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influence to a certain extent the amount of heat absorbed and retained in the film. In the following pages, the influence of some of these factors is specifically determined but no attempt will be made at the present time to establish definitely the precise influence of all of those mentioned.
A Simplex projector of standard design fitted with the usual arc lamp was used; the lower magazine was removed and the calorimeter was placed directly under the head of the machine, as shown in Figure 7. The film, after leaving the projector, passed over two wooden rollers and was forced into the calorimeter by the sprocket mounted thereon. Considerable practice and skill were required on the part of the operator to obtain synchronism between the sprocket motor and the projector motor so as to force the film into the calorimeter at the same rate that it passed through the projector. In future work which is planned along these lines, the calorimeter sprocket will be driven directly from the projector mechanism, thus eliminating this difficulty.
The initial temperature of the film was in all cases equal to room temperature. No attempt was made to hold it at any fixed value, yet special precaution was taken to make sure that the film was at room temperature. Thirty minutes previous to making a test, the film, which was stored in a wooden box situated in one corner of the projection room, was unwound and allowed to lie in a loose mass on the floor. A current of air was circulated through the film and a thermometer, placed in the pile of film, was used to record the temperature. The difference between the temperature of the film itself and the upper magazine of the projector was very small and it is thought that the initial temperature of the film determined in this way is very reliable. In removing the film from the calorimeter after a run, the same general procedure was followed. The film was piled in a loose heap on the floor and allowed to remain there for a half hour or more before winding into rolls and storing away or being placed in the projector magazine for a repeat test. Thus, the film was at all times, except immediately after passingthrough the projector, at room temperature. In every trial, the speed at which the film passed through the projector was regulated and measured. The calorimetric procedure was similar to that previously described in the measurement of specific heat. The temperature of the calorimeter was observed at periodical intervals for a period of half an hour preceding the introduction of the heated film. Temperature measurements were taken at very frequent intervals while the film was being introduced into the calorimeter and for a short time after the maximum temperature had been reached. Temperature readings were thus taken over a period of half an hour or more in order to determine the shape of the cooling curve (Curve 3, Figure 8).
The arc was focused on the gate for ten or twelve minutes before starting the run. Two minutes before starting, the reel of film was placed in the upper magazine, the film threaded through
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