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metallic inclusion, w ill cause a disturbance in this equilibrium, tending to produce different physical properties in the crystal aggregates finally solidified. A study, therefore, of the surface tension phenomena of the crystal aggregates might lead to valuabIe explanation of the inherent noncorrodibility of the known alloys and to the systematic discovery of newer ones. CONCLUSION The purpose of the foregoing remarks is by no means to set up the property of surface tension as the “philosopher’s stone” of corrosion. Undoubtedly galvanic action, oxidation, protective films, etc., play an equally important part. The aim in these remarks is to direct attention to a channel of research hitherto neglected from the corrosion standpoint. For a methodical study, the problems of corrosion may be systemized into two main groups-morphological and histological. Our previous work has all been preponderantly
Vol. 15, N o . 1
morphological. We might also base our division on charrtcteristics of “prenatal” and of “postnatal” origin. The characteristics of the first nature will be such as the effect produced by the formation of definite chemical compounds by proportioning the constituents of the alloying elements, and structural changes produced through addition of small quantities of alloying or purifying agents. As characteristics of “postnatal” origin we may mention change in corrodibility produced by adequate heat treatment, mechanical work, surface polishing, etc. I n the preceding remarks one phase of the corrosion problem has not been touched-namely, the study of protective coatings, such as paints. As this method of protection in no way has a bearing on the properties of the metal t o be protected, it does not come within the scope of the metallurgical aspect of the problem. The study of protective coatings is, no doubt, a fertile and extensive field of research in its own way.
Control of Industrial Processes by Light-Sensitive Means‘ By Lloyd Logan 1000 HAMLINF AvE., ST PAUL, MI“.
application to the control LTHOUGH the literConsiderations of advancing labor costs and of the difficulty of of manufacturing processes, ature on the subject obtaining skilled operators in chemical and industrial works have especially in chemical inof photo-electricity forced more and more strongly on manufacturers the necessity and dustry, appears to have comprises over seven hundesirability of replacing human agencies, wherever possible, by been practically neglected. dred papers published since automatic means. The ready success of the thermostatic control the discovery by Hertz in of the temperature of kilns, furnaces, and buiIdings indicates the TYPESOF PHOTOSENSI1887 of the action of light trend toward the elimination of manual control. Variations in TIVE CELLS on the discharge of elecelectrical conductioity and in the specific gravity of solutions are A brief reference to the tricity and a perhaps even other agencies which have been employed to effect automatic control various types of photosengreater number of articles of industrial processes. sitive cells will indicate how on photo-resistance effects, I t is suggested that another agency may be made a powerful means their properties may be the research in this field of solving many of the most difficult problems in the control of inutilized. These distinctive has been devoted largely dustrial processes. This agency is the extremely remarkable properties consist in all to the important bearing of property of light of producing electrical effects, such as changes of cases of variations of elecphoto-electric action on resistance or electrification of certain substances when it strikes trical characteristics when theories of radiation and them. I t is the purpose of this paper to attempt to outline the possible subjected to light. While subatomic phenomena. field of application of this agency to the control of industrial operano cell yet devised exactly Applied photo-electricity tions and processes and to suggest means for the accomplishment duplicates the color sensimust still be considered a of such control. tiveness of the human eye, comparatively neglected or can be used, without subject. While scientific applications of light-sensitive cells to stellar photometry,2 special means, for the direct measurement of color, neverand-among other applications-the photophone of Bell, theless a rather wide choice of characteristics is available. the automatic lighting of buoys, the electrical transmis- These cells are broadly classified as selective or nonselective, sion of photographs, the control of torpedoes, and the according as they respond only to certain ranges of wave ingenious optophone of Fournier d’Albe for enabling the lengths or to the total amount of radiation. Of the latter type important examples are the bolometer, blind to read, have reached varying degrees of practicability, only relatirely few industrial applications have been pro- the radiomicrometer, and the thermopile, none of which shows posed. While also a few attempts to effect control in increased sensitivity to radiation of a particular region of the certain directions are evidenced by proposed applica- spectrum. Of the selective class one of the oldest types is the selenium tions to the regulation of the luminosity of vacuum-tube lamps13 the regulation of voltage,4 and the control of cell, in which the conductivity of a thin layer of .the leadultraviolet lamps in the sterilization of water,6 the field of gray modification of selenium, formed by slow cooling a t 217” C., is increased by exposure to light. I n one form this 1 Presented before the Division of Industrial and Engineering Chemistry cell consists of a coating about 0.0001 in. thick forming a a t the 63rd Meeting of the American Chemical Society, Birmingham, Ala., high-resistance conductor between two series of metal plates, April 3 to 7, 1922. Chemical and other applications of the methods referred to in this paper are subjects of patents pending. usually German silver, each series being clamped together as 2 Kunz and Stehbins, Phys. Rev., [2] 7 (1916), 62. shown in Fig. 1, to form an electrode of the cell. The cell 8 Moore, U. S. Patent S56,483 (1917). is heat-treated to drive out all moisture and occluded gases. 4 Moskowitz, U. S. Patent 1,150,953 (1915) These precautions are necessary on account of the suscepti5 Pratt, U. S. Patent 1,208,830 (1916).
