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intervals in order to eliminate the possibility of scratching the back of the film. The slats illustrated in Figure 26 are fitted with spring adjusters covered with felt to take care of shrinkage during drying, but the adjusters are unnecessary if the film is not wound tightly and is loosened up frequently. Strips of rubber inset along the edges of the slats also prevent scratching of the film.
A hygrometer is a very essential requisite to the drying room. The best known type of recording instrument is one fitted with a motor driven fan which blows a continuous stream of air over the wet bulb thermometer, and which records the dry and wet bulb readings. A simple wet and dry bulb non-recording instrument is quite satisfactory, though it is better to take wet bulb readings by means of a whirling wet bulb thermometer (sling psy chrometer) .
Methods of Removing Excess Water from the Film
In order to eliminate the possibility of the formation of drying marks it is very necessary to remove all superfluous water from the film. This can be performed in two ways as follows:
(a) By wiping the film with absorbent cotton, sponge, or chamois. Owing to the difficulty of keeping a sponge or chamois free from grit they are not recommended. When using absorbent cotton, thoroughly saturate the wad of cotton with water and swab both sides of the film when transferring from the rack to the drying reel. It is important to keep the film well moistened with water because the purpose of this operation is to wash off grit and dirt from the film. Use only very little pressure. After transference to the reel use a fresh wad of cotton partially wrung out and squeegee both back and front of the film. Repeat the operation with the cotton wrung out as dry as possible. No superfluous moisture should now be visible on the gelatine side of the film. In the case of negative, excess moisture should be removed from the back of the film with a chamois.
During drying the film shrinks more or less so that it is necessary to loosen up the film two or three times during drying. It is also necessary to change the position of the film relative to the slats, which is usually taken care of during loosening, because the edge of the slat which is wet dries more slowly than the film and wherever the film touches the slat the rate of drying of the film is retarded and drying marks are apt to be produced.
(b) Excess moisture may be removed from the film when transferring to the drying reel by means of a blast of air or a suction squeegee. The compressed air supply should be well filtered from dust and oil. This method is to be recommended because it eliminates any danger of scratching the film.
Rate of Drying
The time required to dry the film depends on the following factors :
1. The weight of water absorbed by the gelatine film. The quantity of water absorbed is proportional to the thickness of the swollen film or the degree of swelling, so that if the developing solutions and wash water are warm the film swells abnormally and the time required to
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