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432 DANGER OF FIRE [Cn. XI
The time required for igniting a film was examined. It was found that an ordinary film, partly black and partly transparent when held in the condenser focus would first curl and later burst into flame. The time required for each was noted, first with, then without a water-cell.
No water-cell With water-cell
Image of arc Curl Burn Curl Burn
20 Ampere D. C. Arc
Concentrated spot 1.3 sec. 2.6 sec. 5 sec. 10 sec.
Small spot 2 sec. 3.5 sec. 7 sec. 12 sec.
24 Ampere A. C.
Concentrated spot 6 sec. zosec. over 30 sec.
35 Ampere A. C.
Spot large enough to project
picture, film dead black ...3 sec. 12 sec. over 60 sec.
With 35 amperes alternating current and the crater image large enough to project the full size of picture, the film curled in 3 seconds and burst into flame in 12 seconds. When a water-cell was used the film was merely slightly warped and not in the least injured after an indefinite exposure. With larger installations the watercell could not be relied on to protect the film indefinitely, though it would much reduce the risk.
The data given in ยง 848 (fig. 342), shows the effects of the water-cell in reducing the radiant energy.
Examination was made of the probable security afforded by the fire-trap of a fire-proof film magazine. A short piece of film was put through the fire-trap of a film magazine. This fire-trap consists in a flat tube, the lower end of which is nearly closed by a pair of metal rollers. The flame would not follow the film through the metal tube. When, however, the film was pulled rapidly through the fire-trap it might or might not be extinguished by the rollers.
With the upper magazine, where the film hangs down, the rising flames heated the film to such an extent that when pulled upward through the fire-trap it continued to burn on the other side. When the film projecting from the lower magazine was ignited and pulled down through the fire-trap, it was extinguished just as a strip of