SPECIAT, METHODS OF DISTRIBUTION. 103 
way of obtaining the lower voltage desired. The three- 
wire system may often be used to advantage in doing the 
terminal work connected with a high voltage interurtan 
line. In conjunction with boosters it may also be decid- 
edly useful in working a long and heavily loaded double 
track line. Fig. 60 shows this arrangement, which it will 
be readily seen is really a modified five-wire system. The 
main generators would then operate directly an ordinary 
three-wire railway system, while with the assistance of the 
boosting dynamos they would furnish current for working 
a heavy suburban or interurban line in the manner just 
described. 
Such are the principal devices for operating extended 
railway lines from a single power station without any trans- 
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Fi1c6. 60. 
formation of voltage. They are easy of application and 
fairly economical, although the voltages dealt with are not 
really high enough for the purposes to which they are 
sometimes applied. There is a steady growth of long lines 
which cannot be economically operated by any of these 
simpler methods, which at best partake something of the 
nature of makeshifts. ‘The time comes when a road be- 
comes too long to be successfully worked from a single 
‘power station even with the assistance of auxiliary dyna- 
mos. A choice has then to be made between operating inde- 
pendent power stations at points along the line, and sub- 
stations similarly located supplied with power from a 
single generating plant by the means usual to the long 
distance transmission of power. The principles involved 
in these important cases it is now our purpose to discuss.