International photographer (Jan-Dec 1934)

Four The INTERNATIONAL PHOTOGRAPHER November. 1934 Motion Picture Sound Recording CHAPTER XIV By CHARLES FELSTEAD, Associate Editor HE Mrcond of this series of chapters on Him recording, published last month, completed the de^ scription of the constructional detail of the film recording machine employed in the Western Electric sound recording system. That chapter also discussed the constant light source, the two lens systems, and the electromagnet assembly on which the light valve is mounted. The photo-electric cell amplifiers used for PEC monitoring were also described. In this chapter, the construction of the light valve, which is the light modulating device used in the W. E. film recording system, will be examined in considerable detail ; but the theory of the functioning of the light valve will not be discussed until next month in the final chapter on film recording. The related operations of stringing and tuning the light valve will also be described in the present chapter. Mounting of the Light J alve The frame of the light valve is formed of a flat slab of metal that just fits across the two end arms of the £-shaped core of the electromagnet. Two screws with knurled knobs hold it firmly and exactly in position when it is placed on the core. This flat slab of metal thus forms the armature of the electromagnet described in the chapter last month ; and it is equipped with a central projecting wedge of metal that is identical in size and shape with the wedge-shaped end of the center leg of the electromagnet. Like the wedge-shaped projection on the electromagnet, this central projecting wedge on the light valve frame has a slot cut through it. On the front face of the wedge-like elevation on the light valve frame, the opening is eight mils wide by 256 mils long, and it expands to an opening 204 mils wide by 256 mils long in the outside face of the armature. This exactly corresponds to the dimensions of the tapered slot in the electromagnet core. The narrow portions of the two slots face each other when the light valve is mounted in position on the electromagnet. The flaring of the slots is to provide space for the converging and diverging light beam on the two sides of the ribbons of the light valve. The light beam is focused by the condensing lens mounted next to the recording lamp and converges through the wedge-shaped opening in the center leg of the electromagnet onto the ribbons of the light valve. The light beam then passes between the ribbons and is permitted to diverge in the flare of the slot through the wedge-like elevation on the light valve frame. This divergent beam of light is collected by the objective lens system and focused on the moving film in the exposure chamber of the recording machine. Thus we see that when the light valve is fastened in place on the core of the electromagnet, the wedge-shaped elevation in the center of the frame of the light valve is precisely opposite the similar wedge-shaped end of the center leg of the E-shaped core ; and the small openings of both slots are facing each other and are exactly parallel. With the light valve thus in operating position, the two elevations are spaced eight mils apart when there is no current flowing through the winding of the electro magnet ; but when the electromagnet is energized by a current from the battery, this air gap is reduced to seven mils. This is crudely illustrated in the accompanying diagram. Construction of the Light Valve On the face of the light valve frame, which is the side that has the central elevation, there are two screwlike arrangements called windlasses mounted near one end. These windlasses are slotted to hold the two ends of a loop of duralumin ribbon ; and they fit their mountings tightly enough so that they cannot turn of their own accord and permit the ends of the ribbon to come loose. One windlass is insulated from the light valve frame, but the other one is grounded directly to the frame ; and they are connected to the two binding posts on the back of the frame that form the two terminals of the light valve. The center of the loop of duralumin ribbon is drawn tight by a miniature pulley mounted on an arm at the other end of the light valve frame. This pulley is insulated from the frame ; and it is arranged so that a spring makes it keep a constant tension on the loop of ribbon. The strength of the pull of the spring on the ribbon can be regulated by means of a screw that projects through to the back of the light valve frame. Insulated bridges are mounted on each side of the wedge-like central elevation on the face of the frame. The windlasses and the pulley are arranged so that the two sides of the loop of ribbon are supported by the bridges just three mils above the face of the projecting metal wedge. These bridges are comparable to the bridge that supports the strings of a violin, only in the case of the light valve the bridges are smooth on the top instead of notched. The positions of these bridges should be evident from the accompanying photograph. Position of the Ribbon Loop Four adjustable insulated pincers confine the two sides of the loop of ribbon to the center portions of the insulating brdiges, which serves to place the sides of the loop directly along the longitudinal edges of the slot in the face of the projecting wedge. When the two ribbons are properly centered over the slot and spaced exactly one mil apart (one mil is one-thousandths of an inch) by adjusting the four pincers, they shut off most of the width of the opening; and if the light valve is inter Face of the light valve of the Western Electric recording system. A and A1 are the windlasses. B is the insulated pulley. C and C1 are the pincers; the insulated bridge is just below them. D is the slot in the wedge-shaped elevation through which the beam of light shines. Courtesy Bell Telephone Laboratories. Chapter XIV. Please mention The International Photographer when corresponding with advertisers.