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The Use of the Polariscope in Testing High Tension 



Insulators. 



By 0. Francis Harding. 



It has long been known that glass internally strained, when placed 

 in a polariscope in the path of polarized light produces upon the screen 

 a chromatic effect, and that the colors thus produced rotate across the 

 field as the analyzer of the polariscope is rotated. If, however, this 

 peculiarity of glass has ever been put to practical use along engineering 

 lines, such usage has not been common and its results have not been made 

 accessible. 



Without going deeply into the theory of the action of polarized light 

 upon crystalline bodies or the similar phenomenon produced by the action 

 of polarized light upon the imaginary planes into which the molecules are 

 forced to arrange themselves within glass subjected to internal stresses, 

 the writer has found the above mentioned color rotation in glass which 

 is internally strained of the utmost value in testing glass insulators. Any 

 ordinary piece of glass which shows no color rotation in the polariscope, 

 when compressed in a vise or clamp and subjected again to the polarized 

 light test exhibits streaks of purple and brown radiating from the points 

 where the pressure is applied. The greater the pressure the brighter and 

 more far-reaching the color effects seem to be, and when the analyzer is 

 turned each color field seems to rotate about the point of application of the 

 compressing force as a center. Similarly if a piece of ordinary glass 

 showing no such effect be heated to a molten state and allowed to cool 

 suddenly, the internal stresses due to irregular and unequal cooling will 

 produce similar color rotation upon the screen. 



With these facts at hand, several high tension glass insulators of 

 different makes designed for a 33,000 volt transmission line were sub- 

 jected to the polarized light test. Th©se of the No. 1 type, manufactured 

 by one company, showed no color rotation in any portion, while those of 

 type No. 2, designed and manufactured by another company for the same 

 service, showed very marked effects. Some of the latter showed results 

 more marked than others. In some the principal peculiarity noticeable 



