SPECIAL METHODS OF DISTRIBUTION. 91 
leads must have this area the total weight of copper neces- 
sary is as 11 to 13 compared with the 500 volt arrange- 
ment. ‘The enormous conductivity of the neutral, however, 
renders the matter of balance of comparatively little im- 
portance in this case. 
The somewhat anomalous character of this result has 
its origin in the fact that the track, which is or ought to be 
a first class conductor, is fully utilized in Fig. 52, while in 
Fig. 53 it can only come to the rescue when the system is 
unbalanced. 
Thisarrangement of the three-wire system iscapable of 
accomplishing a notable saving of copper only when the track 
is so poor as a conductor that 14 or 15 has to be used as the 
constant in the computation concerning Fig. 52. There is, 
however, another distinct species of three-wire system which 
is capable of giving greater economy for certain work. 
Suppose we make connections as in Fig. 54. Here 
the outside wires are connected one to an overhead line, 
the other to the track, while the second overhead line 
serves as the neutral. ‘The motors may then be connected 
either on 500 or 1000 volts, an arrangement which would 
be valuable for interurban work with special motors. 
This arrangement is not, however, suited for general use, 
and should be regarded as a mixed system, which, how- 
ever, is excellently fitted for certain otherwise difficult 
work. It is closely related to a booster system, the 
booster not being used in the ordinary way to compensate 
for line loss, but to give a higher working voltage on 
lines where it is needed. ‘The two dynamos need not be 
of the same voltage. 
The regular three-wire system of Fig. 53 is capable of 
saving from twenty to forty per cent in copper according 
to the character the track return. If well balanced it, of 
course, tends to greatly diminish the electrolytic action on 
buried conductors and hence is desirable per se. The 
saving in copper, while by no means as great as in the 
three-wire system for lighting is still enough to pay for 
the extra trouble of installation and the expedient has been