124 POWER DISTRIBUAION FOR EILECIRIC RAILROADS. 
about 5500 volts, at which it is transmitted to Ayer’s Mills 
and to Nashua. The frequency of the thre= phase current 
is about thirty complete cycles per second—3600 alterna- 
tions per minute. 
The four generators are habitually operated in parallel 
as is.the case with many of the recent polyphase stations, 
  
FIG. 69. 
and the whole plant from end to end is substantially in 
parallel. The use of composite generators in this case 
seems to be somewhat unnecessary, as there is little 
reason to suppose that they will ever be diverted from 
their present function to supply an extra demand for con- 
tinuous current. The generators, too, are belted to a 
common shaft instead of being direct belted or coupled. 
Altogether the generating plant could have been decidedly 
improved by more specialization. On the other hand the 
substations are well planned. ‘That at Ayer’s Mills is a 
compact frame building divided into two rooms. - One of 
these contains the bank of reducing transformers and the