CHAPTER II. 
THE RETURN CIRCUIT. 
The outgoing circuit of an electric railway has just 
been discussed in its more general relations. Before invest- 
igating the proportioning of the working conductors it is 
necessary to look into the return circuit. Up to this point 
it has been assumed that this is similar to the outgoing 
system as it is in the case of motor systems in general. 
In nearly all electric railway practice it has been the 
custom to employ the rails and earth as the return circuit, 
since the former are good conductors and mnecessarily in 
contact with the car wheels, and the latter is as necessarily 
in contact with the rails. 
In a few casestwo running contacts are employed as 
in the double trolley system, some recent elevated roads, 
and the like, but in most instances the total circuit of any 
railroad consists of the outgoing system of copper conduc- 
tors and a return circuit consisting of the rails and their 
environment. 
Now the conductivity of an iron or steel rail is com- 
puted with tolerable ease, but the rest of this heterogeneous 
system is most uncertain. It consists, near the surface, of 
bond copper, tarnished surfaces, iron rust, rock, dirt, dirty 
“water, mud, wet wood and promiscuous filth, and deeper 
down of all sorts of earthy material, and in cities various 
sorts of pipes for gas, water, etc. 
In the early days of electric railroading the resistance 
of this strange assortment was assumed to be zero on the 
theory that the earth was the conductor concerned and was 
practically of infinite cross section. ‘This was shockingly 
far from the truth and although data are rather scarce, we