446 BELL SYSTEM TECHNICAL JOURNAL 



intermediate between the initial absorption and desorption slopes, 

 thus indicating some reversibility in the properties of the cotton which 

 determine these slopes, due to the drying effect after the initial desorp- 

 tion test. Water-boiled cotton does not show this effect, the slope 

 of the second absorption curve being identical with that of the initial 

 desorption curve, under the conditions of drying used for these tests. 

 This behavior is consistent with some experiments made to determine 

 if the initially high insulation resistance observed in some cases with 

 water-boiled cotton could be restored by some simple means after the 

 resistance had been adversely affected by exposure to high atmospheric 

 humidities. 



In the course of some I.R. tests made on washed cotton the control 

 samples of raw cotton used to check each I.R. experiment to assure 

 the same humidity and temperature conditions were found to have 

 suddenly changed from 4.5 kilomegohms — their normal value under 

 the test conditions — to 1.8 kilomegohms under these conditions. 

 These controls had been exposed to atmospheric humidity conditions 

 of 83 per cent R.H. at 32° C, while a new set of washed cotton samples 

 were being prepared for test. Since it was particularly desirable to 

 continue the use of the same control samples, an attempt was made to 

 restore them to their original conditions by drying. Air at less than 

 0.1 per cent R.H. at 25° C, was passed over these samples for 40 

 hours at room temperature. When subsequently measured their 

 resistances had increased from 1.8 to 2.9. Further drying for 48 

 hours at 105° C. caused a further gain of but 0.1 kilomegohm. Con- 

 versely, similar tests on washed cotton showed no improvement. 

 A bundle of washed cotton was dried at 105° C. Instead of giving an 

 I.R. of between 100 and 400 kilomegohms, normal for other similarly 

 washed and dried samples, the resistance was but 23 kilomegohms. 

 Chemical analyses of this cotton gave no indication that this low value 

 was due to electrolytic contamination. Neither redrying of this 

 cotton in a vacuum oven at 80° C, nor drying in an air-oven at 105° C, 

 gave any improvement; in fact the resistance after such redrying was 

 but 18 kilomegohms. 



However, this washed cotton was greatly increased in I.R. by 

 simply rewetting with excess water and drying rapidly at 105° C.^^ 



From this discussion of the data it is seen that three types of linear 



equations may be used to express fairly accurately the relation between 



insulation resistance and the moisture-absorbing properties of cotton 



over a range of atmospheric relative humidity from saturation down 



" Samples A and B used to secure the data in Tables II and III were from this 

 test. After rewetting and oven-drying at 105° C, sample A gave 108 kilomegohms 

 and B gave 63 kilomegohms at 75 per cent R.H. — 25° C. 



