The motion picture projectionist (Nov 1930-Oct 1931)

28 Motion Picture Projectionist January, 1931 Figure 3 where the reflections will fall. We may assume that in a full size auditorium sound will reflect exactly as light reflects in the small model with the exception that the sound will be diffused to a greater degree than is the light. This discrepancy is of little importance since the surfaces which are causing bad reflections are to be determined rather than the areas over which this disturbance can be heard. An area of disturbance indicated in the model will be slightly less than that actually obtained in the theatre. However, the indicated center of disturbance will coincide with the actual center of disturbance. To determine more definitely from which surfaces the reflected sound or light is proceeding it is very helpful to insert pins upright in the paper in the positions which would be occupied by auditors. Each one of these pins will cast a shadow which will point directly away from the source of light or from the speaker on the stage. That shadow is caused by the direct light coming from the source. The pin may also cast another or several other shadows which will all point in different directions. Each of these shadows will then be pointed away from some reflecting surface which is causing a disturbance. If the direction of the shadow is prolonged through the pin, the resulting line will touch the surface which is producing the disturbance. (See dotted line in Fig. 1.) Reading the Results The surfaces which are indicated by these shadows should then be treated with a very highly soundabsorbent material. It is evident that since the dead spots are caused by reflections from these surfaces the more efficient the acoustic material covering these surfaces can be, the more thoroughly the dead spots will be eradicated. Consequently the very best or most highly absorbent acoustic material should be used .for the treatment of these spots. A material having not less than 70 per cent, sound absorption over the range of from 512 to 1,024 vibrations per second should be used. Figure 3 is an actual photograph of a set-up; and shows an auditorium in which "dead spots" occurred in the section of the house just in front of the balcony. Book Review PHOTOCELLS and Their Applications— By V. K. Zworykin and E. D. Wilson. 209 pages, 98 illustrations, and 114 references to the literature ■which are found at the end of each chapter. Published by John Wiley & Sons, New York City. Price, $2.50. Both authors are research engineers in the Westinghouse Research Laboratories and are eminently qualified to prepare a work of this nature. Dr. Zworykin is very well known to workers in the field of light sensitive cells because of his many valuable contributions to this highly specialized art. Tn the preface the authors use the terms "photocell" and "phototube" to designate the device more commonly known as the "photo-electric" cell, as distinguished from the "photo-conduction" and "photo-voltaic" types of cells. This subject of correct nomenclature for light sensitive cells has been discussed many times in these columns, thus it is unnecessary to go further into the matter at this time. Suffice it to say that the Institute of Radio Engineers has adopted the terms "phototube" as expressive of any type of light sensitive cell which comes under the classification of the "Hallwach effect," i.e., photo-electronic emission. The reasons for this reviewer's disagreement with this nomenclature are well known to readers of this publication. With the exception of one chapter dealing with photo-conduction and photo-voltaic types of cells, this book is devoted exclusively to the phototube. In fact, it is an ideal elementary treatise on the phototube and as a source of information on that specific form of cell is recommended highly. All the data presented on this form of cell is accurate, very well presented, and is buttressed by profuse and interesting reference matter. Other Types Ignored However, this reviewer cannot ignore the implications contained in the single chapter which treats of the photo-conduction and photo-voltaic types. In addition to being decidedly vague, this particular chapter leaves with the reader an impression that both these forms of cells have no contemporary significance and should be classified as "has beens." We disagree emphatically with this contention. As a matter of fact, selenium cells (photo-conduction type) are now being applied commercially for theatre work in sound pictures. The photo-voltaic cell, too, is offering stern .competition to the phototube, the former requiring no pre-amplifier and practically eliminating background noise. These two types of cells cannot be dismissed or shoved aside by a mere statement that they are useless; performance is what counts. The photo-conduction and photovoltaic cells are, for some purposes (other than television), very much better than the phototube. As a matter of fact, some of the oxides of metals, which are classified under photo-conducuon properties, are sufficiently fast in response to function in television transmitters, which indicates their worth. Good Elementary With the exception of this data on forms of cells other than the phototube, we are entirely in agreement with the subject matter of this book and consider it a splendid elementary guide on the phototube. — Samuel Wein. W. E. Service System Far-Flung To service the 4,789 installations of the Western Electric sound system in the United States, Electrical Research Products now has over 450 service engineers who make their headquarters in 220 towns and cities throughout the country. New York with 34 resident engineers leads all other cities; Chicago is second with 26; Los Angeles, third, with 18 and Philadelphia is fourth with 16. Boston has 15, Kansas City 10. In servicing Western Electric equipped theatres these 450 service men travel a total of 93,000 miles every week, or an average of 205 miles for every service man each week.