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photoelectric cell

photoelectric cell or photocell, device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material. A battery or other voltage source connected to the electrodes sets up a current even in the absence of light; when light strikes the semiconductor section of the photocell, the current in the circuit increases by an amount proportional to the intensity of the light. In the phototube, an older type of photocell, two electrodes are enclosed in a glass tube—an anode and a light-sensitive cathode, i.e., a metal that emits electrons in accordance with the photoelectric effect. Although the phototube itself is now obsolete, the principle survives in the photomultiplier tube, which can be used to detect and amplify faint amounts of light. In this tube, electrons ejected from a photosensitive cathode by light are attracted toward and strike a positive electrode, liberating showers of secondary electrons; these are drawn to a more positive electrode, producing yet more secondary electrons—and so on, through several stages, until a large pulse of current is produced. Besides its use in measuring light intensity, a photomultiplier can be built into a television camera tube, making it sensitive enough to pick up the visual image of a star too faint to be seen by the human eye. The photovoltaic type of photoelectric cell, when exposed to light, can generate and support an electric current without being attached to any external voltage source. Such a cell usually consists of a semiconductor with two zones composed of dissimilar materials. When light shines on the semiconductor, a voltage is set up across the junction between the two zones. A phototransistor, which is a type of photovoltaic cell, can generate a small current that acts like the input current in a conventional transistor and controls a larger current in the output circuit. Photovoltaic cells are also used to make solar batteries (see solar cell). Since the current from a photocell can easily be used to operate switches or relays, it is often used in light-actuated counters, automatic door openers, and intrusion alarms. Photocells in such devices are popularly known as electric eyes.

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photoelectric cell

photoelectric cell (photocell) Device that produces electricity when light shines on it. It used to be an electron tube with a photosensitive cathode, but nearly all modern photocells are made using two electrodes separated by light-sensitive semiconductor material. Photoelectric cells are used as switches (electric eyes), light detectors (burglar alarms), devices to measure light intensity (light meters), and power sources (solar cells).

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photoelectric cell

pho·to·e·lec·tric cell • n. a device that generates an electric current or voltage dependent on the degree of illumination.

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"photoelectric cell." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 10 Sep. 2018 <http://www.encyclopedia.com>.

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Photoelectric Cell

Photoelectric Cell

A photoelectric cell, also called sometimes a phototube, electron tube, or electric eye, is an electronic device that is sensitive to incident radiation, especially visible light, which is used to generate or control an output of electric current. During the latter half of the nineteenth century, many scientists and engineers were simultaneously observing a strange phenomenon: electrical devices constructed from certain metals seemed to conduct electricity more efficiently in the daytime than at night. This phenomenon, called the photoelectric effect, had been noted years earlier by French physicist Alexandre-Edmond Becquerel (18201891), who had invented a very primitive device for measuring the intensity of light by measuring the electrical current produced by photochemical reactions.

The photoelectric effect is the process in which electromaganetic radiation such as visible light, x rays, or gamma rays strike matter and cause an electron to be ejected. The ejected electron is called a photoelectron.

At the time that Becquerel was performing his experiments, it was becoming evident that one metal in particularseleniumwas far more reactive when exposed to light than any other substance. Using selenium as a base, several scientists set out to develop a practical device for measuring light intensity.

A number of them succeeded. In 1883, American inventor Charles Fritts created a working photoelectric cell; that same year German engineer Paul Nipkow used a photoelectric cell in his Nipkows diska device that could take a picture by measuring the lighter and darker areas on an object and translate them into electrical impulses. The precursor to the modern photoelectric cell was invented by German physicists Hans Geitel (1855923) and Julius Elster (18591920) by modifying a cathode-ray tube (CRT).

Strangely, the explanation for why selenium and other metals produced electrical current did not come until 1902, when German physicist Phillip Lenard (18621947) showed that radiation within the visible spectrum caused these metals to release electrons. This was not particularly surprising, since it had been known that both longer radio waves and shorter x rays affected electrons. In 1905, GermanAmerican physicist Albert Einstein (18791955) applied the quantum theory to show that the current produced in photoelectric cells depended upon the intensity of light, not the wavelength; this proved the cell to be an ideal tool for measuring light.

The affordable Elster-Geitel photoelectric cell made it possible for many industries to develop photo-electrical technology. Probably the most important was the invention of transmittable pictures, or television. Employing a concept similar to that used in Nipkows scanning disk, a television camera translates the light and dark areas within its view (and, later, the colors within) into a signal that can be sent and decoded into a picture.

