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Feb., 1930] FlU! PERFORATION AND ITS MEASUREMENT 213
The first is to count 100 holes on the film and make a measurement with the ordinary ruler between the corresponding edges of the first and one-hundredth perforations. This gives a measurement of between 476 and 468, which is great enough in length to be easily recognized when compared with the standard. It will tell just how much in millimeters the film has shrunk between the individual perforation holes.
The second method gives a similar measurement by comparison with the standard but consists of taking the piece of film the length of 40 perforations and stretching it between brass slides. The distance between the corresponding edges of the first and forty-first holes is measured with micrometers and is approximately 190 millimeters. If this space is divided by 40, you again have the difference between the individual perforations. This is merely a modification of the first method mentioned, only using a shorter piece of film.
The third method is to measure the perforation space or pitch with a calibrated microscope. It is essential in using this method to make several measurements before striking an average. There is always the possibility of a minute variation between the individual perforation holes which could be detected by the microscope but which would not be an accurate measurement of the shrinkage of the film, nor be great enough to cause any difficulty with it.
In this connection we might mention that under the old system of one-hole perforation it was essential to determine the exact distance between each pair of holes, as a consistent variation would give an unsteady picture and a noticeable change in the frame line on the screen. At the present time, to the best of our knowledge, all of the manufacturers are using the four-hole system of perforation, which punches four holes on each side of the film at one time with a gang of eight punches. The film is then drawn forward by a shuttle and eight pilots, engaging and filling exactly the previous eight holes punched, thereby very accurately placing the film into position before the next eight holes are punched. With this system it is practically unnecessary to check the distance between the individual holes unless a marked recurrent unsteadiness is noted on the screen. We might also mention another interesting point in this connection — that tests have proven that film having a uniform thickness shrinks uniformly throughout its length, so that the relationship of the perforation holes one to the other is not changed.
We do not believe that any of these three methods described above