from the larger crystal. Since the faster a clock’s oscillator vibrates, the more precise the clock, a crystal-based clock is more precise than a large pendulum clock. By keeping the crystal under constant temperature, the clock is kept accurate, but it eventually loses its stability and slowly wears out. In 1948, Harold Lyons and his colleagues at the National Bureau of Standards (NBS) constructed the first atomic clock, which used the ammonia molecule as its oscillator. Such a clock is called an atomic clock because, when it operates, a nitrogen atom vibrates. The pyramid-shaped ammonia molecule is composed of a triangular base; there is a hydrogen atom at each corner and a nitrogen atom at the top of the pyramid. The nitrogen atom does not remain at the top; if it absorbs radio waves of the right energy and frequency, it passes through the base to produce an upside-down pyramid and then moves back to the top. This oscillation frequency occurs at 23,870 megacycles (1 megacycle equals 1 million cycles) per second. Lyons’s clock was actually a quartz-ammonia clock, since the signal from a quartz crystal produced radio waves of the crystal’s fre- quency that were fed into an ammonia-filled tube. If the radio waves were at 23,870 megacycles, the ammonia molecules absorbed the waves; a detector sensed this, and it sent no correction signal to the crystal. If radio waves deviated from 23,870 megacycles, the ammonia did not absorb them, the detector sensed the unabsorbed radio waves, and a correction signal was sent to the crystal. The atomic clock’s accuracy and precision were comparable to those of a quartz-based clock—one part in a hundred million—but the atomic clock was more stable because molecules do not wear out. The atomic clock’s accuracy was improved by using cesium 133 atoms as the source of oscillation. These atoms oscillate at 9,192,631,770 plus or minus 20 cycles per second. They are accurate to a billionth of a second per day and precise to nine decimal places. A cesium clock is stable for years. Future developments in atomic clocks may see accuracies of one part in a million billions. Impact The development of stable, very accurate atomic clocks has farreaching implications for many areas of science. Global positioning satellites send signals to receivers on ships and airplanes. By timing the signals, the receiver’s position is calculated to within several meters of its true location. Chemists are interested in finding the speed of chemical reactions, and atomic clocks are used for this purpose. The atomic clock led to the development of the maser (an acronym formicrowave amplification by stimulated emission of radiation), which is used to amplify weak radio signals, and the maser led to the development of the laser, a light-frequency maser that has more uses than can be listed here. Atomic clocks have been used to test Einstein’s theories of relativity that state that time on a moving clock, as observed by a stationary observer, slows down, and that a clock slows down near a large mass (because of the effects of gravity). Under normal conditions of low velocities and low mass, the changes in time are very small, but atomic clocks are accurate and stable enough to detect even these small changes. In such experiments, three sets of clocks were used—one group remained on Earth, one was flown west around the earth on a jet, and the last set was flown east. By comparing the times of the in-flight sets with the stationary set, the predicted slowdowns of time were observed and the theories were verified.