Another interesting application of photoelectric cells was the invention of motion pictures. As a film is being shot, the sound is picked up by a microphone and converted into electrical impulses. These impulses are used to drive a lamp or neon light tube that causes a flash, and this flash is recorded on the side of the film as a sound track. Later, when the film is played back, a photoelectric cell is used to measure the changes in intensity within the soundtrack and turn them back into electrical impulses that, when sent through a speaker, become sound. This method replaced the old practice of playing a gramophone recording of the actors voices along with the film, which was very difficult to time to the action on the screen. Stored on the same film, a soundtrack is always perfectly synchronized with the action.

The photoelectric cell has since proven useful in many different applications. In factories, items on a conveyor belt pass between a beam of light and a photoelectric cell; when each item passes it interrupts the beam and is recorded by a computer, so that the exact number of items leaving a factory can be known simply by adding up these interruptions. Small light meters are installed in streetlights to turn them on automatically when darkness falls, while more precise light meters are used daily by professional photographers. Alarm systems have been designed using photoelectric cells that are sensitive to ultraviolet light and are activated when movement passes a path of invisible light. Cousin to the photoelectric cell is the photovoltaic cell which, when exposed to light, can store electricity. Photovoltaic cells form the basis for solar batteries and other solar-powered machines.

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Photoelectric Cell

Photoelectric cell

During the latter half of the nineteenth century many scientists and engineers were simultaneously observing a strange phenomenon: electrical devices constructed from certain metals seemed to conduct electricity more efficiently in the daytime than at night. This phenomenon, called the photoelectric effect , had been noted years earlier by the French physicist A. E. Becquerel (1820-1891), who had invented a very primitive device for measuring the intensity of light by measuring the electrical current produced by photochemical reactions. It was becoming evident that one metal in particular—selenium—was far more reactive when exposed to light than any other substance. Using selenium as a base, several scientists set out to develop a practical device for measuring light intensity.

A number of them succeeded. In 1883 the American inventor Charles Fritts created a working photoelectric cell; that same year a German engineer, Paul Nipkow, used a photoelectric cell in his "Nipkow's disk"—a device which could take a picture by measuring the lighter and darker areas on an object and translate them into electrical impulses. The precursor to the modern photoelectric cell was invented by the German physicists Hans Geitel (1855-1923) and Julius Elster (1859-1920) by modifying a cathode-ray tube.

Strangely, the explanation for why selenium and other metals produced electrical current did not come until 1902, when Phillip Lenard showed that radiation within the visible spectrum caused these metals to release electrons. This was not particularly surprising, since it had been known that both longer radio waves and shorter x rays affected electrons. In 1905 Albert Einstein (1879-1955) applied the quantum theory to show that the current produced in photoelectric cells depended upon the intensity of light, not the wavelength; this proved the cell to be an ideal tool for measuring light.

The affordable Elster-Geitel photoelectric cell made it possible for many industries to develop photoelectrical technology. Probably the most important was the invention of transmittable pictures, or television . Employing a concept similar to that used in Nipkow's scanning disk, a television camera translates the light and dark areas within its view (and, later, the colors within) into a signal that can be sent and decoded into a picture.

Another interesting application of photoelectric cells was the invention of motion pictures . As a film is being shot, the sound is picked up by a microphone and converted into electrical impulses. These impulses are used to drive a lamp or neon light tube that causes a flash, and this flash is recorded on the side of the film as a sound track. Later, when the film is played back, a photoelectric cell is used to measure the changes in intensity within the soundtrack and turn them back into electrical impulses that, when sent through a speaker, become sound. This method replaced the old practice of playing a gramophone recording of the actors' voices along with the film, which was very difficult to time to the action on the screen. Stored on the same film, a soundtrack is always perfectly synchronized with the action.

The photoelectric cell has since proven useful in many different applications. In factories items on a conveyor belt pass between a beam of light and a photoelectric cell; when each item passes it interrupts the beam and is recorded by a computer, so that the exact number of items leaving a factory can be known simply by adding up these interruptions. Small light meters are installed in streetlights to turn them on automatically when darkness falls, while more precise light meters are used daily by professional photographers. Alarm systems have been designed using photoelectric cells that are sensitive to ultra-violet light and are activated when movement passes a path of invisible light. Cousin to the photoelectric cell is the photovoltaic cell which, when exposed to light, can store electricity. Photovoltaic cells form the basis for solar batteries and other solar-powered machines.

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"Photoelectric Cell." The Gale Encyclopedia of Science. . Encyclopedia.com. 10 Sep. 2018 <http://www.encyclopedia.com>.

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"Photoelectric Cell." The Gale Encyclopedia of Science. . Retrieved September 10, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/photoelectric-cell

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Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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