International projectionist (Jan-Dec 1935)

April 1935 INTERNATIONAL PROJECTIONIST 19 the A. C. arc to $2222.85 for the Suprex D. C. arc where the present motor generator set is retained and used in an overloaded condition. Conversion Costs Harking back to installation costs consider the following conversion costs. The conversion of present lamps requiring two suitable transformers and two arc controllers represent a maximum cost of $500, as against its nearest competition (rectifiers and new lamps) at from $1100 to $1640 list. Moreover, the latter great expenditure would net no better results, as far as flicker is concerned (referring back to Mr. Kalb's oscillographs). Furthermore, after the capital expenditure for new equipment was made, there still remains the comparative annual operating costs of $773.80 for the A. C. arc as against $1587.85 for the Suprex D. C. arc using rectifiers, a difference of $814.05 in favor of the A. C. arc. The A. C. arc obviously is anathema to Mr. Finn, but even where results would be much worse than he admits are possible, the accompanying data make it impossible for me to accept his opinion as to (1) the acceptability of conversion jobs, and (2) the worth of the A. C. arc for small-theatre projection. ARC DATA SUMMARY James J. Finn THE relative merits of the A. C. and the D. C. arcs having been discussed in detail in these columns last month, Mr. Hoffman requested, and was granted, the opportunity to present figures which, he asserted, would justify use of the A. C. arc on the basis of installation and operating costs. He presents these figures herein. Mr. Hoffman evidently takes leave of his favorite topic, the A. C. arc, with considerable reluctance, being unable to resist the impulse to make a few more (Continued on page 21) ANOTHER OPPOSING VOTE ON L. I. LAMP CONVERSION J. E. McAuley J. E. McAULEY MANUFACTURING COMPANY Appended hereto is the second contribution to the symposium sponsored by I. P. to determine the desirability, or lack of it, of converting existing low-intensity lamp equipment for use with Suprex carbons — a topic which continues to furnish the projection field with much lively discussion. Mr. J. E. McAuley is particularly fitted to discuss this interesting and very important topic by reason of his long experience as a designer and manufacturer of arc lamps which have been used for many years throughout the world. — Editor. IT is with great satisfaction that I accept your kind invitation to present my views on the conversion of existing low-intensity lamps for use with Suprex carbons. In providing the forum for the presentation of all shades of opinion on this question, I. P. is performing a real service for manufacturers, projectionists and exhibitors. The cubical contents of the L. I. lamphouse, as well as the ventilating stacks, are not of sufficient dimensions to carry off the smoke fumes generated by the Suprex type arc. None of the L. I. lamps have means to protect the reflector from sooting up when the arc is being struck or while the carbons are being burned in. Low-amperage lamps carry the arc so close to the reflector that excessive and costly mirror breakage inevitably results since the mirrors used for low-amperage arcs are not of heat-resisting glass. The magnification of low-amperage lamp mirrors produces a proper size spot at the aperture when employing a 12 mm. positive carbon. When 7 mm. positive carbons, such as the Suprex, are used with this mirror, the crater magnification is not great enough to produce a spot that properly covers the aperture, hence very poor light distribution is had on the screen. The result is that the center of the screen is bright, whereas the light on either side of the screen center falls off tremendously. TABLE B. Comparative Hourly Operating Costs Type Arc Arc Line or Amps. Volts Gen. Volts Current Current Used Cost Carbon Combined Cost Cost Low-Intensity 30 50 80 5.3 kw. 26.5c 5. c 31.5c I. P. Figures 30 50 80 3.96 " 19.8 5. 24.8 A. C. Arc 80 25 220 2.2 " 11. 10.2 21.2 I. P. Figures 80 29 232 2.7 " 13.5 12. 25.5 Suprex D. C. With 3-Phase Rectifier 45 35 220 3.5 " 17.5 21.4 and 4.6 bulbs 43.5 I. P. Figures 45 32 220 2.2 " 11. 14.4 & 2. for bulbs 27.4 The relatively small diameter of Suprex carbons necessitates a fixed relationship between the negative and positive carbons at all times, if proper crater formation on the positive is to be had. Thus, a slightly warped Suprex carbon, if used with a low-intensity lamp, will require constant realignment as the carbons are consumed. The feed screws on low-amperage lamps are arranged for an equal ratio of consumption of positive and negative carbons, whereas the burning rate between the positive and negative Suprex carbons may vary from 2-to-l to 4-to-l, depending upon the current employed. A special feed screw to provide a fixed ratio of negative feed to positive feed could be installed (and is in conversion jobs — Ed.), but this would necessitate operating the lamp at one fixed amperage and would not permit the flexibility of operation that is essential in arc lamps using Suprex carbons. The operation of practically all lowintensity arc controls is governed by changes in the voltage across the arc, which principle is unapplicable to a Suprex carbon arc. By the time sufficient alterations are made in a low-voltage lamp to make it a satisfactory device for operation with Suprex carbons, the cost entailed would amount to almost as much as a lamp designed specially for operation with Suprex carbons, and of course, the conversion job would never give the results obtainable from a lamp designed expressly for the purpose. There are, of course, many other reasons why such a conversion inevitably must result in a "botched up" job, most important of which is that people attempting such work are not experienced manufacturers, have none of the drawings of the original parts (necessary if proper dimensions of substituting parts were to be supplied), and the known fact that such manufacturers come and go as frequently as the seasons. I am afraid that anybody buying such an orphan would be unable to find repair parts when he needed them.