24 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
the full network is developed. At each stage of compli- 
cation the system may be considered as composed of one 
or more mains with branches, without sensible error, the 
inaccuracy of the assumption being negligible compared 
with the uncertainty produced by the irregular load. 
T‘he variations of load in an electric railway system 
are so prodigious as to render the most careful calculations 
only roughly approximate. ‘They are, in general, of three 
kinds. First, the momentary variations due to accidental 
changes of load incident to the nature of the service. 
Second, periodic general variation of the aggregate load 
caused by the varying conditions of service throughout the 
day. ‘Third, shifting of the load to various points of the 
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TG, A5, 
system, concurrent with the daily variations in total load, 
but bearing to them no simple relation. 
The momentary variations are constantly occurring 
from minute to minute, almost from second to second. 
They are most considerable in street railway systems oper- 
ating but few cars, and their amplitude may then be equal 
even to the maximum total load, and occur in a fraction of 
a minute. Such a condition may easily exist in a plant 
operating eight or ten cars. As the number of cars in~ 
creases, the chance of so great variations diminishes, 
although somewhat slowly. In very large systems, the ex- 
treme amplitude of these oscillations of load may be re- 
duced to twenty or twenty-five per cent. of the total load, 
but they can never disappear entirely. Their effect on the 
design of the conducting system is but small, for the volt- 
age does not have to be kept closely uniform, and the con-