A POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
the Boston network which lies within a mile radius from 
the Post Office as a center. It conveys an idea, better than 
any words, of the sort of network that occurs in practice. 
It differs totally from the networks usually met with in elec- 
tric lighting, in that it is without any pretense of sym- 
metry, either in configuration or load. 
In all large installations one is likely to find all three 
types of distribution, usually a network in the center, and 
branched and linear distribution in the outlying districts. 
In laying out the system as a whole, each type must con- 
form, as far as practicable, to its own conditions of economy, 
while the general feeding 
system must consider 
them all. 
The starting point in 
any discussion of a con- 
ducting system for any 
purpose is Ohm’s law in 
its simplest form 
Sl 
-—E. 
In problems of distri- 
bution such as we are 
considering, the term in- 
volving R 1is usually the 
quantity sought, since 
the.current and loss of potential are generally known 
or assumed. It is therefore desirable to transform this 
simple equation into some form which allows the ready 
substitution of the known quantities to determine the un- 
known. ‘The resistance of any conductor may be written 
  
FIG. 4. 
) 
R= K—‘I&fi— in which A is the cross section, L the length 
and K a constant depending on the material considered and 
the units in which I, and A are measured. If I,is in feet 
and A in square inches the constant is obviously different 
from what it would be if I, were taken in miles. The con- 
stant is, in practice, so taken that R will be in ohms when