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