Julius Elster and Hans Friedrich Geitel’s pioneering work on the photoelectric effect and photoelectric cells was of decisive importance in the development of the electron theory of metals.
The photoelectric effect was known to science in the early nineteenth century when Alexandre-Edmond Becquerel
What came to be known as the photoelectric cell, a device connecting light and electricity, had its beginnings in the 1880’s. At that time, scientists noticed that a metal plate charged negatively lost its charge much faster when it was subjected to light (especially to ultraviolet light) as opposed to darkness. Several years later, researchers demonstrated that this phenomenon was not an “ionization” effect because of the air’s increased conductivity, as the phenomenon took place in a vacuum but did not take place if the plate was positively charged. Instead, the phenomenon had to be attributed to the light, which excited the electrons of the metal and caused them to fly off:
After Sir Joseph John Thomson’s
In 1888, Wilhelm Hallwachs observed that an electrically charged zinc electrode loses its charge when exposed to ultraviolet radiation if the charge is negative, but it is able to retain a positive charge under the same conditions. The following year, Elster and Geitel discovered a photoelectric effect caused by visible light; unlike Hallwachs, however, they used the alkali metals potassium and sodium for their experiments instead of zinc.
The Elster-Geitel photocell (a vacuum emission cell, as opposed to a gas-filled cell) consisted of an evacuated glass bulb containing two electrodes. The cathode consisted of a thin film of a rare, chemically active metal (such as potassium) that lost its electrons fairly readily; the anode was simply a wire sealed in to complete the circuit. This anode was maintained at a positive potential in order to collect the negative charges released by light from the cathode. The Elster-Geitel photocell resembled two other types of vacuum tubes in existence at the time: the cathode-ray tube,
The new cell, then, emitted electrons when stimulated by light and at a rate proportional to the intensity of the light, hence a current could be obtained from the cell. However, Elster and Geitel found that their photoelectric currents fell off gradually; they therefore spoke of “fatigue” (instability). It was discovered later that most of this change was not a direct effect of a photoelectric current’s passage; it was not even an indirect effect. Rather, the change was caused by oxidation of the cathode by the air. Given that all modern cathodes are enclosed in sealed vessels, that source of change has been completely abolished. Nevertheless, the changes that persist in modern cathodes often are indirect effects of light that can be produced independent of any photoelectric current.
The chief sources of instability in the Elster-Geitel cell arose from changes in the cathode caused by change of temperature or bombardment by positive ions and changes in the field as a result of charges on the walls of the cell. These changes are connected with the incidence of light and the passage of a photoelectric current because light is usually accompanied by heat and because a photoelectric current may generate positive ions. As long as the constitution of a cathode is unchanged, its emission is independent of temperature within wide limits, except in the neighborhood of the threshold; however, change of temperature may alter its constitution by causing the evaporation or deposit of surface films. Positive ion bombardment may produce the same effects as rise of temperature as well as other effects.
Those cathodes that are least stable are ones whose emission depends most closely on the presence of volatile surface layers. Plain metals are relatively stable. Metals sensitized by the Elster-Geitel process are very unstable. First, these metals are subject to chemical change. The alkali metal absorbs hydrogen on its surface during sensitization; this hydrogen tends to diffuse into the unchanged potassium below. If a cell is badly prepared, the hydrogen may diffuse away entirely, leaving the surface bright once more; although this diffusion does not occur in a well-prepared cell, progressive change in sensitivity—usually a loss—might occur over long periods of time. Second, heating of the cathode causes the alkali metal to distill to the cooler parts of the cell and produces an irreversible change in its sensitivity. Cooling of the cathode (less usual) may cause the reverse change, but not a restoration of the original sensitivity. Such changes of temperature of the cathode relative to the rest of the cell are most likely to occur when the cathode is supported in the center and is therefore highly insulated.
The Elster-Geitel photocell was for some twenty years used in all emission cells adapted for the visible spectrum, and throughout the twentieth century and into the twenty-first, the photoelectric cell had a wide variety of applications in numerous fields. For example, if products leaving a factory on a conveyor belt are passed between a light and a cell, they can be counted as they interrupt the beam. Personnel entering a building can be counted also, and if invisible ultraviolet rays are used, they can be detected without their knowledge. Simple relay circuits can be arranged to switch on streetlights automatically when it grows dark. The sensitivity of a cell with an amplifying circuit enables it to “see” objects too faint for the human eye, such as minor stars and certain lines in the spectra of elements excited by a flame or a discharge. The fact that the current depends on the intensity of the light made possible the development of photoelectric meters that can judge the strength of illumination without human error—for example, in order to get the right exposure for a photograph. Throughout the history of science, men and women have searched for devices more reliable than their own faculties for estimating size, weight, temperature, loudness, and so on; with the photoelectric meter, light joined the list.
The cell also made talking motion pictures possible. The earlier systems for making films with sound had depended on gramophone records, but it was very difficult to keep the records in sync with the action on the film. With the development of the photocell, it became possible to record the waves of speech and music in a “sound track” by turning the sound into current through a microphone and then into light with a neon tube or magnetic shutter and photographing the variations in the intensity of this light on the side of the film. Through the reverse process of running the film between a light and a photoelectric cell, the visual signals could be converted back to sound.
John Logie Baird
Photoelectric cells have also played a role in the ongoing search for alternative energy sources; for example, they have been used to transform solar radiation into other forms of energy.
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Campbell, Norman Robert, and Dorothy Ritchie. Photoelectric Cells: Their Properties, Use, and Applications. 3d ed. London: Pitman, 1934. Although dated, this informative book discusses the theory of photoelectricity, contemporary technical advances in the field (including Elster and Geitel’s photocell), and applications that introduce or illustrate important principles. Includes illustrations, graphs, and references following each chapter. -
Lange, Bruno. Photoelements and Their Application. Translated by Ancel St. John. New York: Reinhold, 1938. An early digest of knowledge in the field of photoelectricity. Written for the layperson as well as the engineer or scientist, gives a useful historical introduction to the development of photoelectricity. Includes graphs, charts, illustrations, and bibliography. -
Morgan, Bryan. Men and Discoveries in Electricity. London: John Murray, 1952. Provides an overview of the history of electricity and magnetism. Focuses on the work of scientists whose work has shaped that history, discussing their work in the larger context of the history of ideas that led up to and followed their breakthrough discoveries. Includes copious illustrations and an appendix listing the most notable scientists in the field of electricity up to the 1950’s. -
Sommer, A. Photoelectric Cells. Brooklyn, N.Y.: Chemical Publishing, 1947. This very brief work is devoted entirely to photoelectric cells of the emission type, as opposed to cells of the barrier-layer and photoconducting types. A brief survey of the principles of photoelectric emission is followed by a more detailed description of the manufacture of photocathodes. Includes numerous graphs and illustrations as well as a bibliography. -
Summer, W. Photosensitors. London: Chapman & Hall, 1957. Presents useful information on the wide range of photoelectric devices that were in use at the time of publication. Also of interest is the discussion of the many applications of these devices to industry. Includes many illustrations and a bibliography.
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