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Oct., 1943] THE PROJECTION LIFE OF FILM 299
was started, but a recheck showed that the gate tension was heavier than normal and might have caused too great a pull and elongation of the film. Therefore, a second series was run alter the gate tension was reduced, with the results shown in Table II.
TABLE u
"Normal" Gate Tension
Projections with Sprocket Film Base Film Pitch No. 1 No. 2 No. 3 No. 4
Nitrate— Sample 1 0.1869 1215 1250 2350 1935
Nitrate— Sample 2 0.1867 810 1575 2439 2340
Safety— Sample 1 0.1869 205 545 450 445
Safety— Sample 2 0.1866 679 1263 1386 1390
This table shows a considerable increase in projection life over the previous conditibns, but, surprisingly, it shows no reduction on the average in sprocket diameter for maximum projections.
Such effects are difficult to explain. In the first series the indication of a maximum is so definite, at a sprocket pitch greater than that of the static film dimensions, that a general drop in the sprocket pitch for maximum life was anticipated with a reduced gate tension. Table II does not show any such drop.
However, the favorable showing on projection life is seen to be definite regardless of an explanation of these details of the data.
A study of the worn perforations of the film showed different, typical tears, depending on the relative pitch of film and sprocket during the test. When the sprocket is small in comparison with the film pitch, the entering tooth rubs the sprocket-hole, tending to break or tear it with a push toward the surface of the film and away from the sprocket itself. Conversely, when the sprocket is large in comparison with the film, the film drags on the tooth as it withdraws from the film tending to break or tear it by a pull toward the film surface next to the sprocket. In the case of the best fit no predominant tear could be found; in fact, many perforations had a notch worn in them the width of a sprocket tooth and a few thousandths of an inch deep.
The results of the tests emphasize the fact that small differences of pitch can be important in determining limits of projection life, and they suggest that much greater projection life is possible than is usually achieved under theater and exchange conditions.