for use as part of a talking picture the sound do accompany the photograph the action must be recorded for reproduction when the picture is shown the most desirable material for this purpose has been found to be the photographic film the finished film employed by reproduction in the motion picture theater therefore carries not only the picture of the action that is to be seen as it did in the silence that today's but also the photographic record of the positive brand which is used in the theater to reproduce the original picture and sound since the photographic film is affected only by light it is necessary for the recording of sound to convert the sound waves the corresponding changes in light as i speak the sound waves produced by my boys are transmitted through the air to the microphone where these sound waves are converted to changes in an electric current these variations in the electric current i then amplified and used to control the light falling on the photographic film the light that falls upon the photographic  in the form of a fine bright line extending transversely of the direction in which the film moves the part of the film on which the line of light falls to make the photographic sound wreck are and it is called the sound track when the line of light is so placed as normally to extend part way across the sound track and the sound control variations cause the position of one end of the line to move transversely of the track when both ends of the line of light are caused to move transversely of the track record known as a bilateral variable area sound track is produce when the line of light extends completely across the sound track and the thickness of the line is varied by the sound control the current variations i record known as a variable density soundtrack is pretty  all of the sound records gun thing photographic records of light variations corresponding to the pressure variations in the original sound if either a variable area or a variable density sound record  is run past the narrow line of light from a fixed source and the light after passing through the film is visually observed the amount of light reaching the observer is constantly varying from instant to instant if the i were capable of quantitatively measuring the light it would see that the amount of light reaching it from either sound record varies in exactly the same way there are in common use today system to which you are now listening contains such a photo electric cell this varying beam of light falling on the photoelectric cell produces variations in the electric current which are directly proportional to the variations in the light beam as the varying electrical current in the photoelectric cell is small a vacuum tube amplifier is required to increase it to the point where it will operate a loud speaker the loud speaker converts the varying electrical current into sound waves which are the counterpart of the sound waves falling on the microphone in the recording studio to review in the recording process the original sound waves are converted to varying electrical current the varying electrical current this converted to variations in the light beam and the varying light beam produces a photographic sound record in the reproducing process the photographic sound record is used to produce a varying beam of light the varying beam is converted into varying electrical current and the varying electrical current is then converted back into sound waves the advent of successful talking pictures in the late twenty's nearly all variable density photographic sound recording has been done by means of a device known as the light that out of the light valve contains two very thin metallic ribbons tightly stretched and spaced very close together to form a line transmitting slot these ribbons lying in a strong magnetic field and the amplified sound current flows through the ribbons the electric current reacts with the magnetic field causing the ribbons to move like shutters displacing them in direct proportion to the amplitude of the current in order to ensure a high quality sound recording the vibrating length of these thin ribbons is limited by bridges over which the ribbons are tightly stretched the pension of the ribbons is adjusted by tensioning means the ribbons are so delicate and their separation and movement so slight that the action of the ribbons can be observed only with the aid of a microscope the separation between the ribbons is actually less than a hair's bread system to form a very fine line of light on the photographic veil since the ribbons act there's light controlling shutters any change in the sound current flowing through the ribbons corresponding change in the size of the light transmitting slot defined by their closely spaced edges and therefore in the amount of light falling on the film the device does act like a valve controlled by the sound current for controlling light in the explanation of the light valve action that follows that tension will be directed particularly to the portions of the ribbons that control the light and these portions and their movement will be shown s  as has been explained the ribbons line in the magnetic field and the amplified sound current flows through the ribbon the electric current reacts with the magnetic field causing the ribbons to move like shutters displacing them in direct proportion to the amplitude of the current a steady light is concentrated on the ribbons and these ribbons acting as shutters cause variations in the light corresponding to the pressure variations in the original sound this varying light buzzes variation in the exposure of the film the exposed film when developed and printed is the sound recording the soundtrack used for the reproduction of music will be shown on the screen and the soundtrack will be changed from a variable area to a variable density track simultaneously on both the picture and sound record the photoelectric  cell and the entire reproducing system is the same in all cases