1996 UV Waterworks Ashok Gadgil, a scientist at the Lawrence Berkeley National Laboratory in California, invents an effective and inexpensive device for purifying water. UV Waterworks, a portable, low-maintenance, energy-efficient water purifier, uses ultraviolet light to render viruses and bacteria harmless. Operating with hand-pumped or hand-poured water, a single unit can disinfect 4 gallons of water a minute, enough to provide safe drinking water for up to 1,500 people, at a cost of only one cent for every 60 gallons of water—making safe drinking water economically feasible for populations in poor and rural areas all over the world.
Barely stifled yawns greeted the electronics novelty that was introduced to the public in mid-1948. "A device called a transistor, which has several applications in radio where a vacuum tube ordinarily is employed, was demonstrated for the first time yesterday at Bell Telephone Laboratories," noted an obviously unimpressed New York Times reporter on page 46 of the day's issue. Barely stifled yawns greeted the electronics novelty that was introduced to the public in mid-1948. "A device called a transistor, which has several applications in radio where a vacuum tube ordinarily is employed, was demonstrated for the first time yesterday at Bell Telephone Laboratories," noted an obviously unimpressed New York Times reporter on page 46 of the day's issue.
The roots of the triumph reach deep. Germanium and silicon, along with a number of other crystalline materials, are semiconductors, so-called because they neither conduct electricity well, like most metals, nor block it effectively, as do insulators such as glass or rubber. Back in 1874 a German scientist named Ferdinand Braun identified a surprising trait of these on-the-fence substances: Current tends to flow through a semicon-ductor crystal in only one direction. This phenomenon, called rectification, soon proved valuable in wireless telegraphy, the first form of radio communication. The roots of the triumph reach deep. Germanium and silicon, along with a number of other crystalline materials, are semiconductors, so-called because they neither conduct electricity well, like most metals, nor block it effectively, as do insulators such as glass or rubber. Back in 1874 a German scientist named Ferdinand Braun identified a surprising trait of these on-the-fence substances: Current tends to flow through a semicon-ductor crystal in only one direction. This phenomenon, called rectification, soon proved valuable in wireless telegraphy, the first form of radio communication. When electromagnetic radio waves traveling through the atmosphere strike an aerial, they generate an alternating (two-way) electric current. However, earphones or a speaker must be powered by direct (one-way) current. Methods for making the conversion, or rectification, in wireless receivers existed in the closing years of the 19th century, but they were crude. In 1899 Braun patented a superior detector consisting of a semiconductor crystal touched by a single metal wire, affectionately called a "cat's whisker." His device was popular with radio hobbyists for decades, but it was erratic and required much trial-and-error adjustment.Another route to rectification was soon found, emerging from Thomas Edison's work on the electric lightbulb. Back in 1883 Edison had observed that if he placed a small metal plate in one of his experimental bulbs, it would pick up an electric current that somehow managed to cross the bulb's vacuum from the hot filament. Not long afterward, a British engineer named John Ambrose Fleming noticed that even when the filament carried an alternating current (which Edison hadn't tried), the current passing through the vacuum always traveled from the hot filament to the plate, never the other way around. Early in the new century Fleming devised what he called an "oscillation valve"—a filament and plate in a vacuum bulb. It rectified a wireless signal much more reliably than Braun's crystals. By then the nature of the invisible current was understood. Experiments in the 1890s by the British physicist Joseph John Thomson had indicated that a flood of infinitesimally small particles—electrons, they would be called—was whizzing through the vacuum at the incredible speed of 20,000 miles per second. Their response to signal oscillations was no less amazing. "So nimble are these little electrons," wrote Fleming, "that however rapidly we change the rectification, the plate current is correspondingly altered, even at the rate of a million times per second.„In 1906 the American inventor Lee De Forest modified Fleming's vacuum tube in a way that opened up broad new vistas for electrical engineers. Between the filament and the plate he inserted a grid like wire that functioned as a kind of electronic faucet: changes in a voltage applied to the grid produced matching changes in the flow of current between the other two elements. Because a very small voltage controlled a much larger current and the mimicry was exact, the device could serve as an amplifier. Rapidly improved by others, the three-element tube—a triode—made long-distance telephone calls possible, enriched the sound of record players, spawned a host of electronic devices for control or measurement, gave voice to radio by the 1920s, and helped launch the new medium of television in the 1930s. Today, vacuum tubes are essential in high-powered satellite transmitters and a few other applications. Some modern versions are no bigger than a pea.
In addition to amplifying an electric signal, triodes can work as a switch, using the grid voltage to simply turn a current on or off. During the 1930s several researchers identified rapid switching as a way to carry out complex calculations by means of the binary numbering system—a way of counting that uses only ones and zeros rather than, say, the 10 digits of the decimal system.
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