144 POWER DISTRIBUTION FOR ELECTRIC RAILROADS, 
glass insulators will give good results. Such insulators 
have been in use on arc circuits of similar voltage for years 
with uniform success. Glassinsulators of special construc- 
tion with extra deep petticoats have been successfully em- 
ployed with even 10,000 volts alternating. A pole head 
equipped with such insulators is shown in Fig. 82. ‘This 
  
BTG - 82, 
is the type of insulator used in the San Antonio Canyon 
plant, to which reference has been made. 
At so high a pressure, however, porcelain is much to 
be preferred, owing to its higher insulating properties, par- 
ticularly after protracted weathering, and to its great 
mechanical strength. It should be distinctly understood 
that poor porcelain is worse than glass and that to be effect- 
ive as an insulator the porcelain must have not merely a 
surface glaze, but must be strongly vitrified clear through.