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Feb., 1930] FlLM PERFORATION AND ITS MEASUREMENT
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of the first pair of holes exactly, we fix two pins accurately at that point. Now we must decide about the length of the metal bar and what degree of accuracy is desirable. On the bar, the distance between the black marks will be determined by the two foregoing factors. Let us, therefore, glance at Table I, which will give the distance of the reading point from the pins, S, for various values of X, and A , figuring the value of B in this equation as 2 mm., since that is the standard width of a positive perforation hole:
TABLE I Distances of Reading Points from Rule Pins
a = 4.76
Marking Distance on the Ruler 4.77 4.78 4.79 4.80
Perforation dis
tance of the
film. x = 4.68
119.0
106.0
95.6
87.1
80.0
4.69
136.0
119.3
106.2
95.8
87.3
4.70
158.7
136.3
111.5
106.4
96.0
4.71
190.4
159.0
136.7
119.8
106.7
4.72
238.0
190.8
159.3
136.9
120.0
4.73
317.3
238.5
191.2
159.7
137.1
4.74
476.0
318.0
239.0
191.6
160.0
4.75
952.0
477.0
318.7
239.5
192.0
4.76
954.0
478.0
319.3
240.0
It will be noted that it is possible to measure perforation distances between 4.68 and 4.76 mm. with a rule,A75 n?ra. in length, provided the distance between the marks is 4.79 mm. A greater distance between the marks will reduce the length of the scale. A smaller distance will increase the accuracy, but also lengthen the ruler.
By establishing the distance between the black markings on the upper row as 4.79 mm. it is possible to read a variation between 4.76 and 4.73 mm., and, similarly, by establishing the distance on the lower row at 4.76 mm. it is possible to measure between 4.73 and 4. 68 mm.
We have assumed, up to the present, that the pins have been placed in a position which will coincide exactly with the correct position of the first pair of holes on the strip of film to be measured ; however, if they are placed a little to the left, we can facilitate the reading of slight variations, or, if these pins are moved to the left 1 mm., the original equation will be changed as follows :