SUBSTATIONS. 125 
high tension switches and accessory apparatus, including a 
little direct current motor for furnishing the air blast which 
is used to facilitate dissipation of heat from the transformers. 
Fig. 69 shows the interior of this transformer room. The 
other room is devoted to the two seventy-five kilowatt ro- 
tary converters that form the power equipment, toggther 
with the low tension three phase switchboard and the rail- 
way switchboards for the continuous current side of the 
  
  
  
  
  
FIG. 70. 
machines. This pair of rotary converters is well shown 
in Fig. 70. 'These machines have proved to be singularly 
convenient, being highly efficient, capable of enduring 
severe overloads without difficulty, and generally quite 
unexceptionable in their performance. 
The Nashua substation, C, was, so far as apparatus is 
concerned, practically a duplicate of that at Ayer’s 
Mills.