146 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
in which a voltage of 20,000 is designed to be used. The 
insulators were tested at 40,000 volts and very few had 
to be rejected for failure. They depend for their insu- 
lating power on the quality of the porcelain and on well 
designed double petticoats. Such insulators are admir- 
ably adapted for work up to at least 15,000 volts, pro- 
vided the climate is reasonably 
good. In very moist climates 
where the insulators are exposed 
to frequent searching mists and 
nearly constant dampness still 
further precautions are desirable, 
    
\ 
  
      
  
| 
Z l’j and probably the best results can 
. e 
//// il be secured by using oil insulators 
    
| of which a very good specimen 
""| is shown in section in Fig. 84. 
i 
  
  
I The pin, P, of iron, is cemented 
'W into the body of the insulator, I, 
”“;{ which is made thick and solid. 
The thick bell of the insulator is 
turned inwards and upwards at 
its lower edge so as to form a cir- 
cular cup, C. 'This cup is filled 
with highly insulating oil, which 
is exceedingly efficient in stop- 
ping leakage along the surface of the insulator to the iron 
pin. Insulators similar to this were used in the famous 
Lauffen-Frankfort experiment and proved to be excellent. 
In dry and dusty weather, however, the oil accumulates 
dirt and is likely to be reduced to a species of mud, quite 
destroying its insulating value. 
With respect to the general arrangement of a trans- 
mission line too much care can hardly be taken in keeping 
the circuit away from danger of accidental contact to per- 
sons and things. Bare wire is preferable to insulated since 
it does mot encourage a feeling of false security, and it 
should be distinctly understood that the wires are danger- 
ous and must be let alone. Particular pains should be 
  
FIG. 84.