TRANSMISSION OF POWER FOR SUBSTATIONS. I45 
A good test of quality is to chip through the external sur- 
face and place the point of a well filled pen on the break. 
If the ink flows and produces a spreading stain, and partic- 
ularly if it works under the exterior glaze, the porcelain is 
probably worthless as an insulator. 
N 
N 
NI 
AN NN 
  
il v 
One third size l ‘ o \ [ \ 
  
First class porcelain, however, is an ideal substance for 
insulators and should be used on all long, high voltage 
lines. For this purpose the insulators should all be tested 
and should not break down at double the normal voltage. 
An excellent specimen of these high grade porcelain 
insulators is shown in Fig. 83. ‘This particular form 
was developed for the Niagara-Buffalo transmission line