CHAPILER 1. 
FUNDAMENTAIL PRINCIPLES. 
The distribution of electrical energy for use in propel- 
ling railway cars is, by nature, a special problem. It deals 
with magnitudes and distances greater than are usual in 
other branches of electrical engineering, and, in addition, 
with the difficulties of a load that constantly shifts in 
amount and position. Consequently, the design of a dis- 
tributing system is of singular difficulty. 
In computing the area of conductors, one ordinarily 
assumes the load to be the only independent variable, but 
i1 this case the distance of transmission must be so consid- 
ered, and both quantities are of the most erratic character. 
The general equations can therefore only be solved 
within limits, except in special cases, and even then only 
by very judicious assumptions. It is therefore worth while 
to investigate these limits, their extent and the causes 
which impose them. 
The conducting system of an electric railway, large or 
small, consists of three somewhat distinct parts—the work- 
ing conductor, the return circuit and the feeders. By the 
first is meant that part of the total circuit from which the 
moving contact, carried by the car, immediately derives its 
current. Physically it is a wire or bar, uninsulated as re- 
spects the moving contact, and supported in any position— 
overhead, on the ground or under the ground—that cir- 
cumstances may require. 
The return circuit is, in a large proportion of cases, 
that which receives current from the wheels of the car, and 
is composed, partly or wholly, of the rails. In certain 
cases, conduit roads, double trolley roads and telpher sys- 
tems, the working and return conductors are alike and of