Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1921) fuge 45 As to resistance of cells, they may be divided into two classes (1) high and (2) low resistance cells. The former type have a dark resistance (when they are not illuminated), varying usually from 100,000 to 500,000 ohms, while the low resistance cells from 100,000 to about 10,000 ohms. There is very little difference between the two, although high resistance cells often have greater sensitivity. For engineering work, due to various circuit constants as well as ordinary electrical apparatus, cells of fairly low resistance are often to be preferred. The theory of light action in selenium is explained by Fournier d'Albe.G as due to the ionization of the selenium atoms. The current due to the acting light varies as the square root of the illumination. Selenium is more responsive to light of longer wavelength than any of the photoelectric cells of the alkali metal type, this sensitivity also extending into the infra-red spectrum. It will be noted that the increased conductivity of selenium under the action of light is fairly rapid, but the return to the dark resistance value is much slower, varying from several minutes to a number of hours, according to the intensity and duration of the excitation. This fatigue characteristic is more or less noticeable, depending largely on the fulfillment of the conditions of design and construction mentioned in the beginning of this section. However, both the fatigue and lag of cells prevents their satisfactory use for many applications although they can be employed in many cases, especially in places where they are used in connection with a relay for short intervals of illumination with relatively longer rest periods between successive illuminations. The following facts regarding the care of cells should always be observed : 1. Keep cells cool. The heating effect of passing too large currents through the cell, or from the exposure to intense radiation, will cause the formation of selenides of the metal electrodes. Gold selenide is indicated by the appearance of dark brown spots on the surface of the selenium. 2. Apply the lowest voltages that will give the desired results. The use of high-resistance relays is preferable, or a limiting resistance may be placed in series with the cell for protective purposes. In general, ten milliamperes or more should not pass through the cell, but it is always best to follow the manufacturer's instructions in this matter. 3. Cells should not be exposed to intense light for long intervals of time. The cell becomes fatigued and becomes temporarily (or even permanently) insensitive to light. 4. Keep cells dry. If not sealed in to exclude moisture, they should be kept in a box containing a few pieces of calcium chloride. Fig. 2. The Case Thalofide cell, which is particularly sensitive for the infra-red end of the spectrum. 5. When not in use, cells should be kept in the dark, but they may be exposed to light regularly, for short periods, to aid in retaining their sensitiveness. 6. If the resistance of a cell drops greatly, it can be raised, at least temporarily, by applying pulsating or alternating currents. Case7 discovered that thallium sulphide is not only sensitive to light, but also has a maximum in the infra-red region of the spectrum and, therefore, such a cell can be controlled by radiations invisible to the human eye. In his earlier work, Case made use of thallium sulphide, but later refers to thallium oxysulphide, having found that the slightly oxidized compound is more active under illumination. Case fuses the material at about 650 deg. C, the fusing point of the compound, in the presence of air, thus forming thallium oxysulphide. The thickness of the material is usually between 0.3 and 0.5 mm. After fusing, it is immediately and rapidly cooled. The Case cell (Fig. 2) of this type is called the "Thalofide" cell. The construction consists of a quartz disc, %inch in diameter, which is coated with a film of lead and this latter is cut in a grid form similar to that shown in Fig. 1. Leads are soldered on after the thallium compound has been applied. The assembly is sealed into an evacuated glass bulb. The actual light-sensitive surface exposed is about 2 mm. wide and from 10 to 12 mm. long. The average sensitiveness of these cells is such that the dark resistance is lowered by 50 per cent., in 0.02 footcandle when the source of light is a tungsten filament. In some of the best cells this drop is obtained in 0.004 footcandle. The dark resistance of different cells may be anywhere from 5 to 50 megohms, depending primarily on the nature of the material used and to the grid spacing. This high resistance could be reduced by making the grid of 100 or more lines per linear inch. Thallium oxysulphide appears to undergo a slow photo-chemical change without the use of a color filter. This is supplied with the commercial cell. The thalofide cell responds quickly to the exciting radiation, usually completed after a lapse of 15 seconds. On longer exposures, the galvanometer deflection increases slowly and sometimes irregularly. Although the time —two minutes to obtain maximum response — is shorter than molybdenite, the thalofide cell behaves somewhat like it, requiring twice as long — about four minutes — for complete recovery. For small deflections, as an exposure of one minute, two minutes for recovery is sufficient. //. Photoelectric Cells Allen8 has traced their historical development from the early Elster and Geitel alkali hydride cell down to all but the more recent types developed in this country. Some of these are of particular interest and will be described here. Hughes9 designed a cell which suggests the black-body inclosures used in the study of heat radiation. A pearshaped flask traps practically all of the light that enters the cell through the window and by progressive reflections from the interior walls produces electronic emission wherever it impinges on the light-sensitive cathode. Small cells — two, three or four inches in diameter — are usually suitable for all applications except television. For such communication both large or small cells are used depending on the method of scanning. The two methods are often termed direct and reflection scanning. In the former case, the subject to be transmitted is strongly illuminated and placed before the scanning apparatus; for instance, the conventional Nipkow disc. An image of the subject is thrown on the disc by means of a lens. The photoelectric cell collects the light that falls upon it through the apertures in the disc. This optical system is less efficient than that employed in the reflection method where a small beam of light is made to pass over the subject by the rotation of the scanning disc and large photoelectric cells collect the diffused light reflected from 'The Moon-Element," Chop. TIT. \Phys. Rev., Vol. 15, p. 289; 1920; Jour. Opt. 80c. Am.., Vol. 6, p. 398 ; 1922. s"Photo-electriciti/," Chap. XVII; 1925. 9 Philo. Man., Vol. 35, P. 679: 1913 also his "Report on Photo-electricity," p. 104.