TRANSMISSION OF POWER FOR SUBSTATIONS. 139 
quent variations and in actually making contracts these 
variations are very arbitrary in character. Only after the 
bids are opened is it possible to make any fine discrimina- 
tions on the economics of one system or another. It is 
sometimes for the interest of a bidder to cut prices on some 
particular arrangement of apparatus or to raise them on 
another, quite overturning the buyer’s preconceived 
notions on the subject. 
The transformers used in this heavy transmission work 
are very different in appearance from the familiar little 
ones that decorate the poles of 
electric lighting companies, al- 
though, of course, identical in 
principle. 
The output of an alternating 
current transformer, the general 
features of the design remaining 
the same, would naturally, save 
for the question of heating, in- 
crease rather faster than in pro- 
portion to itsaggregate weight of 
copper and iron. But, other 
things being equal, the weight 
increases as the cube of the linear dimensions, while the 
surface increases only as their square. Hence the heat into 
which the energy losses in a transformer are converted has 
less chance to escape by radiation in a large than in a 
small transformer, the available surface area per watt being 
much reduced. Therefore unless there are special precau- 
tions taken the large sizes will run too hot and endanger 
  
FIG. 79. 
the insulation. 
So the ordinary small transformer, of which the core 
and coils are shown in Fig. 79, cannot be indefinitely in- 
creased in size without taking care to provide means for 
compensating the lack of proper radiating surface for get- 
ting rid of the heat. 
There are several methods of doing this. One of the 
best is by filling the transformer case with oil. This by its