nted by Edison, such as the multipolar, the
disk dynamo, and the armature with two windings, for sub-station
distribution; indeed, it is not possible within our limited space to
present even a brief digest of Edison's great and comprehensive work on
the dynamo-electric machine, as embodied in his extensive experiments
and in over one hundred patents granted to him. We have, therefore,
confined ourselves to the indication of a few salient and basic
features, leaving it to the interested student to examine the patents
and the technical literature of the long period of time over which
Edison's labors were extended.

Although he has not given any attention to the subject of generators for
many years, an interesting instance of his incisive method of overcoming
minor difficulties occurred while the present volumes were under
preparation (1909). Carbon for commutator brushes has been superseded
by graphite in some cases, the latter material being found much more
advantageous, electrically. Trouble developed, however, for the reason
that while carbon was hard and would wear away the mica insulation
simultaneously with the copper, graphite, being softer, would wear
away only the copper, leaving ridges of mica and thus causing sparking
through unequal contact. At this point Edison was asked to diagnose the
trouble and provide a remedy. He suggested the cutting out of the mica
pieces almost to the bottom, leaving the commutator bars separated by
air-spaces. This scheme was objected to on the ground that particles
of graphite would fill these air-spaces and cause a short-circuit. His
answer was that the air-spaces constituted the value of his plan, as
the particles of graphite falling into them would be thrown out by the
action of centrifugal force as the commutator revolved. And thus it
occurred as a matter of fact, and the trouble was remedied. This idea
was subsequently adopted by a great manufacturer of generators.

XI. THE EDISON FEEDER SYSTEM

TO quote from the preamble of the specifications of United States
Patent No. 264,642, issued to Thomas A. Edison September 19, 1882: "This
invention relates to a method of equalizing the tension or 'pressure'
of the current through an entire system of electric lighting or other
translation of electric force, preventing what is ordinarily known as a
'drop' in those portions of the system the more remote from the central
station...."

The problem which was solved by the Edison