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BELL SYSTEM TECHNICAL JOURNAL 



possible to maintain the thermocouples to within 0.01° C. of each 

 other, and the temperature at any point within the thermostat re- 

 mained constant to at least 0.01° C. 



For several years prior to the development of the flowing air stream, 

 or "dynamic" method of testing textiles, insulation-resistance measure- 

 ments had been made on samples mounted on electrodes in a closed 

 vessel in which 76 per cent R.H. was maintained by saturated NaCl 

 solution. This vessel, in turn, was placed in an air thermostat nearly 

 surrounded by a water bath maintained at 25° C. ± 0.1° C. Since 

 the atmosphere above the salt solution is relatively stationary as com- 

 pared with that in the flowing stream method, this procedure is 

 defined as a "static" method. A statistical analysis, made by Dr. 

 W. A. Shewhart of these laboratories, on data taken with both the 

 static and dynamic methods, using samples from the same spool of 

 cotton, clearly showed the superiority of the dynamic method."^ 



Experimental Data 



Table I contains equilibrium data on moisture content and insulation 



resistance measurements of raw cotton made at a series of different 



relative humidities at 25° C, in both absorbing and desorbing cycles. 



Tables II and III contain similar data for two samples of water- 



TABLE I 

 Moisture Content and Insulation Resistance Data on Raw Cotton in 

 Equilibrium with Constant Atmospheric Humidities during Re- 

 peated Absorption and Desorption Cycles at 25° C. 



Equilibrium Relative 

 Humidity at 25° C. 



% 



Moisture Content 



7o M.C. log % M.C. 



Insulation Resistance per 

 5-in. Length of 30/2- 

 ply Cotton Thread 



megohms log megohms 



First Cycle of Increasing Humidity — Absorption 



17.6 



26.3 (2 hours) 



26.3 (overnight — 20 hours) . . . . 



45.7 



6L0 



7L5 (3 hours) 



7L5 (overnight — 21 hours) . . . . 



82.3 



87 5 



92.1 '{d hours). '.'.'.'.'.'.'.'.'.'.'.'.'.'.'. 



93.0 (overnight — 24 hours) . . . . 



99.2 



Saturated air (1 hour exposure) 



.76x109 

 .18x10^ 

 21 xlO" 

 03x10" 

 81 XlO^ 

 .33x10^ 

 .84X103 

 .61 X103 

 05X10'^ 

 58X10-' 

 6 

 .0 

 .75 

 17 



9.25 

 8.34 

 7.34 

 7.31 

 5.76 

 4.80 

 3.95 

 3.94 

 3.02 

 2.41 

 1.62 

 1.58 

 0.76 

 0.62 



'" This analysis has been published by Dr. Shewhart, as an illustration of testing 

 control in a book, "Economic Control of Quality of Alanufactured Product," I). 

 \'an Nostrand, 1931. His conclusion regarding this analysis was, "We assume, 

 therefore, upon the basis of this test, that it is not feasible for research to go much 

 further in eliminating causes of variability." Page 21. 



