CHAPTER XXIV
CONCLUSION
254. Numerous apparatus, structural features and principles
have been invented and more or less developed, but have found
a limited industrial application only, or are not used at all, be-
cause there is no industrial demand for them. Nevertheless a
knowledge of these apparatus is of great importance to the elec-
trical engineer. They may be considered as filling the storehouse
of electrical engineering, waiting until they are needed. Very
often, in the development of the industry, a demand arises for
certain types of apparatus, which havQ been known for many
years, but not used, because they offered no material advan-
tage, until with the change of the industrial conditions their
use became very advantageous and this led to their extensive
application.
Thus for instance the commutating pole ("interpole") in
direct-current machines has been known since very many years,
has been discussed and recommended, but used very little, in
short was of practically no industrial importance, while now
practically all larger direct-current machines and synchronous
converters use commutating poles. . For many years, with the
types of direct-current machines in use, the advantage of the
commutating pole did not appear sufficient to compensate for
the disadvantage of the complication and resultant increase of
size and cost. But when with the general introduction of the
steam-turbine high-speed machinery became popular, and higher-
speed designs were introduced in direct-current machinery also,
with correspondingly higher armature reaction and greater need
of commutation control, the use of the commutating pole became
of material advantage in reducing size and cost of apparatus,
and its general introduction followed.
Similarly we have seen the three-phase transformer find gen-
eral introduction, after it had been unused for many years; so
also the alternating-current commutator motor, etc.
Thus for a progressive engineer, it is dangerous not to be famil-
iar with the characteristics .and possibilities of the known but
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