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THE PROJECTION ROOM 263
The bottom crosspiece is made in the same way but its groove is one inch deep with (80) a half -inch strip of soft sheet rubber laid in it edgewise to deaden the noise when the shutter is dropped. (81) It is possible to purchase ready-made metal port casing already provided with shutter grooves and a groove for the cover glass of the observation port. These casings are adjustable for different wall thicknesses. Their use is recommended though hand-made jobs are equally satisfactory.
Shutter Suspension
The chief service of the shutter is to prevent audiences from recognizing fire. To do this successfully the shutters must drop quietly within two or three seconds after the fire starts. The idea of placing metallic fuse links near the ceiling is correct theoretically, but it does not always work out well in practice, especially if the fire produces plenty of smoke but very little heat.
(82) To be sure of a quick-acting shutter it is essential that a quick-action fuse be installed where the flames will reach it the instant a fire blazes up. This is a very important precaution since a delay, even if measured in fractions of a second, may provoke considerable harm.
(S3) No unusual powers of reasoning are required to understand that a metallic fuse is almost totally useless for prompt action where a film fire is confined to a closed projector magazine, from which it exudes relatively little heat but an enormous volume of smoke.
It is wholly useless if it is located at any considerable distance from the magazine. Its location on the ceilingdoes not lessen its ineffectiveness. As soon as a fire starts, projectionists must attend to closing the shutters at once. It is a moment when life must be considered as more important than property.
Glass in Ports
(84) Since the advent of sound it has become necessary to confine all noises incident to projection to the projection room itself. (85) Optical glass is used to