SEISMIC METHODS 819 



quality white stock. The film support is seldom employed in field work 

 due chiefly to three factors: (a) greater cost; (b) developing difficulties 

 encountered in warm weather due to separation of the sensitized emulsion 

 from the film support; (c) greater bulk and weight. Occasionally when 

 very high recording speeds are desired, film is used, because it usually 

 can be obtained in a greater range of emulsion speeds than paper. 



The sensitized emulsion employed for recording is a high contrast 

 emulsion, usually of silver bromide in gelatin. This material is superior 

 to the usual silver chloride emulsion employed in photography, because 

 it is relatively unaffected by the processing procedure. Thus, records ob- 

 tained with the silver bromide emulsion are uniform and not appreciably 

 dependent upon the processing, t 



The developer employed for processing the film should be a high 

 contrast material, with sufficient potassium bromide to restrain fog. 

 Practically all manufacturers of paper supply their own packaged de- 

 velopers. These frequently are found more satisfactory than the bulk 

 materials, chiefly due to their convenience in use, their uniformity, and 

 the elimination of waste and errors in weighing chemicals. 



The developing time should be long enough to give a proper density 

 in the portions of the record which have been properly exposed. The 

 record may be examined occasionally during the development provided 

 a safety lamp is used. The ordinary red light is seldom safe for the 

 modern high speed recording papers. To determine the safeness of a 

 red light, it is convenient to expose half of a test piece of the paper to the 

 light for a few minutes and then develop and fix. Excessive fogging of 

 the exposed portion of the test strip, as compared to the unexposed portion, 

 will indicate that the light should be changed or moved further from the 

 paper during the processing. 



If the developing temperature is maintained at about 65° F., a more 

 concentrated developer will usually give a more dense record. At higher 

 temperatures, the more concentrated developer may produce less effective 

 contrast due to the increase in fog density. The developing speed, also, 

 is greatly dependent upon temperature ; an increase to 85° F. will often 

 cut the developing time in half, while a decrease to 50° F. may necessitate 

 more than twice the normal developing time. The higher temperatures 

 accentuate fog and also cause discoloration due to oxidation of the de- 

 veloper. For best results, the temperature should not exceed 70° F. Often- 

 times, it is difficult in field work to control the developer temperature, and 

 the best compromise is to vary the strength of the solution, i.e., use rela- 

 tively strong solutions at the lower temperatures and diluted solutions at 

 the higher temperatures. The most satisfactory method for uniform 

 records is to employ a tank, the temperature of which is maintained con- 

 stant by a surrounding water bath. Thermal insulation (cork or glass 



t F. A. Tompkins, "Fundamental Photographic Processing Operations Influencing Production 

 of Seismograph Records," Geophysics, 1936, Vol. 1, No. 1, pp. 107-114; "EfTect of Development 

 Time and Developer Temperature on the Production of Photographic Seismograph Records," Geo- 

 physics, 1936, Vol. 1, No. 3, pp. 313-318. 



