Journal of the Society of Motion Picture Engineers (1930-1949)

528 C. F. VlLBRANDT Vol 48, No. 6 enter the perforation, come into contact with its edge, and finally disengage from the perforation. The manner in which these actions occur depends on the relative pitches of the sprocket and the film, and also on the function of the sprocket in question. In a paper presented before the Society in 1923, Jones2 described the theoretical principles involved. Their application to 35-mm intermittent sprockets was reported and discussed by Talbot3 in 1Q45. Since the considerations involved are essentially the same for 16-mm and 35-mm sprockets, much of the terminology suggested by Talbot is used in this paper. It may be of interest to review the behavior or movement of film on both drive and holdback sprockets for each of two cases, i.e., FIG. 3. Action of film on a holdback sprocket when the pitch of the film perforations is greater than the pitch of the sprocket teeth. Case 1 — in which the pitch of the film is greater than that of the sprocket; Case 2 — in which the pitch of the film is less than that of the sprocket. The holdback sprocket, because of the way in which the tension is applied, acts as a driven sprocket. When the pitch of the film is greater than that of the sprocket, the leaving tooth is driven, as shown in Fig. 3. Here the entering tooth engages the film without touching the edge of the perforation, and the film seats at the base of the tooth before it comes into contact with the face of the tooth. As it disengages from the driven tooth, the film slides forward on the sprocket until it reaches the next tooth. Theoretically, these conditions should result in longer life of the film than when the film pitch is shorter than the sprocket pitch, which condition is shown in Fig. 4. Here the edge of the perforation must be forced down on the tooth by a shoe