Projection engineering (Jan-Dec 1931)

Page 16 PROJECTION ENGINEERING snap back into position again. A modification of Lenz's law states that a ivire carrying a current in a magnetic field tends to move in, a direction at right angles both to the direction of the field and to the direction of the current. This is sometimes known as the motor rule, because it is this principle that causes electric motors to function. In Fig. 2a, the reaction of the fields of the ribbons and of the electromagnet causes most of the lines of force from the electromagnet to be bent around the outsides 'of the two ribbons. Now, in this case, the strength of the field on the outsides of the ribbons is the sum of the two separate fields, while the strength of the field between the ribbons is the difference of the two fields. If we think of the lines of force acting like stretched rubber bands, as did Faraday, then we will see that the energy exerted by these lines of force in an effort to take straight paths between the poles of the electromagnet will tend to bring the two ribbons closer together. In Fig. 2b, which represents the current flow on the other half of the a-c. cycle, exactly the opposite action takes place and the ribbons are forced farther apart by the reacting magnetic fields. In these illustrations, a cross-section of the ribbons is shown; and the X indicates the current flowing into the page, while the dot represents the current flowing out of the page. Vibration of Ribbon So we see that an a-c. speech current passing through the ribbon loop causes the two sides of the loop to be alternately drawn toward each other and forced apart by the reacting magnetic fields. In an ideal recording system, this movement of the ribbons is of the same frequency as the frequency of the a-c. speech current, which in turn has the same frequency as the sounds producing it; and the amplitude of the movement of the ribbons corresponds precisely to the amplitude of the speech current." This narrowing and widening of the light slit by the ribbons alternately reduces and increases the amount of light from the lamp that falls on the moving film. This results in narrow and wide bands, or striations, of varying density running transversely of the sound track on the film, as may be seen in the photograph (Fig. 3) of a strip of sound track of the variable density-fixed area type. This sound track is of the full 128-mil width. The higher the pitch of the sound producing the modulation of the light beam, the narrower and the closer together will be the lines of the sound track; and, the louder the sound, the greater will be the contrast in density between adjacent light and dark lines on the film. Operation of Recorder The operation of a film recording machine during the actual making of a sound motion picture requires considerable experience and close attention to detail on the part of the recorder operator. The various departments in the sound studio — stage, monitoring, amplifying, and recording — are connected by a system of colored signal lights and intercommunicating telephones. The recorder operator loads his recording machine with a 1000-foot roll of film ; punches the start mark in it ; numbers it properly ; sets the footage counters on the recording machine at zero, and then writes down all the details, such as picture number, director, film roll number, and film recorder number on a special form card. He turns on the current to the various recording circuits and adjusts the rheostats to the positions that provide the proper meter readings. When the director and cast on the stage are ready to make a sound "take," the microphone man calls the recorder operator on the telephone, gives him the scene number and calls for "interlock." The recording technician then throws the first of the two motor switches mounted on the wall near the recording machine and turns on the recording room signal light. This motor switch interlocks all of the motors and brings them to the starting position. The microphone man calls out, "Interlock," when he sees the recording room light go on, and when everything is quiet on the stage, he turns on his stage signal light, and warns, "We're running." At sight of the stage signal light, the recorder operator closes the second motor switch, which sets all of the interlocked motors into rotation. He adjusts the motor rheostat that compensates for changes in the amount of load on the master Selsyn motor in the generator room, and watches the extension milliammeter in the motor control circuit until it shows a reading that indicates the motors are up to their normal speed of 1200 r. p. m. Then, when the motors have reached their full speed, he turns on the bull's eye running light. At this, the microphone man calls out, "Speed," the scene is "shot," and the stage signal light is switched off. The recorder operator then turns off the motors ; writes down the scene number, whether or not it was ok, and the number of feet of film run through the machine, and makes the necessary preparations for the next take. AAA Future Developments in Moving Pictures jytR. LESLIE ROWSON, of London, England, returned recently to that city after an American trip which included a visit in Hollywood. The following is that portion of Mr. Rowson's report of his observations which deals with probable future developments : "Many improvements loom dimly ahead — wide film, natural colors, stereoscopy and television. Which one of these will come first? Twelve months ago wide film in one form or another seemed to be a certainty for the immediate future : the public, however, showed no interest in it, and it has been quietly shelved, temporarily, at any rate. "That a new standard size of picture will eventually be adopted seems probable, but what it will be, one can hardly say. Certainly not the wide, flat shape which met with so little success last year. "On the purely photographic side, I think we can hope for very much faster film. Eastman Kodak have recently placed on the market in America a new film, three times faster than the Type 2 Panchromatic at present in use, and I am sure that the limit in this direction has by no means been reached. "As regards lenses, I do not think we can expect any very great advances as regards speed, as covering power and depth of focus both become very poor at apertures larger than f.2. "Stereoscopy, I am certain, we will have within the next few years, though I do not believe there is any optical principle at present known by which it can be achieved, but I am certain that some new principle will be discovered which will startle us all by its simplicity. "Television is knocking at the door, and the only factor which is preventing its immediate entrance is, I understand, the purely mechanical difficulty of obtaining a picture of adequate size. "Natural color, too, is just around the corner ; Eastman Kodak, Multicolor, and other important companies arc working on it, and the solution cannot be long delayed. "Anyhow, if I may be allowed tc speculate as to the future, I predict stereoscopic pictures in color projected by television from a single print to a whole chain of theatres on to a screen whose shape continually changes to suit the composition of the scene which is being projected. "But of one thing I am certain. Whatever new forms cinematograph entertainment may take, there is one man at least who will always find himself striving to improve his knowledge and increase his mastery of his medium, and that is the cameraman."