03 February 2009

Atomic bomb

The invention: A weapon of mass destruction created during World War II that utilized nuclear fission to create explosions equivalent to thousands of tons of trinitrotoluene (TNT), The people behind the invention: J. Robert Oppenheimer (1904-1967), an American physicist Leslie Richard Groves (1896-1970), an American engineer and Army general Enrico Fermi (1900-1954), an Italian American nuclear physicist Niels Bohr (1885-1962), a Danish physicist Energy on a Large Scale The first evidence of uranium fission (the splitting of uranium atoms) was observed by German chemists Otto Hahn and Fritz Strassmann in Berlin at the end of 1938. When these scientists discovered radioactive barium impurities in neutron-irradiated uranium, they wrote to their colleague Lise Meitner in Sweden. She and her nephew, physicist Otto Robert Frisch, calculated the large release of energy that would be generated during the nuclear fission of certain elements. This result was reported to Niels Bohr in Copenhagen. Meanwhile, similar fission energies were measured by Frédéric Joliot and his associates in Paris, who demonstrated the release of up to three additional neutrons during nuclear fission. It was recognized immediately that if neutron-induced fission released enough additional neutrons to cause at least one more such fission, a selfsustaining chain reaction would result, yielding energy on a large scale. While visiting the United States from January to May of 1939, Bohr derived a theory of fission with John Wheeler of Princeton University. This theory led Bohr to predict that the common isotope uranium 238 (which constitutes 99.3 percent of naturally occurring uranium) would require fast neutrons for fission, but that the rarer uranium 235 would fission with neutrons of any energy. This meant that uranium 235 would be far more suitable for use in any sort of bomb. Uranium bombardment in a cyclotron led to the discovery of plutonium in 1940 and the discovery that plutonium 239 was fissionable— and thus potentially good bomb material. Uranium 238 was then used to “breed” (create) plutonium 239, which was then separated from the uranium by chemical methods. During 1942, the Manhattan District of the Army Corps of Engineers was formed under General Leslie Richard Groves, an engineer and Army general who contracted with E. I. Du Pont de Nemours and Company to construct three secret atomic cities at a total cost of $2 billion. At Oak Ridge, Tennessee, twenty-five thousand workers built a 1,000-kilowatt reactor as a pilot plant.Asecond city of sixty thousand inhabitants was built at Hanford, Washington, where three huge reactors and remotely controlled plutoniumextraction plants were completed in early 1945. A Sustained and Awesome Roar Studies of fast-neutron reactions for an atomic bomb were brought together in Chicago in June of 1942 under the leadership of J. Robert Oppenheimer. He soon became a personal adviser to Groves, who built for Oppenheimer a laboratory for the design and construction of the bomb at Los Alamos, New Mexico. In 1943, Oppenheimer gathered two hundred of the best scientists in what was by now being called the Manhattan Project to live and work in this third secret city. Two bomb designs were developed. A gun-type bomb called “Little Boy” used 15 kilograms of uranium 235 in a 4,500-kilogram cylinder about 2 meters long and 0.5 meter in diameter, in which a uranium bullet could be fired into three uranium target rings to form a critical mass. An implosion-type bomb called “Fat Man” had a 5-kilogram spherical core of plutonium about the size of an orange, which could be squeezed inside a 2,300-kilogram sphere about 1.5 meters in diameter by properly shaped explosives to make the mass critical in the shorter time required for the faster plutonium fission process. A flat scrub region 200 kilometers southeast of Alamogordo, called Trinity, was chosen for the test site, and observer bunkers were built about 10 kilometers from a 30-meter steel tower. On July 13, 1945, one of the plutonium bombs was assembled at the site; the next morning, it was raised to the top of the tower. Two days later, on July 16, after a short thunderstorm delay, the bomb was detonated at 5:30 a.m. The resulting implosion initiated a chain reaction of nearly 60 fission generations in about a microsecond. It produced an intense flash of light and a fireball that expanded to a diameter of about 600 meters in two seconds, rose to a height of more than 12 kilometers, and formed an ominous mushroom shape. Forty seconds later, an air blast hit the observer bunkers, followed by a sustained and awesome roar. Measurements confirmed that the explosion had the power of 18.6 kilotons of trinitrotoluene (TNT), nearly four times the predicted value. Impact On March 9, 1945, 325 American B-29 bombers dropped 2,000 tons of incendiary bombs on Tokyo, resulting in 100,000 deaths from the fire storms that swept the city. Nevertheless, the Japanese military refused to surrender, and American military plans called for an invasion of Japan, with estimates of up to a half million American casualties, plus as many as 2 million Japanese casualties. On August 6, 1945, after authorization by President Harry S. Truman, the B-29 Enola Gay dropped the uranium Little Boy bomb on Hiroshima at 8:15 a.m. On August 9, the remaining plutonium Fat Man bomb was dropped on Nagasaki. Approximately 100,000 people died at Hiroshima (out of a population of 400,000), and about 50,000 more died at Nagasaki. Japan offered to surrender on August 10, and after a brief attempt by some army officers to rebel, an official announcement by Emperor Hirohito was broadcast on August 15. The development of the thermonuclear fusion bomb, in which hydrogen isotopes could be fused together by the force of a fission explosion to produce helium nuclei and almost unlimited energy, had been proposed early in the Manhattan Project by physicist Edward Teller. Little effort was invested in the hydrogen bomb until after the surprise explosion of a Soviet atomic bomb in September, 1949, which had been built with information stolen from the Manhattan Project. After three years of development under Teller’s guidance, the first successful H-bomb was exploded on November 1, 1952, obliterating the Elugelab atoll in the Marshall Islands of the South Pacific. The arms race then accelerated until each side had stockpiles of thousands of H-bombs. The Manhattan Project opened a Pandora’s box of nuclear weapons that would plague succeeding generations, but it contributed more than merely weapons. About 19 percent of the electrical energy in the United States is generated by about 110 nuclear reactors producing more than 100,000 megawatts of power. More than 400 reactors in thirty countries provide 300,000 megawatts of the world’s power. Reactors have made possible the widespread use of radioisotopes in medical diagnosis and therapy. Many of the techniques for producing and using these isotopes were developed by the hundreds of nuclear physicists who switched to the field of radiation biophysics after the war, ensuring that the benefits of their wartime efforts would reach the public.