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was the lack of uniform darkening and lighting of the entire field projected 

 upon the screen as the analyzer was turned; while in others, especially 

 in those thai were whole, bright streaks of violet, purple and brown were 

 seen, and found to rotate as the analyzer was rotated. In some cases 

 these colored streaks were radial, and in still others they formed con- 

 centric rings about the knob of the insulator as a center. In order 

 to prove more conclusively that these phenomena were caused by internal 

 stresses, which were in turn produced by poor annealing, a portion of in- 

 sulator No. 1, which showed no initial color rotation, was poorly annealed, 

 and when tested again in the polariscope the color rotation effect was 

 found to have been introduced. Conversely wheu a portion of insulator 

 No. 2 was properly annealed the color rotation initially present was found 

 to have disappeared. 



The insulators which were first tested were those which had actually 

 broken while in service upon the line, the parts of which were found on 

 the ground near the poles where they were formerly installed. When 

 later whole insulators of the same lot were tested it was found that in 

 the latter the stresses were much more marked than in the broken parts. 

 This fact caused the writer to suspect that some of the internal stresses 

 produced by poor annealing were relieved by the breaking of the insulator, 

 and to test this belief a whole insulator showing very marked color rota- 

 tion was broken and the various parts placed in the polariscope for in- 

 spection. It was found that in spite of the fact that the same portion of 

 the insulator which showed the most marked stresses was used when 

 broken out, practically all the color rotation had been eliminated, although 

 the stresses were still present to a less degree in the remainder of the 

 insulator. In turn each quadrant of the umbrella of the insulator was 

 broken out, and in each case the stresses were found to have been either 

 reduced to a minimum or entirely eliminated. A further proof of the poor 

 annealing was found in the fact that in insulators where the greatest 

 stresses were present the umbrella shivered to bits when broken; while 

 from insulators showing lesser stresses a whole quadrant could be broken 

 out in a single piece. 



Although it is very probable that insulators which are improperly 

 annealed fail in service because of sudden temperature changes due to the 

 weather and leakage of current over their surface, it seemed advisable to 

 show, if possible, what effect, if any, the internal stresses had upon the 

 mechanical strength of the insulator in order to determine whether the 



