140 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
mere presence in the case assists in transferring the heat 
from the core and coils to the case whence it can be radi- 
ated, and may increase the possible output for the same 
heating by ten per cent or more. In large transformers it 
is usual to go further and to cool the oil artificially either 
by a worm through which cold water is kept circulating or 
by circulating the oil itself through a cooling worm. 
An excellent example of the former practice is shown 
in Fig. 8o, which is a 100 k. w. Westinghouse substation 
  
  
  
  
  
TG, 80; 
transformer taken apart to show the construction. ‘The 
case is an iron cylinder, in which the core and its coils are 
placed. The case is then filled with paraffine oil. Just 
inside the case, between it and the coils, is the cooling 
worm of galvanized iron through which a constant stream 
of cold water is kept flowing. ‘This keeps down the 
temperature so that a large output can be obtained 
without loss of efficiency.  For the efficiency depends on 
the ratio between the output and the sum of the losses 
in the core and the coils. ‘The losses in the former 
are nearly constant, so that if they form a considerable por- 
tion of the total loss the efficiency may even increase with: 
increase of output. 
Another equally effective method of obtaining a high