DESIGN' FACTORS IXFLUKN'riN'O IIKMA HI I,rr> oK UKI-AVS !)(S3 



serious conditions. A sroup of coils aw su])jo('<o(l to 90 per ccul iclnlixc 

 humidity at 85°F willi iH'.ualixc polciilial ap])li('(! 1o (lie imici- \\iii(liii<i 

 and positive potential lo I lie outer w iiidiii.u. No ciiniMit flows in I he w iiid- 

 iugs. Dc^peiidiiiiL!; upon the l\pe of a|)para1us in which the coil is used, 

 other parts of the stiucture may he made positix'e or ne<2;ati\'(» to sinuilatc! 

 actual service conditions. When electrolytic action takes place, c()i)per 

 is always eaten away from the positive electrode. Consecjuently in prac- 

 tice where there is a choice, it is Ix^tter to keej) the winding negative with 

 •espect to its surroundings. During a 8.") to 40 day period, continuity 

 checks are made periodically using a Wheatstone Bridge ha\'ing a battery 

 supply of U A'olts in a series with 10,000 ohms. This method of test does 

 not pro\i(le high enough voltage nor permit flow of sufficient cuiicnt 

 to establish continuity through a miinite length where the wire may be 

 corroded through, nor does it cause a reduced section of wire to burn out. 

 In other words, this method of test does not restore continuity in a cor- 

 roded through section nor does it destroy metallic continuity. Thus more 

 consistent results are obtained than if higher voltages or currents were 

 to be used. The marked superiority of the cellulose acetate insulated 

 coil is apparent, and experience with its use in service has shown that 

 corrosion failures have been eliminated. 



From time to time the question arises as to how the cellulose acetate 

 insulated coil compares with coils vacuum impregnated with a varnish 

 and employing other types of insulation for use in equipment for the 

 Armed Services where atmospheric conditions are more severe than those 

 ordinarily encountered in the telephone plant. Frequently specifications 

 for these applications require impregnation of the windings. Tests have 

 showii that impregnation will extend the life of a coil employing inferior 

 materials, but that corrosion will take place in a shorter period than 

 where cellulose acetate is used throughout without impregnation. 



llesults of such tests are shown in Fig. 4 where the various groups of 

 coils were kept in a humidity chamber at 95 per cent relati\'e humidity. 

 The temperatiu'c within the chamber was raised and lowered between 

 limits of 85° and 150°F in cycles so as to produce severe condensation 

 on the coils. Each cycle, plotted as abscissae represents 24 hours of expo- 

 sure. The top two curves IIIA and IIIB show the failure rates of two 

 groups of coils constructed exactly alike except that one group was im- 

 pregnated while the other was not. They were cellulose acetate filled coils, 

 but used lead-out wires insulated with commercial grades of l)i-aided 

 cotton. The two curves II A and IIB represent the results on two groups 

 of spool wound coils ha\'ing cellulo.se acetate core and interwinding insu- 

 lation, but provided with x'incellatate muslin covers and red-rope paper 



