and closely fitted
joints between the several parts thereof, the magnetic resistance
(reluctance) of the iron parts of the magnetic circuit was reduced to
a minimum, and the required magnetization effected with the maximum
economy. At the same time Mr. Edison announced the commercial necessity
of having the armature of the dynamo of low resistance, as compared
with the external resistance, in order that a large percentage of the
electrical energy developed should be utilized in the lamps, and only a
small percentage lost in the armature, albeit this procedure reduced the
total generating capacity of the machine. He also proposed to make the
resistance of the supply-mains small, as compared with the combined
resistance of the lamps in multiple arc, in order to still further
increase the percentage of energy utilized in the lamps. And likewise to
this end the combined resistance of the generator armatures in multiple
arc was kept relatively small by adjusting the number of generators
operating in multiple at any time to the number of lamps then in use.
The field-magnet circuits of the dynamos were connected in multiple with
a separate energizing source; and the field-current; and strength of
field, were regulated to maintain the required amount of electromotive
force upon the supply-mains under all conditions of load from the
maximum to the minimum number of lamps in use, and to keep the
electromotive force of all machines alike."

Among the earliest of Edison's dynamo experiments were those relating to
the core of the armature. He realized at once that the heat generated in
a solid core was a prolific source of loss. He experimented with
bundles of iron wires variously insulated, also with sheet-iron rolled
cylindrically and covered with iron wire wound concentrically. These
experiments and many others were tried in a great variety of ways,
until, as the result of all this work, Edison arrived at the principle
which has remained in the art to this day. He split up the iron core of
the armature into thin laminations, separated by paper, thus practically
suppressing Foucault currents therein and resulting heating effect.
It was in his machine also that mica was used for the first time as an
insulating medium in a commutator. [27]

[Footnote 27: The commercial manufacture of built-up sheets
of mica for electrical purposes was first established at the
Edison Machine Works, Goerck Street, New York, in 1881.