Projection engineering (Sept 1929-Nov 1930)

Page Jf6 Projection Engineering, September, 1929 the subject. The advantages and disadvantages of these systems are fully discussed by Gray and Ives.10 Various combinations of cells and vacuum tubes in a single device have been proposed. Often these include separate elements for both cell and tube, or a grid element, serving the same function as in the thermionic tube, may merely be inserted between the electrodes. The advantages of such combinations are questionable. Where a filament is employed, the photoelectric cell has a life which terminates when the filament is Fig. 3. Mounting for photoelectric cells. (Courtesy of Cambridge Instrument Co., Inc.) destroyed, as far as its use in the combination is concerned ; otherwise, it has been noted, a cell will greatly outlast a vacuum tube. The heating effect of the filament on the alkalimetal surface may also be harmful. Furthermore, the current-illumination characteristic of a cell is linear from the highest to lowest values. The current-voltage characteristic of a vacuum tube is not linear. This, therefore, limits the wider usefulness of the cell. An assisting grid in a cell may be necessary in certain physical research, but the idea is not new; Lenard in 1902 was probably the first investigator who employed it. There are two very significant patents regarding this structure: Huth, Rosenbaum and Loewe (German Pat. 304,325; Sept. 28, 1917) and Langmuir (U. S. Pat. 1,282,439; Oct. 22, 1918). Dr. Robert C. Burt, of Pasadena, makes a quartz central-anode cell, but the alkali-metal can also be volatilized and deposited on the central electrode so that the cell can be used for stellar photometry as a central-cathode type with a saturation point at about 8 volts. In working with these cells it is often desirable to mount them as shown in Fig. 3. Here the cell is inclosed in a light-tight wooden case which is fitted with an iris diaphragm and shutter. Thus the size of the light beam that falls on the light-sensitive cathode can be regulated and also the time ' of exposure may be either "instantaneous" or the shutter con trolled by tbe bulb, as is the case with a photographic camera. III. Photovoltaic Cells The literature referring to photovoltaic phenomena is less extensive than that of either the photoelectric or photoconductive effects. Neither has the commercial development of cells of this sort been undertaken to any great extent in spite of the fact that there is no doubt of their practical value. In regard to the literature on the subject, there seems to be no better introduction to it than the interesting paper of Carl W. Tucker.11 Allen12 also lists a number of modern references. Tucker describes the typical photovoltaic cell as follows : "If a photosensitive substance is placed upon two metal electrodes which dip into some solution, the potential difference between these two electrodes, in the dark, may be reduced to zero. But if one of these electrodes is illuminated while the other is darkened, the photochemical changes, which take place on the illuminated surface, may be expected to produce a potential difference between the exposed and darkened electrodes. What the magnitude of this voltage will be and what will be the sign of the charge on the illuminated electrode may be expected to vary with the nature of the photosensitive substance and its lightreaction." Becquerel13 first prepared and named the photovoltaic cell. The photovoltaic effect is also known as the Becquerel effect. This investigator employed silver chloride, bromide, and iodide as the light-sensitive material on plates of silver and platinum, which were immersed in dilute sulphuric acid. Becquerel observed, as Tucker reports, that (1) "the sign of the charge on the illuminate electrode varied with the thickness of the photosensitive material" and that (2) "the observed voltage was inconstant during the illumination and that its value varied with the intensity of the illumination." Minchin1* also investigated the silver halides, as well as Wilderman.15 More recently, Case16 prepared cells of cuprous oxide on copper plates in a solution of 2.5% copper formate and 0.4% formic acid. Thus, as far as investigations in this field have been carried out, it seems that the silver halides or copper oxide are the most satisfactory materials for practical photovoltaic cells. One of these cells has been de veloped by Samuel Wein, of New York City. It has a semi-cylindrical copper plate, which is treated chemically, and a lead electrode placed in a bulb containing a colorless solution (Fig. 4). When the copper plate is exposed to diffused sunlight, a current of approximately 0.5 milliampere will pass between the electrodes. Such cells can be charged, just as an ordinary storage battery is charged, and for a time a much larger current flows, although this effect rarefy lasts longer than an hour. However, this charging is not at all necessary as the ordinary current output is large in comparison with the photoelectric cells. The chief advantages of these cells are that they will operate relays directly, that they do not require external batteries in operation and that they do not exhibit the lag noticed in photoconductive cells. They will also respond to intermittent light comparable to voice frequencies, and hence are suitable for photo-telegraphy and talking motion pictures. The upward frequency is not definitely known — at least, not to the writer — and it is thus uncertain whether they will respond to the wide frequency band required for the production of intelligible television signals. However, they are excellent for every other purpose. 10 Jour. Opt. Soc. Am., Vol. 16, p. 177; 1928: Vol. 17, p. 428; 1928. 11 "A Study of Photovoltaic Cells" (Jour, of Phys. Cliem., Vol. 31, pp. 1357-1380 Sept.. 1927). 12 "Photo-electricity," 2nd. ed., 1925. 13 La Lumi&re, II., 121. "Phil. Mag. (5) Vol. 31, p. 207: 1891. 16 Z. physik. Chem., Vol. 59, pp. 553, 703 ; 1907. 10 Trans. Electrochem. Soo., Vol. 31, p. 351 ; 1917. Fig. 4. The Wein photovoltaic cell. (Courtesy of Radiovision Corp.) It is also surprising to find that the cells are not subject to progressive fatigue in so far as rough tests can determine. In confirmation of this statement, an experimental cell was observed at intervals over a period of seven or eight months. It does not appear that there has been any diminution of the photoeurrent in that time.