238 BELL SYSTEM TECHNICAL JOURNAL 



fiber and in turn upon the relative humidity of the atmosphere. The 

 more water present in any given fiber, the poorer are the electrical 

 properties. If the amount of water in fibers were the sole determinant 

 of electrical characteristics we would expect the animal fibers to be, 

 at a given humidity, the poorer electrically of the classes enumerated 

 above. But this is emphatically not the case. With respect to 

 electrical properties, we find that the vegetable fibers are inferior to 

 the more hygroscopic animal fibers and are also inferior to the least 

 hygroscopic variety of commercial fiber, viz., cellulose acetate. To 

 make the existing contrast clear we have plotted in Fig. 4 for the 

 several textiles. Log Insulation Resistance vs. Log Moisture Content. 

 When so plotted the values for each kind of fiber fall approximately 

 on straight lines throughout the range of actual measurement. The 

 relative position of the curves for animal fibers to the right and above 

 that for cotton ^^ means that the animal fibers have the better insu- 

 lating qualities in spite of higher hygroscopicity. 



The slopes of these lines have an even greater significance for they 

 indicate the relative sensitivity of the fibers to an increment of 

 moisture. Since the slope for the animal fibers is greater, it is evident 

 that the animal fibers are more sensitive electrically to moisture than 

 cotton. LTnder a given set of conditions they are not only wetter 

 than cotton, but are more sensitive to the effects of further increments 

 of moisture and yet they have a higher insulation resistance. 



In one respect alone can we say that cotton is preferable as an 

 insulating material. It has the merit of being more nearly uniform 

 in behavior under a variety of weather conditions. When the amount 

 of moisture taken up from the surrounding atmosphere by silk or 

 wool is doubled, the electrical leakage increases by a factor of 50,000 

 to 100,000, while that for cotton rises by a factor of only 600. 



The position and slope of the values for cellulose acetate are of 

 great interest as the curve coincides with that for cotton, indicating 

 that moisture affects these two fibers in a very similar manner. The 

 essential electrical difference between these two appears to be satis- 

 factorily accounted for by the fact that cellulose acetate absorbs less 

 water than cotton at any given relative humidity of the atmosphere. 

 The conversion of cotton which is essentially cellulose into cellulose 

 acetate by the process of acetylation, has, as could be predicted on 

 chemical grounds, reduced its hygroscopicity but apparently has not 

 modified its structure greatly. Cellulose acetate is therefore put in a 

 subgroup rather than in an independent classification. 



1^ The threads referred to are approximately, but not precisely, of the same size. 

 This variable is by no means sufficient to account for the higher position of the 

 animal fibers as compared with cotton. 



