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-