International projectionist (Oct 1931-Sept 1933)

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14 INTERNATIONAL PROJECTIONIST September 1933 PHOTOLYTIC CELL COJ.OK I^ESPONIE FIG.15 3000 4000 sooo 6000 700 WAVE-LC//6TH-AHeSTR0MS PHOmOMC CELL f/6.16 r'HOTR.ONK CELL §^« 0 ^ /^ f ^ * ? ■/ FIG. 17 FOOT-CANDLES leaks out of the case in time. Many installations originally equipped with this cell have been changed over to use a photo-emissive cell. The color sensitivity curve of the Photolytic cell is somewhat similar to that of the human eye. It is shown in Fig. 16. Both in this country and abroad the electronic type of photo-voltaic cell has been developed and commercialized. In these cells electrons are displaced by the action of light, which results in an e.m.f. at the terminals of the cell. A lie plate upon which is a layer of sentypical construction consists of a metalsitive cuprous oxide or other material. On top of this layer is a semi-transparent conducting film. The metal plate and the conducting film are the two electrodes. The electronic type of photo-voltaic cell is capable of furnishing a continuous current over a long period of time without depreciation. This cell represents a direct converter of light energy to electrical energy. No battery or other source of potential is used in the external circuit of the cell. The current output of a typical cell (Weston), is shown in Fig. 16. It will be noted that the linearity of response and magnitude of the output current is lessened as the load resistance is increased, the reason being that the internal conductance of the cell acts as a by-pass for a part of the generated current, and the greater the external load resistance the larger the proportion of the current which is shunted internally. Furthermore, the resistance of the cell decreases as the light intensity increases (not linearly, however), which accounts for the flattening of the response curve. In Fig. 17 is shown the open circuit voltage output characteristic. It is an interesting fact that if two similar cells are connected in parallel to a load circuit of resistance R, the current is twice that of one cell, provided that R is small. Also, if two cells are connected in series with a load circuit of resistance R, the current 0 PHOTROmC / \ CELL / \ / FIG. 18 \ 4000 sooo eooo 7000 WAVE-Lef/STfJ—ANOSTROMS IS approximately the same as for one cell connected to a load of 1/2 R. The color response of this cell IS shown in Fig. 18. The red sensitivity is low, but the characteristic is not greatly different from that of the human eye. The frequency response is very poor due to the high shunt capacity of the two cell electrodes. By making the cells of smaller size this difficulty is reduced to some extent. [Note: This is the second of a series of articles by Mr. McMaster on types and characteristics of light-sensitive cells. The concluding installment next month will include a comparison of merit of the three types of such cells. — Editor.l Schroeder Opposes Fit tin a; Richardson Procedure On Brush The following letter anent certain observations by F. H. Richardson on a recent contribution! by Mr. Schroeder to these columns is self-explanatory. IVT Y attention has been directed to certain comments in a recent issue of Better Theatres in the course of which Mr. F. H. Richardson, answering an inquiry from Mr. J. L. Schrock of Missouri, criticizes my remarks on fitting brushes. Evidently Mr. Richardson does not like my particular method. He says that the method is O.K. theoretically (which means that it can't be so very wrong), but he implies that it just won't work out practically. Mr. Richardson's remarks are very inconsistent. He thinks that projectionists will not go to the trouble of raising the brush before pulling the sandpaper back; but in the next breath he suggests that the brush be held with a screwdriver by "an assistant" while the projectionist pulls the paper back and forth. Further along in his comments he says that after sanding the brush and running it in for a half hour, it should be removed two or three times and the shiny spots sanded off each time. "Six of One—" Does Mr. Richardson think that a man who will not raise a brush while sanding it will go to the trouble of pulling the brush out two or three times and very carefully sand off the shiny spots until the fit is perfect? Or maybe he thinks that his method is not so much work as raising the brush for the return of the paper. He does admit, however, that pulling the paper in both directions, as he suggests, might not be the best way. He says: "However, if you want something a bit better, have an assistant hold a screwdriver against the brush to prevent it from tilting." Now, it's all right with me, and one may take his choice: either pull the sandpaper in one direction, as I ^ "The Maintenance of Motor Generators (Part II: Fitting Brushes Accurately)", Int. Projectionist, February, 1933, Vol. 4, No. 4, p. 10. described it, or hold the brush with a screwdriver, as Richardson suggests, and pull the paper both ways. On some machines, however, the construction is such that the brush cannot be kept from tilting by using the screwdriver method, and, in fact, it is very difficult to utilize this method on any machine. There is one statement in Mr. Richardson's comment from which I most emphatically dissent. He says that certain advice (on projection) is of little value because so few men will heed it. This thought, carried to its logical conclusion, would mean inefficient projectionists and generally poor projection — a clear case of "Oh, what's the use?" Article Served Useful Purpose Mr. Richardson must admit that at least one man has benefitted by my article, and that man is his correspondent, Mr. Schrock. The latter says that he has fitted brushes by pulling the paper both ways for a long time — maybe for years — and apparently he has not given the matter much thought. But, and this is the important point, he read my article and started to think about the subject. "Maybe Schroeder is right and I am wrong," he might have thought. In any event, Mr. Schrock went to the trouble to write a letter to Mr. Richardson and ask for his views on the matter— an act which definitely bespeaks interest on the part of the former. The chances are that Mr. Schrock now will go to a bit more bother in fitting his brushes. He may even go to the trouble of following Mr. Richardson's advice and pull the brush out two or three times in order to sand the high spots. At least he knows more about the subject than he did before he read my notes, even though they appear to be "too finely spun" to suit Mr. Richardson. A. C. Schroeder Los Angeles, Calif.