ELECTRICAL CONDUCTIVITY OF COTTON 449 



ditioned at 100 per cent R.H., gave an I.R. when tested at 75 per cent 

 R.H. at 25° C, of 25 kilomegohms.-' Its insulation resistance on 

 being brought directly from the wet state to 75 per cent R.H. at 25° C, 

 was but 3.7 kilomegohms, being in this case lower than the resistance 

 of raw, unwashed cotton in Table IV. Of course, if the raw cotton 

 could be wet with water without undergoing any change due to 

 reduction in ash content, no doubt its resistance would be much lower 

 than that of similarly treated water-washed cotton, since this effect 

 appears to be structural and certainly is not dependent upon electro- 

 lytic impurities. 



Effect of Temperature of Drying Wet Cotton on its Insulation Resistance 



The higher the temperature at which wet, water-boiled cotton is 



dried, the higher is its insulation resistance. Such cotton, dried at 



105° C, 120° C, and 162° C, from the wet state, gave 139, 171, and 



201 kilomegohms respectively, when subsequently equilibrated at 75 



per cent R.H. at 25° C. 



Theory 



The most important fact to be derived from these experimental 

 data is that cotton may have a range of insulation resistance values 

 for any single moisture content over at least the average atmospheric 

 humidity range, from about 15 to 85 per cent R.H. Another 

 interesting fact is that the insulation resistance of cotton when 

 measured at definite test conditions depends to a surprising extent 

 upon the previous exposure of the material to prevailing atmospheric 

 humidity and temperature conditions, prior to such tests. 



This behavior suggests that the absorption of appreciable quantities 

 of moisture causes changes in the cotton structure, which affect the 

 mechanism of current conduction. This change in structure, no 

 doubt a result of swelling, an effect investigated by Collins,^* appears 

 to be a difificultly reversible alteration in the colloidal gel structure of 

 the cellulose, even after subsequent removal of the moisture by drying. 

 These effects, rather small to be detected by ordinary methods, are 

 revealed by the extremely sensitive electrical tests, since very small 

 changes in moisture content cause large changes in insulation re- 

 sistance. 



Since the substitution of acetyl for hydroxyl groups in cellulose is 

 accompanied by a marked reduction in the moisture adsorption, -^ 



-* This oven-dried material gave 80 kilomegohms when not exposed to the 100 

 per cent R.H. before test. 



2" Collins, Jour. Text. Inst., 21, T311, 1930. 



25 Wilson and Fuwa, Jour. Ind. and Engg. Clieni., 14, 913, 1922. (This lower mois- 

 ture adsorption of cellulose acetate has been observed in our own experiments. See 

 also reference '^.) 