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January, 1923
bility of selenium t o the injurious effects of moisture in the air. The increase in conductivity is explained by Pfund6 as being due to the emission of additional electrons by resonance set up within the atom, these electrons having the properties of free electrons during the absorption of light. Although a certain lag in the recovery of the original resistance occurs, selenium cells capable of detecting fluctuations of several hundred per second are readily constructed. I
ALKALI-METAL CELL SELENIUM CELL Fro. I-DIAGRAMSOF LIGHT-SENSITIVB CGLLS
Another. important type of cell illustrative of the selective class, is the alkali-metal, or photo-electric, cell developed largely through the efforts of Elster and Geitel in Germany, and of Kunz and Ives in this country. I n this type of cell, when light, strikes the negatively charged surface of the metal, electrons available for carrying current are thrown off, the effect being proportional to the intensity of the light. It is well known that certain wave lengths of light affect certain cells of this type more than others, and it is of interest to note that in general these cells are sensitive only to a limited range of wave lengths, the maximum sensitivity of the most electropositive elements occurring with radiation of the greater wave lengths. Thus, the maximum sensitivity of sodium occurs in the blue, while most of the less electropositive metals and the nonmetals respond only in the ultraviolet. To increase the sensitiveness, such cells are generally filled a t low pressure with an inert gas such as argon or helium, or with hydrogen, so that, by applying a sufficiently high potential, ionization by collision and consequently a greatly amplified current are produced. The formation of a hydride on the surface#also greatly multiplies the photo-electric current. By the w e of 3-electrode vacuum valves, or audions, it is possible to further increase the current to 2 or 3 milliamperes, which is sufficient to operate a relay. Another type of cell of more recent development, whose sensitivity, on the other hand, extends into the infra-red, is the so-called “thalofide cell,”7 containing a sensitive surface of slightly oxidized thallium sulfide. On account of the extremely high resistance of this cell, the current is preferably amplified by means of a vacuum valve to a magnitude sufficient to operate a relay. Several other types of light-sensitive cells have been devised, depending on the Becquerel effect, involving the illumination of the negative electrode of a cell containing an electrolyte, and on the Hertz effect, involving the illumination of the negative terminal of a spark gap to facilitate the passage of the spark. The sensitiveness of photosensitive cells is obviously one of some concern, especially where exact colorimetric control is required. It may suffice to show that the photoelectric cell has proved entirely adequate for the photometry of light from the most distant stars. It is probable that the sensitiveness of specially designed cells may be made quite equal, or superior, to that of the human eye. RELIABILITY OF METHOD The next point to be considered in the evaluation of such a method of controI is its reliability. Attention will doubtPhys. Rev., [I] 28 (1909), 324. Case, Ibrd., [2] 15 (1920), 289. 1,316,350 (1919). 6
7
U. S. Patents 1,301,227 (1919) and
41.
less be called to the fatigue, mentioned frequently in the literature, of certain cells under prolonged illumination. This effect in the earlier types of cells has been shown to be due largely to such extraneous effects as presence of moisture and occluded gases, and has been largely eliminated in the more recently developed cells. Any difficulty, however, with regard to photo-electric fatigue can be readily avoided by the arrangement of the apparatus in such a manner that the continuous sample is compared as to its optical quality or qualities with a standard sample of the desired compositjionthrough the use of a common source of light, so that any difference in the samples will produce a difference in the relative resistances of the cells and operate the control through the medium of a relay suitably installed in a Wheatstone bridge arrangement. The infinitesimal quantity of luminous energy required to produce an appreciable effect, the ease and certainty with which the light may be controlled, and the comparative independence of other conditions in the operation of the current-varying substance, are characteristics of the selenium, thalofide, and alkali-metal cells which lend themselves readily to the immediate solution of many industrial and chemical problems involving control of production. Such problems, on examination, generally resolve themselves into checking and maintaining within certain limits, throughout all steps of the process, the following characteristics of the intermediate and final products: 1-Quantity of product or number of production units. or shape of product. 3-Quality of product. (a) Chemical composition. ( b ) Physical properties.
