International projectionist (Nov-Dec 1933)

December 1933 INTERNATIONAL PROJECTIONIST 11 r ■ FIGURE 5. Diagrammatic representation of the functioning of rectifiers, showing how A.C. is rectified to pulsating B.C. but one direction. This use as a rectifier is the only one to which the twoelectrode tube lends itself. Two of the four tubes in both the 42-A and 43-A (W. E.), amplifiers are employed in this manner to provide plate voltage for the other two tubes that serve as amplifiers. Three-electrode tubes are used for this purpose, so that fewer types of tubes will be needed, but they are connected as two-electrode tubes, their grids being left unconnected. Subsequent installments will discuss the functioning of that all-important electrode, the grid, and the operation of vacuum tubes as amplifiers. Appended hereto is a group of questions which will serve as a review of the information given in the article just concluded. Questions, Article I: 1. What is the "Edison effect"? 2. What is the purpose of the filament in a vacuum tube? 3. Why must a center tap be used on the secondary of the transformer when A.C. is used for the filament supply? 4. What is the "space charge"? 5. What substances are used on an oxide-coated filament? 6. How are electrons emitted from the surface of a hot metal in vacuum? 7. Why is a vacuum tube said to be a unidirectional conducting device? 8. From what portion of a thoriated filament does the electron emission come? 9. Why must a positive potential be applied to the plate to obtain normal operation of a vacuum tube? 10. What is the difference in the rectified output of the half-wave and full-wave rectifiers shown in Figures 3 and 4? PERFORATED SCREEN NO BAR TO GOOD PROJECTION J. S. Ward DIRECTOR OF OPERATIONS, ELECTRICAL RESEARCH PRODUCTS THE statement that the advent of sound revolutionized the motion picture industry has become a bromidium; everybody knows that radical and fundamental changes have taken place since 1926. And not the least of these has been in the character of the projection screen. In the early days of the silent picture era, the screen was almost the last detail to be given attention. For years any flat, light colored surface was deemed good enough for a screen; but once the fact was recognized that the screen played an important part in a picture's presentation, its development was rapid. Then came sound pictures, and the newly perfected screen was shot full of holes, both figuratively and literally. Naturally. For the sound had to come from the direction of the picture, hence the only logical place for the horn — the source of the sound — was back of the screen. And when the sound tried to get through the solid screen, it didn't sound like the same sound. Therefore, the only thing to do was to make holes in the screen, and with characteristic enterprise the industry set forth to make the best of the holes. Early Position Experiments At first, sound picture loud speakers were placed at various and sundry locations about the screen. Between the twin problems of illusion and sound distribution, it was soon found that no "standard" position relative to the screen would insure satisfactory results in the many contrasting types of theatres. Hence, Electrical Research Products, Simplex Booklet Available International Projector Corp. has available for readers of INTERNATIONAL Projectionist a limited supply of booklets containing a general description of and operating instructions for their latest model projector, the Super Simplex. Copies of this booklet may be had without charge by addressing the projector company at 92 Gold Street, New York, N. Y. First come, first served. at an early date began experimentation with porous and perforated screens. Types enabling good sound results to be had while still providing acceptable pictures, were soon available. The problem yielded readily to attack. In the beginning, however, when the matter of sound transmission was considered all-important, the perforations were made so large and so numerous that the picture suffered decidedly. It was soon found that only a relatively small percentage of open space was necessary, and in the present screens the reduction in area ranges from 5.8 per cent, to 8.5 per cent, with the easily tolerable loss of approximately 3 decibels in the sound transmission. Two physical objections to the perforated screen present themselves: loss of brightness, due to loss of screen area; and appearance, due to the perforations themselves. Now, brightness is a function chiefly of area and of illumination. Tests show that screen illumination varies greatly from place to place, being frequently 50 per cent and in some cases as much as 100 per cent. If we take 8 per cent as the average decrease in effective screen area, it will at once be apparent how relatively unimportant this factor becomes as compared with the probable variation in illumination value. Punch Visibility Unimportant The objection that perforations may be visible from the auditorium, thus marring the appearance of the projected picture, will usually be unimportant. The limiting distance at which perforations one-sixteenth of an inch in diameter are barely visible is eighteen feet. Not many seats are as near the screen as this, and in many makes of screens the perforations are of even less diameter. We may conclude, then, that the disadvantages due to the perforated screen are relatively small, and it will readily be appreciated from this brief review of the subject that in the interest of providing their audiences with convincing sound picture operation, practical showmen will adhere to the use of screens which insure, at the same time, a satisfactory picture and sound of natural quality and pleasing illusion. ERPI PROMOTIONS Electrical Research Products has announced the following promotions, effective immediately: Harry Dodge, Central Division General Manager to General Manager of the Western Division; Stanley Hand, Northeastern Division Sales Manager to General Manager of the Central Division succeeding Dodge; Bert Sanford from Merchandising Manager to Northeastern Division Sales Manager succeeding Hand.