246 



BELL SYSTEM TECHNICAL JOURNAL 

 Characteristics of Purified Insulations 



Obviously, the first consideration in the insulation of electrical 

 conductors is to provide an insulating medium of sufficient dielectric 

 strength to withstand the working potentials to which it is sub- 

 jected. Also, the d.c. insulation resistance must be high enough 



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2 3 4 

 TIME 



5 6 7 

 IN DAYS 



Fig. 2 — D.C. insulation resistance of 50 ft. of twisted pair wire insulated with double 

 servings of equal thickness. 



to prevent undue d.c. energy loss. A comparison of the electrical 

 resistance of the cotton and silk at relative humidities ranging upward 

 from 65 per cent to 90 per cent and down again to 65 per cent, before 

 and after washing, is shown by the graphs in Fig. 1 as determined 

 by samples prepared and tested by the method and testing apparatus 