%Form
APPLICATIONS I n the following discussion it is proposed to indicate means for applying light-sensitive cells to the control of these factors affecting production in such a manner that variations in such factors will produce variations in the intensity or character of light or radiation which will photo-electrically operate suitable means for automatically exercising control over the process. The general method is to produce such a change in the intensity or character of the illumination of the cell as to produce a corresponding change in the current sufficient to operate a relay, which in turn controls the appropriate valves, switches, or other means necessary to effect the desired operation.
FIG.2
Among the properties of the final or intermediate products which may be utilized for this purpose are the following: I-Absorptive power for white or mixed light. %-Selective absorptive power for light of various colors. 3-Index of refraction. +Power of scattering light. &Rotation of t h e plane of polarization of plane polarized light, or change of pattern of other forms of polarized light. 6-Dispersion of light; spark, flame, and absorption spectra. 7-Reflective power for white light, or for light of various colors. 8-Intensity of emission of light a t a given temperature. 9-Phosphorescence and fluorescence. 10-Combinations of the foregoing.
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The choice of the method of control will obviously de- character of mechanical movements, safety devices, interpend on the nature of the factor it is desired to control. locking devices for the prevention of operations out of their I n the case of the control of the quantity or physical form proper sequence, and the control of electrical characteristics. of the product, directing purely mechanical operations, The third class, that of control of the quality of the product, the actuation of the light-sensitive cell can in many cases offers an almost endless variety of applications in chemical be most readily effected by simple obstruction of white light, industry. A partial list has been prepared by the writer, due to the opacity of the product. of the large number of possible applications of this type of While the first two classes of control, involving the quantity control in over a hundred important industries, including and the form or shape of the product, are applicable to a wide such varied processes as the manufacture and purification range of mechanical engineering and production problems , of acids, sugar, paper, gas, and oils, and the control of high mention of a few typical applications that have been the sub- temperature operations. ject of study will indicate their possible usefulness in the field of chemical industry. Following the foregoing classification of types of control according to factors of quantity, shape to be governed, or quality, the field of possible application of the control of the quantity of a product may first be considered. This group of applications is concerned with the control of operations or processes in such manner that when a predetermined dimension or quantity of production has been reached any desired step in the cycle of operations will take place. Such steps may be the opening or closing of valves, starting or stopping of machinery, operating alarms or indicating devices, or any mechanical or other operation. As illustrating the method of accomplishing these objects, the accompanying diagrams will serve to show particular applications. The device indicated in Fig. 2 utilizes a combination of means for the projection of approximately parallel light rays through a hole in a rotating element of a machine so that a light-sensitive cell will be illuminated twice for each revolution, with a counting device adapted to respond to the electrical impulses transmitted by the cell through a relay. This counting device is arranged to close a circuit when a certain number of impulses have been received and to automatically reset to repeat the cycle. The circuit closed by the “counting” mechanism may contain any such electrical device as a solenoid valve for shutting off the supply of material to a machine, or a mechanism for performing any such function as stopping or starting a machine, closing or moving containers, or effecting such operations 8s would otherwise be accomplished through the medium of gearing, cams, or an attendant. In the control of the quality or the constitution of the Another typical application is that to the measurement and product, the choice of the method of control will depend flow of liquids, Fig. 3 representing a method of controlling mainly on the degree in which the substance possesses optical the level of liquids of high refractive index. This comprises qualities differentiating it from other substances, or the ease the use of a container with a hollow transparent prism with which it can be caused to produce, through the action attached thereto by pipes and so arranged that when the of a reagent, a distinctive optical test. liquid in the container rises to a predetermined level, paralOf this class, an illustration of the regulation of chemical lel light is deflected a t right angles, owing to internal reflec- operations in which the production of an opaque test, stain, tion, and caused to impinge on a light-sensitive cell. On or coloration by the action of a suitable reagent serves suffiaccount of the lowered resistance of the cell,,there is trans- ciently to identify the substance to be produced, or the immitted through a relay a current which operates to close purity to be avoided, may be found in the protection of the solenoid inlet valve until the level has dropped below a catalysts, as, for instance, in the contact process for the manupredetermined point. facture of sulfuric acid. I n this case, as is well known, the A second large class of control by light-sensitive means platinum catalyst is very susceptible to “poisoning” by even comprises the gobverning of operations or processes in such a minute traces of arsenic in the burner gases. The visual manner that a variation in the form or shape of the material detection of arsenic by observation, through a window in the in process or of the final product, or of the path of motion burner gas main, of its presence as a white cloud has been causes a predetermined step in the cycle of operatiofis to found ineffective and unreliable. The method, proposed take place. Such operations may be the starting or stopping and shown diagrammatically in Fig. 4, employs the Marsh of machines, the regulation of the characteristics of electric test for arsenic in connection with a light-sensitive cell so currents, the operation of alarms or indicating devices, or arranged that when arsenic is present a black stain will be the accomplishment of any similar desired object. produced on a transparent water-cooled cell interposed Although a discussion of the many groups of applications between the cell and a source of light. The light will then of this class investigated is beyond the scope of this paper, be cut off, operating, on account of the consequent increase it may be mentioned that these include automatic examination in resistance of the cell, a relay controlling a 3-way valve for and sorting of articles in process of manufacture as to size the purpose of by-passing the gas for further treatment, and shape, automatic stops, controlling and recording the thus preventing the arsenic from reaching the catalyst
