448 BELL SYSTEM TECHNICAL JOURNAL 



resistance of both raw and washed cotton when measured at 75 per 

 cent relative humidity and 25° C, ajter exposure to the 88 per cent 

 relative humidity conditions and dried,^^ are given in percentage of 

 the initial 75 per cent — 25° C. insulation resistances. 



TABLE V 



Percentage Reduction in the Insulation Resistance of Raw and Washed 

 Cottons at 75 Per Cent Relative Humidity and 25° C, after Suc- 

 cessive Exposures to 88 Per Cent Relative Humidity at t° C. 



Temperature (<° C.) of the % Reduction in Insulation Resistance at 75% — 



Successive 88% R.H. Cycles 25° C. after each 88% R.H. Cycle 



Washed Raw 



22° C 39% 37% 



30.2° C 64% 58.5% 



38.0° C 72% 64.5% 



22° C 63% 59.5% 



Exposure of cotton to high humidity (in this case 88 per cent) alters 

 the properties of the material in such a way that its insulation resistance 

 when subsequently measured at 75 per cent relative humidity and 

 25° C, is lower than the insulation resistance measured at the 75 per 

 cent condition before such exposure to 88 per cent humidity. This 

 decrease in insulation resistance observed at 75 per cent humidity and 

 25° C, becomes progressively greater the higher the temperature of 

 the 88 per cent humidity exposure, but on again exposing the cotton 

 to 88 per cent humidity at the reduced temperature of 22° C, after 

 exposure at 38° C, the insulation resistance subsequently measured 

 at 75 per cent humidity and 25° C, is greater than after the 88 per 

 cent — 38° exposure, but less than when measured at this condition after 

 the original exposure to 88 per cent humidity and 22° C, thus indi- 

 cating that some reversal occurs in the temperature effect. 



The fact that in each test the percentage reduction is of the same 

 order of magnitude for raw and washed cotton, suggests that the 

 effect is structural and not related to the quantity of electrolytic im- 

 purities which may be present. 



An important feature of the data recorded in Table IV is that the 



insulation resistance of washed cotton is reduced by exposure to 88 



per cent R.H. A natural question is — What would be the resistance 



of this cotton if exposed to 100 per cent R.H. instead of 88 per cent, 



or brought directly to equilibrium with 75 per cent R.H. at 25° C, 



from the wet state without oven-drying? Tests have been made to 



determine these points. Washed cotton, dried at 105° C, then con- 



^^ The samples were dried with a stream of very dry air at 25° C. after each 

 exposure to the 88 per cent humidity conditions to avoid the hysteresis effect, which 

 would occur if the samples were brought back to the 75 per cent humidity condition 

 directly from the higher humidity. Before starting the test all samples were similarly 

 dried. 



