149 
possible unequal strain from the line wire could be considered a cause 
for breakage. Only four whole insulators were available for this test, 
two of which had marked internal stresses, while in the other two the 
stresses were almost negligible. The insulators were broken by placing 
them upon an iron pin as in service and by exerting a strain upon them 
in the direction of the line wire. One insulator which was poorly an- 
nealed broke at 960 pounds, while the others failed at 1,890, 1,675 and 
2,220 pounds respectively, the latter being one which was also poorly an- 
nealed. While this test did not show very conclusively that the poorly 
annealed insulators were weak mechanically, it is believed that if the 
pull in the latter case, could have been in such a direction as to cause the 
insulator to break alon& strained internal planes as was probably the case 
in the first test, the latter insulator as well would have been found to 
have been weak mechanically. For conclusive evidence of this fact, how- 
ever, a much larger number of tests should be available. 
It will be seen from the foregoing, therefore, that a very practical 
use has been made of the phenomenon which has so long been only an in- 
teresting physical experiment. With the aid of the polariscope it is not 
only possible to determine some of the causes for the unsatisfactory service 
given by certain glass insulators, but it is also possible to make preliminary 
acceptance tests upon new insulators and to eliminate all of those which 
show signs of improper annealing and which for this reason would be un- 
desirable for installation where they must be subjected, not only to severe 
electrical and mechanical strains, but also to vibration and sudden tem- 
perature changes. Although porcelain is rapidly supplanting glass for 
high tension insulators, it is expected that this method of test will be used 
in the future to advantage and that it will prove of equal, if not greater 
value, than it has in this particular instaice. 
[11—21368] 
