CHAPIER IV, 
SPECIAI, METHODS OF DISTRIBUTION. 
It is quite obvious that the use of about 500 volts as 
working potential for railway purposes entails a very 
serious cost of copper on lines of any considerable length, 
for in general the cost of copper for a given proportion of 
energy wasted varies inversely with the square of the 
voltage. 
For instance, to deliver 500 amperes at ten miles dis- 
tance would require, even with a gross drop of 150 volts, 
about 2,000,000 ¢. m. of copper area weighing about three 
tons per rooo ft.; in all over 150 tons, costing not far from 
$45,000, about $225 per kilowatt of energy delivered. 
It is, of course, highly desirable to find means for 
reducing this excessive cost and all sorts of expedients 
have been tried to thatend. ‘T'he gross loss above assumed 
is about as great as can be permitted, since on a line with 
distributed load more loss and greater overcompounding is 
likely to interfere with the proper performance of the 
motors and the regularity of the schedule. Very heavy 
overcompounding increases the cost of the generators and 
leads to extremes of voltage. In dealing with such a case 
as that just cited the most frequently advantageous 
method would be to fall back on some of the regular 
methods of power transmission which will be desctibed 
later, but under some circumstances the substation involved 
in these methods is undesirable, and one must either stand 
the heavy expenditure for copper or adopt some special 
means for reducing it. 
There are several of these that are in fairly successful 
use. Of those which require no special devices in connec-. 
tion with the motors the most generally applicable are the