66 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
erty to put but one interpretation upon average drop. By 
it we should mean in every case that a certain specified 
proportion of the energy delivered to the line during a par- 
ticular period is to be lost in the transmission. On this basis 
we can design the system for conditions of maximum econ- 
omy, knowing approximately the probable cost of energy 
per kilowatt hour and the price of copper. Starting with 
this definition, we can then intelligently work out the re- 
lation of this average energy loss to the loss in volts at the 
various parts of the system. It is necessary however to 
bear in mind, first, that the same conditions of economy 
with respect to loss in transmission do not necessarily hold 
for all parts of a given system, and second, the question of 
economy in transmission is quite subordinate to that of 
successful operation. 
Asregards the former consideration, the average energy 
delivered to an electric railway system 1is a very different 
thing from either the maximum energy or the average 
energy during the hours of heavy load. The load factor, 
i. e., the ratio between average and maximum output on a 
railway system is generally rather unsatisfactory, as has al- 
ready been indicated. It ranges in general from .3 to .6, 
varying greatly with the size of the system, the character 
of the service and the habits of the people who ride. In 
cities many interesting facts appear from the load curve 
of an electric railway—the movements of workingmen, 
the crowd of shoppers going downtown in the forenoon, 
the migration in the early afternoon, the homegoing at 
six and the theatre crowd an hour and a half later. All 
these factors of load operate with varying force, not only in 
different places, but in different parts of the same system. 
The changes from day to day are considerable, but on the 
whole the same line preserves its character remarkably 
well. ‘The result of a varying load factor is a necessary 
limitation in the permissible loss of energy. For if we 
have a load factor of .3, the average loss of energy, what- 
ever economy of transmission may indicate must not be 
enough to cause at maximum load a drop in voltage suffi-