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width of the interior was sufficient to allow a small reel to be inserted with just suffici nt clearance to prevent the generation of heat by friction between the walls of the cavity and the reel when this was rotated to wind the film.. The space between the inner and outer walls was liquid tight. The entire instrument was enclosed in a wooden box and packed with wool fibre to prevent loss of heat from the calorimeter to the surrounding air. The water equivalent of this instrument was approximately 2500 gram-calories. The liquid which filled the space between the inner and outer walls was circulated by a small pump that operated on the principle of the ar chimed es screw. The film, upon leaving the projecting machine, was introduced into the calorimeter and wound on a reel. The path of the film on entering the calorimeter was between two very thin silver plates, both of which were in contact with the jacketing liquid. Although the film did not come into actual contact with these surfaces, the clearance was but a few thousandths of an inch. It was thought that this condition would result in a temperature equilibrium between the film and the calorimeter. The reel inside upon which the film was wound was driven by a motor externally situated. A thermometer was inserted in the circulating liquid between the outer and inner walls for measuring the change in temperature of the system. When this calorimeter was tried out, the results obtained were not entirely satisfactory. Precisely repeatable results could not be obtained.
An analysis of the results finally led to the conclusion that the heat carried in by the film was not entirely absorbed by the calorimetric system in a sufficiently short time to yield reliable measurements. Complete temperature equilibrium between the calorimeter and the warm film from the projector was not reached before the film was wound into a solid roll. After the film was wound into a roll, the transference of the heat by conduction through the film itself and by conduction and convection through the air in the inner chamber to the walls of the calorimeter was so slow that the loss of heat from
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Fig. 1. Cooling curve for tightly rolled film 90