147 
Tae Use oF THE PoLARISCOPE IN Trestrinc HicH TENSION 
INSULATORS. 
By C. 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 eftect, 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 yolt transmission line were sub- 
jected to the polarized light test. These 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 
