96 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
relative amounts of copper required for a few voltages, that 
necessary at 500 volts initial pressure being taken as 100. 
The same percentage of drop is assumed in each case. 
Volts. Copper. 
500 100.0 
550 82.6 
600 69.4 
650 59.1 
700 5I.0 
750 44.4 
8oo 39.1 
850 34.6 
Q00 30.8 
950 27.7 
1000 25.0 
The actual results are slightly better, even, than these 
figures indicate, since the track return gets relatively better 
and better as the voltage rises and the current diminishes, 
To show this we may profitably take a concrete example. 
Fifty kilowatts is to be transmitted 25,000 ft. for railway 
purposes. ‘The track is of sixty pound rail, and we will 
for simplicity assume that the bonding doubles its resistance. 
The conductivity of the track return is then that of one 
continuous line of sixty pound rail which equals 1,000,000 
c. m. of copper. At 500 volts initial pressure and twenty 
per cent gross loss, the drop through the track circuit 
would be 27.5 volts, leaving 72.5 volts drop for the over- 
head line. This requires about 379,000 ¢. m. At 1000 
volts initial pressure the drop in the track circuit would be 
but 13.75, leaving 186.25 for the overhead system, which 
corresponds to nearly 74,000 c. m., a trifleless than twenty 
per cent of the copper needed for 500 volts, instead of 
twenty-five per cent as called for by the table. 
Obviously this difference depends on the fact that one 
side of the circuit is a fixed quantity of equal conductivity 
for all voltages and currents, while this conductivity is a 
factor entering the computation of the rest of the circuit 
where different circuits are under consideration. 
The economic value of working at higher voltages 
than customary is thus very evident, even if one may not 
be prepared for boldly advancing to 1000 volts or more.