International projectionist (Oct 1931-Sept 1933)

July 1932 and other lamps of high brilliancy. B. When the glass walls of the lamp become so dark that the gain of the amplifier must be materially raised, it is good economy to replace the lamp with a new one, even though its filament still lights up. The reason for this is that the ground noises of the amplifier become objectionable at high gain, and the advantages of noiseless recording is thereby lost. C. The formation of an oxide as a white deposit inside the glass bulb is an indication of air leakage. A common cause for such leaks is the excessive tightening of the set screw of the exciter holder which process cracks the glass INTERNATIONAL PROJECTIONIST wall where it is cemented to the base. Such a lamp will burn out in no time, and should be replaced immediately. D. From the foregoing it will be noted that a reduction in exciter lamp current increases its life to a greater degree than it reduces its light emission. Therefore, to obtain longer life, it is advisable to operate exciter lamps at some current value lower than the rated. It is best to operate the lamps at a value recommended by your service engineer. Currrent values lower than those recommended are liable to affect the output materially, necessitating operation at higher gains, with consequent background noise. 19 The foregoing discussion was submitted for inspection to Mr. R. E. Farnham of the Engineering Dept., General Electric Co., makers of exciter lamps. Mr. Farnham, who has contributed much interesting lamp data to these columns, offered the following comment: Addendum By R. E. Farnham 1WAS very much interested in the article by Mr. Watkins which was submitted to me for comment by the editor of International Projectionist. I can readily appreciate the difficulty experienced by many projectionists in thoroughly understanding the relationship between exciter lamp life and the quality of reproduced sound. The life of the photocell (exciter), lamps has a very direct connection with the theatre owner's pocketbook; while good quality sound is rather intangible and generally a matter of opinion — hence the temptation to operate exciter lamps below their rated current value. Evaporation Process In the Watkins article, the general discussion of gas-filed lamps is quite good and substantially correct. However, the conclusion reached in Section A thereof does not necessarily follow. All gasfilled lamps, whether they are designed for fifty hours or for one thousand hours life, go through the same phenomena of evaporation and spot evaporation, and the shorter life of the exciter lamps is due to their being designed to operate at a high brightness in order to secure the maximum amount of light through the optical system. The material given in Sections B and C is quite worth while. Section D of the article by Mr. Watkins is contradictory, inasmuch as its first two sentences advocate lower current operation as a means of gaining longer lamp life, while the last sentence advises against it. It is generally recognized among engineers interested in the development of sound picture equipment that a larger amount of light incident on the photocell than is now possible, would be desirable, particularly so because increased light would reduce the amount of amplification necessary, with consequent reduction of background noise. Analysis of a sound optical system shows that the light reaching the film comes from only a small part of the filament area of the exciter lamp; and since increasing the wattage of the lamp merely increases the filament area, no gain in illumination would ensue from such a procedure. ^ Filament Temperature Increasing the brightness of the filament, with consequent increase of filament temperature, is therefore the only method available to secure greater illumination at the cell. Increased filament temperature results in shorter lamp life. Thus, the entire problem resolves itself into securing the best balance between lamp cost and good quality sound with freedom from background noise. Study of this prf)blem by both ourselves (G.E. ), and those interested in the production of sound equipment has shown that when the lamp filament is operating at a temperature that gives an average of fifty hours life, sufficient light reaches the cell to give ample volume of sound without requiring such high am])lification as to raise the extraneous noise to an objectionable level. On this basis, lamp renewal cost is not a serious factor. Lower Current Values As is evident from the accompanying curve sheet (Fig. 1), operation of the 8.5-volt, 4-ampere lamp at 3.9 amperes would not occasion serious trouble and would hardly produce any audible effect. If projectionists stopped at this point, no harm would result. But the temptation to operate at lower current values is often great, and they cut down to 3.8 amperes, then to 3.7 and even lower, with the result that the quality of reproduced sound is most seriously affected — and because the change probably has been gradual, the seriousness of the situation is not noticed. Nevertheless, much of the improved quality of sound pictures effected recently, notably the so-called new noiseless recording development, is destroyed. Managerial Interference Therefore, I firmly believe that projectionists should adopt as a general policy the operation of exciter lamps at the rated current value marked on their bases. I do believe that a majority of projectionists desire to follow this policy, but are hampered in so doing by the interference of managers and owners who have in mind the effecting of a small petty economy — if it can in truth be called economy when it results, as it does, in inferior sound quality. Managers and theatres owners, as well as those projectionists who are in the Brownell Resigns "M. P. P." Editorship i^HARLES E. BROWNELL, well-known in the motion picture ^^ field as a writer on the technical aspects of cinematography and for the past year editor of "Motion Picture Projectionist," has resigned his office with that publication, according to a statement authorized by Brownell. A trained engineer, Brownell's editorship reflected technical intelligence and good writing style, and his resignation, which was entirely voluntary on his part, occasioned much surprise in projection ranks. Mr. Brownell has had a wealth of experience in the technical branch of the industry, having previously served with RCA Photophone, Inc., as an instruction book and specification writer. He has been actively associated with the electrical engineering profession for the past 15 years. The former editor's home is at II Mountainview Place, Montclair, N. J.