ELECTRICAL CONDUCTIVITY OF COTTON 443 



curves indicates that similar hysteresis in the log I.R. — R.H. curves is 

 due to adsorption of different amounts of moisture by cotton, even 

 when exposed to the same relative humidity. The amount of moisture 

 adsorbed is dependent upon the direction from which equilibrium is 

 approached. 



Unfortunately, the behavior of cotton is still further complicated, 

 so that additional precautions must be taken in measuring its electrical 

 properties. 



The difference in the effect of saturation on the area of the hysteresis 

 loops for raw and water-boiled cotton as shown by the log I.R. — per 

 cent R.H. and per cent R.H. — per cent M.C. curves suggests that 

 some change in structure of cotton occurs when it absorbs much 

 moisture, and this change in structure has a more or less permanent 

 effect on the subsequent behavior of the material. Verification of this 

 suggestion is found in the log I.R. — log per cent M.C. relation which 

 will now be discussed. The study of this log relation has led to many 

 improvements in methods now employed in the fundamental in- 

 vestigation of the electrical properties of cotton and in inspection 

 methods employed in the commercial purification of cotton for elec- 

 trical purposes. 



Insulation Resistance- Moisture Content Data 

 The curves expressing the relation between log I.R. — log per cent 

 M.C. are shown, in Figs. 2 and 3, to be curved, and not linear over 

 the whole range as suggested in an earlier paper.^ The data on 

 raw cotton extends over the wider range, and the curve appears to 

 be sigmoid in shape, exhibiting curvature above 10 per cent and below 

 3 per cent moisture content. Only in the middle range between these 

 moisture content limits is the curve sufficiently linear so that equation 

 II applies. The accuracy of the curve below about 5 per cent M.C. 

 progressively decreases, due to difficulties in measuring the extremely 

 high resistances, and about all that can be said of this range at present 

 is that the log I.R. — per cent M.C. relation expressed by equation I, 

 appears to fit the data better than the log I.R.— log per cent M.C. 

 relation as expressed by equation II. 



The definite curvature above 10 per cent M.C, not observed 

 previously,- was found through the use of the dynamic method and 

 the measurement of insulation resistance and moisture content 

 values simultaneously on similar samples of cotton taken from the 

 same supply." 



" In the vicinity of saturation, an effect similar to polarization can cause errors in 

 the measurement of insulation resistance. The errors result in high insulation 

 resistance values, accentuating the curvature of the curve above 10 per cent moisture 



