SPECIAI, METHODS OF DISTRIBUTION. 93 
The three-wire system was tried as far back as 1889 
in Milwaukee and more recently has been successfully em- 
ployed in Portland, Ore.; Bangor, Me.; St. Louis, and 
elsewhere. 
Practically, balancing with all needful accuracy is 
not a difficult matter and, as has already been mnoted, an 
approximate balance is quite sufficient on account of the 
great capacity of the neutral, particularly on double track 
roads. 
On the whole the three-wire distribution is a very use- 
ful one. 'The saving in copper is very material and en- 
tails no sacrifice of efficiency and but little added expense 
if the station is large enough to make the use of two dyna- 
mos instead of one, of little moment. Above a certain size 
the price of dynamos increases almost directly as their out- 
  
put, so thata pair of machines for three-wire work would 
be little if any more expensive than one large one of equal 
capacity. 
A curiously modified three-wire system has been sug- 
gested for heavy interurban work, although it has not yet 
come into use. ‘This is connected like Fig. 53, except that 
both - and — sides of the system are connected to trolley 
wires over the same track. Two trolleys are used on each 
car so that the car is a unit balanced in itself, the two 
motors taking current from the - and — wires respectively. 
Fig. 57 shows this arrangement in diagram. 
Here A and B are the generators connected respect- 
ively to—+and— trolley wires, and the track forms the 
geutral. ‘Themotors C and D upon the same car take cur- 
rent from the trolleys F and F and are grounded upon the 
track neutral in the ordinary way. The neutral only 
comes into service in the case of need for cutting out one