138 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
efficiency by the loss in the raising transformers, say, two 
per cent, and the depreciation of the line copper is less 
than that of the transformers. On the other hand the de- 
preciation in the high voltage generators is somewhat 
greater than in the low voltage ones used with transform- 
ers and the inductive drop in the line is more considerable. 
Setting these respective qualities off against each other we 
  
can say for an approximation that the cost of transmission 
becomes equal by the two methods when three-fourths of 
the cost of copper for transmission at 5000 volts amounts 
to $10 per kilowatt delivered. ‘The distance usually cor- 
responding to this condition is ten or twelve miles. In 
most cases, therefore, transmission of power for railway 
purposes is undertaken only at distances so great as to call 
for the use of raising transformers. 
One cannot well theorize on this matter, however, 
since the prices of copper and apparatus are subject to fre-