INFLUENCE OF MOISTURE UPON INSULATORS 235 



of the cotton over the ordinary ranges of atmospheric humidity. 

 The sensitivity of the washed cotton to continued appUcation of 

 voltage is much less than that of the original, but polarization still 

 occurs. Commercial silks are similarly affected by washing. 



If the mineral contents of cottons which have undergone washing 

 are compared quantitatively with the original contents a decrease is 

 observable, particularly as to potash, but the calcium and magnesium 

 contents are much less altered. Fairly complete removal of potash 

 is apparently essential to good electrical characteristics, but improve- 

 ment electrically has been attended in some cases by an actual increase 

 in content of alkaline earths. This suggests that interchange of 

 electrolytic impurities between the textile and the water is involved 

 as well as actual removal of electrolytes by the water. Thus in general 

 hard natural waters, i.e., those containing calcium and magnesium 

 salts, have proved as good or better than soft waters when used in 

 economically small amounts. Very exhaustive extraction with distilled 

 water gives excellent results, though not vastly superior to washing 

 with very dilute solutions of alkaline earth salts. Sufficiently complete 

 and accurate analyses of samples of textiles brought into equilibrium 

 with washing liquids and of the kind and quantity of electrolytes in 

 the corresponding liquids have yet to be made to determine the 

 precise importance of the composition of the saline residues. Non- 

 saline electrolytes have also to be considered. This matter requires 

 extended study and the experimental data are reserved for future 

 publication. 



The commercial value of such treatments of insulating yarns has 

 proved to be very substantial. The utilization of the products forms 

 the subject matter of another paper'** from the Bell Telephone Labo- 

 ratories. 



Another common property of textiles is known as the Evershed 

 effect. Evershed '^ found in various insulating materials, including 

 textiles, that insulation resistance does not obey Ohm's law but is 

 less if a larger measuring voltage is used. Evershed's finding as to 

 cotton has been verified by us. This result is easily obtainable if 

 conditions are maintained so that little polarization occurs. But if 

 extensive polarization is allowed to take place the reverse effect is 

 observed and the ultimate resistance is higher in proportion to the 

 voltage used. The conditions which favor polarization are, of course, 

 considerable voltages, prolonged application, high relative humidities, 

 and short paths through the insulation. Evershed's work apparently 



1^ Glenn, H. H., and Wood, E. B., This Journal. 

 " Evershed, S., Inst. Elec. Eng. Jl. 52, 51 (1914). 



