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THE ADDITION OF SOUND TO FILMS
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pole piece is formed from the core of the electro-magnet which is bored out through the centre to permit the modulated light beam to pass, via a suitable lens system, to the film. The lower pole piece is secured by an adjustable plate which may be mounted very accurately with relation to the upper pole piece. By this means it is possible to obtain the necessary small gap between the two pole pieces. The lower pole piece is also bored out to permit the steady light beam from the illuminating system to pass to the modulating ribbons of the valve.
The general layout of an early Western Electric recording machine, fitted with the 'ribbon' type light valve, is seen in Figure 89. Light from the recorder lamp is focused on to the objective lens by means of a condenser lens mounted within the lamp house, the light valve previously described is situated between the condenser and the objective lenses and is carefully aligned with the optical centre so that the beam of light will be accurately modulated by the valve. The film is drawn from the upper supply magazine and passed round two large sprocket wheels of equal size. Sound is recorded at a point on the left hand sprocket wheel as far from the inlet and outlet to the film magazines as is possible. This point is selected to ensure that any uneven film motion, due to the drag of the supply spool or to the over-drive of the take-up spool, will be isolated by the right hand sprocket wheel. The motion of the film past the recording point should therefore be as steady as the accuracy with which the sprocket teeth are cut, plus the accuracy with which the perforations are located in the film. Although the manufacturing tolerances on sound negative film are similar to those for picture negative, the tolerances on the size of the sound sprocket teeth are usually closer than those for picture camera
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-SYNCHRONOUS MOTOR
Fig. 89. — Early Western Electric recording machine