The motion picture projectionist (Nov 1929-Oct 1930)

32 The Motion Picture Projectionist November, 1929 mon use. This diode tube was in common use by radio experimenters prior to 1906. For some reason unknown to the writer, this means of amplifying was subsequently dropped by . Lauste in favor of the compressed air loudspeaker. This latter consisted of an air compressor, air storage tank, and electrically-controlled valve. This valve was connected in series with a potential and a light sensitive cell (selenium). The valve varied the compressed air, and successfully produced sound which could be heard for a distance of four or five city streets. A similar compressed air loudspeaker was manufactured by the A^ictor Talking Machine Co. and was sold under the trade name of the "Auxetophone." Lauste's Contributions It cannot be rightly denied that it was Lauste who made the first successful sound pictures. It is a matter of regret that the many improvements made since Lauste's time were not made just a little earlier so that the fruits of this brilliant investiga tor's work might have been reaped by him. As it was, Lauste merely smoothed out the road of those who followed him, as is so very often the case with pioneers in any field of endeavor. Projectionists will find much of interest tc them in the work ofEugene A. Lauste, for it was he who made some of the most important contributions to the development of the motion picture projector. The proof of Lauste's fine mechanical talents is contained in the accompanying illustrations which show conclusively that this man certainly was far ahead of any of his contemporaries. References English Pat. No. 18,057, Aug. 11, 1906. 10,526, May 3, 1912. 16.941, Nov. 4, 1912. 16.942, Nov. 4, 1912. U. S. Pat. Ser. No. 764,946, May 1, 1913. Daily Express (Loncl.), Aug. 28. 1913. Daily Chronicle (Lor.d.1, Aug. 27, 1913. Film Censor Exhib. Rev., Sept. 3, 1913. Cinema (Lond.), Sept. 3, 1913. Scient. Amer-, Dec. 22, '1917. Elect. Exper., Tune, 1918. Pictures & Picture Goer, May 3, 1919. Motion Picture News, July 26, 1919. Science cr Invention, Dec, 1920. Photo Electric Cell Fatigue Recent comments ip technical papers anent the "fatigue" and "lag" characteristics of photo electric cells prompted the invitation to the following workers in the art for their opinions on the subject. We are told, among other things, that cells must not be exposed to light when not in use and that certain cells display a definite "lag" characteristic. Both characteristics are discussed in the following interesting comments.— The Editor. A. J. McMASTER, G-M Laboratories, Chicago, Illinois: — THE interesting question as to whether photo electric cells of the alkali metal type are subject to depreciation when not in use, and particularly when such cells are exposed to light, has been discussed pro and con for a number of years. Some cells do show depreciation of this sort, although in a final analysis it is generally found that the loss of sensitivity is not due to exposure to light but is inherent in the particular cell in question. In normal operation, exposure to light produces emission of electrons from the sensitive surface of the cell, which negative charges are replaced by the source of potential connected in the external circuit of the cell. When the battery is omitted or the circuit is open, electronic emission occurs upon exposure to light, but a state of equilibrium is almost instantaneously reached which inhibits further discharge of negative electricity. Although it is the writer's opinion that no deleterious effects are produced by exposure of a high quality photo electric cell to light when the cell is not in use, it is highly recommended that cells of the alkali metal type not be exposed to strong direct sunlight or intense incandescent sources at close range, due to the marked depreciation caused by the heating of the cell. As the materials used in some types of cells have a very low melting point, the effect of even moderate heat is to practically destroy the usefulness of the device. As mentioned above, some cells show rapid depreciation with time whether in light or in darkness. This effect is due to the chemical action within the cell as a result of improper preparation of materials. Undoubtedly the purity of materials used and the amount of care exercised in evacuating the envelope are of primary importance in the manufacture of a quality product of this kind. The subject "fatigue" has many ramifications and a long story could be written about the same without perhaps convincing anyone of the truth of the matter. However, I believe that your editorial policy in this regard will bring to light interesting information of considerable value to your readers. DONALD H. LOUGHRIDGE, R. C. Burt Laboratories, Pasadena, California: — I agree thoroughly with the recent statement of Samuel Wein in your columns to the effect that a great deal of this talk in regard to the necessity of keeping photo electric cells in the dark is mostly fundamentalism. Our experience with various types of cells has shown that in the modern type of cell fatigue is practically absent, and exposure even to bright sunlight for as long as eight hours with maximum current being taken from the cell at all times shows absolutely no change in the sensitivity of the cell. SAMUEL WEIN, Radiovision Corp., New York City: — Among the comments on photo electric cells which you submitted to me are the following: (1) photo electric cells, if exposed to a strong source of illumination, are subject to fatigue and require some Hi tie time period before they are fit for further use, and (2) it requires several minutes after certain cells are connected to a source of potential for them to function properlyi. That both comments have no basis in fact will be shown presently. Before we go into the subject matter proper, it would be best if we have a definition of terms. Fatigue: Webster's Dictionary tells us that it is a "state of exhaustion" or "tired." Lag: The same source tells us that this is to "move slowly" and also to "stay behind." Let us now see what bearing these definitions have on the photo electric effect. Fatigue as applied to photo electrons would be a gradual decrease in the photo electric emission with the same source of light. Whereas, lag would be an inertia of the cell in responding to a light change. Let us see if it is possible to give a practical analogy to these effects. When a battery is in continual use, its output continuously decreases : this would correspond to fatigue. On the other hand, if a potential is applied to a motor, the latter will require a few seconds for .its armature to reach uniform and maximum speed: this corresponds to lag. As a matter of fact, if the potential applied to a motor is switched on and off several times in quick succession, the motor itself will not respond to these "on and off" periods of potential. Thus we see that there is a marked difference between "fatigue" and "lag," although both these terms are loosely applied by some writers. As a matter of fact, the true photo electric effect will exhibit no lag or fatigue characteristics. However, if a cell does show these characteristics, it will be because of (1) the absorption of occluded gases of the alkali metal or its corresponding hydride, forming, as it does, a chemical reaction, i.e., an "oxide," (2) a gradual or slow seeping of air into the cell (3) a gradual change of the alkali metal surface, and (4) the most probable cause, a faulty amplifier to which the output of the cell is connected. Much of the difficulty experienced with photo electric cells may be traced directly to the "grid resistor" connected in series with the cell and directly across the "grid" of the first amplifier tube. This resistor is very unstable and not only varies its characteristics with the temperature of the room but also with the humidity and the amount of current flowing through it. On the other hand, if a transformer-coupling is used with the cell, this grid resistor difficulty is overcome. One company uses a high resistance unit in series with the photo electric cell and transformer as a means for protecting the cell from over-loading. Another difficulty experienced with photoelectric cell amplification is "current leakage" across the base of the cell. This is sometimes attributed to the cell proper, but it is known that this leakage may be eliminated by taking the leads out of the cell at opposite ends, such as is now done by certain manufacturers. The conclusions to be drawn from the foregoing are that the photo electric cell possesses no inertia or lag and hardly enough fatigue to be noticeable. Such difficulties as have developed are the fault of the apparatus used with the cell. THE SEILER EXPERIMENT:— Miss Seiler1, a student of Professor Kunz at the University of Illinois, undertook to ascertain whether a long period of "illumination" of a "hard vacuum" or "hydride" cell would give rise to fatigue. To do this * Astrophys. Jour. Vol. 52, p. 129, 1920.