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.