International projectionist (Jan-Dec 1937)

22 INTERNATIONAL PROJECTIONIST August 1937 directional characteristics of such a horn. To prevent backstage reflections and unintelligible speech in the auditorium, and to ensure very high efficiency, it is necessary to load the cone of the two units employed with a folded horn. The other type of loudspeaker is for the higher frequencies (400 to 10,000 cycles per second). Here the problem is not so simple. The higher frequencies radiate at a small angle. For practical reasons, it is not desirable to have a large number of sources of sound. Therefore, to insure high efficiency and wide distribution of the higher frequencies a horn has been developed for two units consisting of a number of small horns moulded together with their axes passing through a common point, their number (usually 15) depending on the size of the theatre. Each unit acts as a separate horn directing sound to a particular portion of the house. However, when added all together the result is uniform distribution of the wide range of frequencies with high efficiency. Considerable work has been done on various types of moving coil units to achieve best results. Where a combination of these loudspeakers is employed, a baffle is usually built around them. Such a baffle is employed to increase the acoustic path between the front and back of the speaker unit. Thus, any sound which may issue from the rear of the unit and be reflected toward the auditorium, is delayed so that its effect on the direct sound is minimized. The two electro-acoustic transducers which, on the one hand, convert sound to electrical waves, and on the other, convert electrical current to sound waves, are very similar in design and operation. In both cases the sound waves are directed over a large area and need to be pickedup or created by a relatively small area which very much complicates the problem. In both cases, the electrical currents involved are very small, thus necessitating delicate apparatus. The vital necessity for high efficiency — that is, the transformation of all of the frequencies uniformly without the introduction of extraneous sounds — requires considerable refinement in the design of such equipment. We will now consider another phase of the nature of electricity. Edison discovered that if a cold metal plate was placed in an electric light bulb, and a potential was applied between them, a current could be passed between the plate and the heated filament. This can be explained by the fact that as the metal of the filament is heated, the free electrons, which are in the body of the metal, move faster by reason of their heat velocity. In fact, if the temperature is sufficiently increased, some of the electrons will move so rapidly that they will escape from the body of the metal and fly out into space. This effect is exactly analogous to that of the evaporation of a liquid. When such a liquid is sufficiently heated, some of the molecules, by reason of their heat motion, acquire enough speed to be able to escape from the surface. In just such a way are we able to "evaporate" electrons from an incandescent filament. If the filament is made negative with respect to the cold plate, the electrons will pass from the hot filament to the plate. This constitutes an electric current. When the plate voltage is small, only a few of the electrons evaporated from the filament move over to the plate, the remainder returning to the filament. As voltage is increased, however, larger and larger numbers of the electrons move over to the plate — that is, the current increases. This can be increased up to a saturation point. The saturation point can be raised by increasing the filament current. The greater the vacuum between the filament and the plate, the better it works. DeForest placed a third electrode in a thermionic tube and obtained remarkable results. This third electrode is in the form of a grid located between the filament and the plate. When a voltage is established between the filament and plate in such a three-electrode tube, the electrons in their flight leave the hot electrode, pass between the meshes of the grid, and finally arrive at the plate. A small change of potential between the grid and filament will then greatly affect the electron flow. A small change in grid voltage will cause a large change in the plate current. If, then, we apply an alternating current to the grid of such a vacuum tube, the resultant plate current will vary in just the same way that the input does, but its variation will be of much greater amplitude. Vacuum tube amplifiers and photoelectric cells will be the topic discussed in the next instalment. So that the reader may determine how well he has absorbed the data presented CRAFT DUTY ON EQUIPMENT Nobody is better qualified than the projectionist to drive home to the manager or owner the importance of improved equipment and to cite the probable ill effects on the box-office of inferior reproduction as contrasted with the high quality now being attained by the circuit houses. Here, surely, is ccmpetition with a vengeance, and the independent exhibitor must put his hands up in self-defense. The task of spreading this doctrine of improvement is strictly one for the craft, which is always better informed on technological progress than anyone else in the theatre. in the foregoing article, there are appended six questions which will serve as a rapid review: 1. What is the difference in characteristics between insulators and conductors? 2. Define the following terms: Ampere. Volt. Ohm. 3. What is the well-known fundamental relationship between the three terms referred to in question 2, known as Ohm's Law? 4. What are the two electro-acoustic transducers described, and what is their function? 5. What did Edison discover about an electric light bulb that was important in the development of the vacuum tube? 6. In what respect did DeForest contribute fundamentally to the perfection of the vacuum tube? SUPPLY FIELD NOTES NEW G. E. INTERCOMMUNICATING SYSTEM FOR THEATRES General Electric's radio division in Bridgeport, Conn., announces development of a new intercommunication system suited to film theatres and their executive offices. Essentially a loud speaker phone system, the new apparatus consists of one master station and from one to four remote speaker-phone stations. Latter may be located at any distance within 2,000 feet of the master unit without special arrangements. Therefore, theatres far within this distance measurement, can utilize the system in its essentially manufactured state. NEW RCA SPEAKER COVERS AREA OVER 1 MILE A powerful new loudspeaker capable of projecting speech and music upwards of a mile with full clarity was demonstrated by RCA Victor engineers as a valuable new means of promoting safety on Atlantic City's beaches. The new "sound projector" utilizes only 100 watts of amplification. Heretofore, power speakers have required almost five times as much power to cover similar distances with far less intelligibility. Its remarkable distance range and tone quality is made possible by the development of what is probably the world's largest permanent magnet ever made for the purpose. It weighs 25 pounds and measures 8%"' in diameter. This powerful magnet actuates a new type of molded diaphragm especially designed to withstand the terrific pressure of the sound, and which is impervious to all types of weather conditions. A new throat construction acts as an acoustic transformer of the mechanical vibrations into sound vibrations, and makes possible a remarkable clarity and uniform distribution of sound in the audible range of from 100 to 7000 cycles. The complete unit is 40" high and 20" in diameter. WHOLESALE'S BOSTON STORE Wholesale Radio Service Co., Inc., added another link to their growing chain of modern establishments with the opening of their beautifully fitted display and salesroom at 110 Federal Street, Boston, Mass., with Michael Scott, well known radio merchandiser, in charge.