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Conclusions
It may be well to review briefly some of the more salient points in the paper and to discuss the significance of some of the results.
The initial experiments with the liquid calorimeter demonstrated the remarkable tenacity with which a roll of motion picture film retains the heat which it has collected during projection. This, of course, is a result of the low thermal conductivity of the material. To bring such a solidly wound roll of film to temperature equilibrium with its surroundings requires a very long period of time. When film after being projected is transferred immediately to tightly closed metal containers, necessary from the standpoint of fire protection, it is obvious that they must retain the heat absorbed during projection for some time, and if a roll is projected a second time after a short interval the amount of heat contained therein must necessarily be accumulative so that its temperature will rise progressively after each projection. No experiments have been reported in this paper indicating the ultimate temperature to which a film would rise on being projected repeatedly, and this is one of the points which we hope to report on later.
The air calorimeter designed and constructed proved to be very satisfactory for the purpose. The results obtained with it are very repeatable and of a high order of precision. Its water equivalent was found to be 961 gram calories with a probable error of +6 gram calories.
The specific heat of motion picture film on cellulose nitrate base has been determined and found to be 0.434, when no metallic silver image is present. For a film having an optical density of 3.46, the specific heat was found to be 0.427. The difference therefore, between " clear" film and a film containing a silver image transmitting 0.1% of the visible radiation is only 0.007 which is equivalent to approximately 2%. It will be seen, therefore, that the specific heat of motion picture film is practically independent of the density of the picture. The specific heat for motion picture film on cellulose acetate base was found to be 0.482.
Using a standard projector adjusted as near as possible to practical conditions, the amount of heat absorbed by a motion picture film on cellulose nitrate base was determined for a variety of conditions. The results show that the amount of heat absorbed per foot of film is approximately inversely proportional to the projection speed, as would be expected. The increase in the amount of heat absorbed per foot of film as the arc current is increased is less than would apply if the relation were one of true proportion. This also is to be expected. Increasing the arc current does to a certain extent increase the energy flux density at the gate of the machine but not in direct proportion to the current. The influence of the optical density of the film upon the amount of heat absorbed is very marked. Thus for clear film (containing practically no developed silver) 6.0 calories per foot were absorbed. This increased to 12.9
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