TRANSMISSION OF POWER FOR SUBSTATIONS. 5% 
to give the requisite 120 deg. phase difference. 'The 
windings, are, for convenience or for special purposes, var- 
iously modified in different machines, but the general ar- 
rangements are as just indicated. 
Polyphase generators as a class give a rather better 
output for their weight than single phase machines, owing 
to a better utilization of the armature space by the dis- 
tributed windings. As a rule, too, they represent later 
and better ideas in design, hence areapt to be more efficient 
and to regulate better than ordinary alternators. Perhaps 
the best example of the two phase type is to be found in 
the huge 5000 h. p. Niagara generator, one of which is 
shown in Fig. 86 during the process of assembling, with 
the field ring ready to slip into place. The stationary 
armature has its coils set in deep slots in the laminations 
and is provided with ample ventilating ducts. The arma- 
ture winding does not consist simply of one coil per phase 
per pole as shown in Fig. 78, but each phase winding con- 
sists of a number of coils in adjacent slots, thus occupying 
the armature surface to better advantage. Such a con- 
struction is very often employed in large polyphase 
machines. 
The revolving field is here external to the armature, 
so that its weight gives the effect of a gigantic flywheel. 
The commercial efficiency of this generator at full load 
is almost exactly mninety-seven per cent, a figure due 
to the combination of careful design and immense size. 
These Niagara generators are probably destined to play a 
very important part in the development of electric railroads 
over a radius of many miles. 
Machines with vertical armature shafts are rather rare 
in American practice, the ordinary horizontal arrangement 
being more generally convenient. Hence the usual type 
of polyphase generator is not that found at Niagara, but is 
more nearly akin in appearance to familiar forms. A thor- 
oughly typical example of recent practice in three phase 
generators is shown in Fig. 87. 'This exhibits the dynamo 
room of the three phase transmission plant at Folsom, Cal.,