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January, 1923
chamber, Means are provided for periodically removing the stain with bleaching-powder solution and restoring operation when the impurity has been removed. Arrangements for using other tests for arsenic have also been devised. The selective absorption of colored substances for monochromatic radiation affords another very direct means for
FIG.4
detecting and eliminating impurities. An apparatus for this purpose is illustrated diagrammatically in Fig. 5. For instance, if it is assumed that the undesirable constituent is ferric iron, ammonium thiocyanate might be employed as the reagent. By the use of a color filter excluding red rays, the light-sensitive cell will be normally subjected to light only of other colors. When, however, ferric iron is present in the sample, a red coloration, capable of transmitting only or chiefly red rays, will appear in the transparent vessel. As no red rays are present in the incident light, it will be totally out off from the light-sensitive cell, thus increasing the cell resistance and operating, through a relay, the appropriate valve lor by-passing the product for further treatment. For simplicity in illustration, this device is shown as operating continuously. In practice, however, the apparatus will be
WFPRSS
-
k U i V N m C SAmPL/NG 5€Y/Cf
FIG. 5
arranged for intermittent operation, with provision for flushing out the cell after each test. The control of processes involving the neutralization of acid or alkaline solutions by the use of the selective absorption of indicators is but one of the large number of subgroups of possible application of selective absorption. Considerable attention has been devoted, especially by Houston, Hutchinson and Hurlbut, and Prideaux, to the study of the relation between the wave lengths and absorption coefficients of a limited number of inorganic and organic
43
substances over considerable ranges of concentration and temperature. Further determinations of absorption coefficients, especially for various chemical indicators, would appear to be a contribution immediately useful in the design of control apparatus. In cases where it is found that a substance is not completely specified by any of the foregoing properties, it will be found in many cases that a combination of tests, such as a color test with a reagent, and of the index of refraction will serve to completely identify the substance. Among other possible applications studied, which do not come entirely within any one of the foregoing classes, may be included those of combinations of the preceding methods of control with automatic specific gravity regulation in such processes as oil distillation where both specific gravity and color of the product are of importance, replacement of complicated mechanical movements, by electrical devices controlled by light-sensitive means and of the utilization of the photo-electric method in connection with a suitable optical pyrometer for the control of high temperature operations. It will be observed that in most of the foregoing cases, apparatus in common use is adapted to the control of the process, with only such alterations as are necessary to make the testing or checking process a continuous one. Among the more usual means for checking the chemical composition of substances are chemical reagents and indicators, the polariscope, the colorimeter, the refractometer, and the spectroscope, while such physical properties as tensile and compressive strength, hardness, density, structure, temperature, and pressure are checked by testing machines, the scleroscope, the hydrometer, the microscope, the pyrometer, and the manometer. A study of these means of checking properties of substances shows that the greater number of them, especially those for checking chemical composition, may be registered by optical means in such a manner as to provide the appropriate impulse through the medium of the photosensitive cell to operate an electric relay. The complete development of the possibilities of process control through light-sensitive means involves the selection or development of analytical tests most suitable for affecting light-sensitive cells, the improvement of photosensitive cells, and the solution of such chemical or mechanical problems as might arise in the application of the control to the actual process. The suggestion is ventured that such development of this field may prove an inexhaustible source, not only of industrial improvements, but also of lateral scientific results of fundamental value.
News from Canada Success is attending the development of acetylene black at Shawinigan Falls, the Canada Carbide Company having marketed some quantity, which is finding a favorable reception by the paint trade. The power concentration of the St. Maurice valley will, we are advised, shortly be reinforced by development a t Les Gres, a fall about half-way between Three Rivers and Shawinigan. This is expected to develop in the neighborhood of 200,000 additional horse power for the Shawinigan Water and Power Company. The Canadian China Clay Company a t St. Remi, Quebec, has been investigating the possibilities of colloidal treatment of kaolin, and as a result is disposing of considerable quantities in the rubber industry. The suspension of the clay by colloidal means permits of the removal of impurities providing a specially adaptable clay for incorporation. Col. F. M. Gaudet, C.M.G., who has been Technical Executive Officer of the Honorary Advisory Council for Research, has resigned from that body and has joined the staff of the Canadian Industrial Alcohol Company.
