on having a very large
cross-section, which was a new thing in those days. Great attention was
also paid to all the joints, which were smoothed down so as to make a
perfect magnetic contact. The Edison dynamo, with its large masses of
iron, was a vivid contrast to the then existing types with their meagre
quantities of the ferric element. Edison also made tests on his field
magnets by slowly raising the strength of the exciting current, so that
he obtained figures similar to those shown by a magnetic curve, and in
this way found where saturation commenced, and where it was useless to
expend more current on the field. If he had asked Upton at the time to
formulate the results of his work in this direction, for publication, he
would have anticipated the historic work on magnetism that was executed
by the two other investigators; Hopkinson and Kapp, later on.

The laboratory note-books of the period bear abundant evidence of the
systematic and searching nature of these experiments and investigations,
in the hundreds of pages of notes, sketches, calculations, and tables
made at the time by Edison, Upton, Batchelor, Jehl, and by others who
from time to time were intrusted with special experiments to
elucidate some particular point. Mr. Jehl says: "The experiments on
armature-winding were also very interesting. Edison had a number of
small wooden cores made, at both ends of which we inserted little brass
nails, and we wound the wooden cores with twine as if it were wire on an
armature. In this way we studied armature-winding, and had matches where
each of us had a core, while bets were made as to who would be the first
to finish properly and correctly a certain kind of winding. Care had
to be taken that the wound core corresponded to the direction of the
current, supposing it were placed in a field and revolved. After Edison
had decided this question, Upton made drawings and tables from which the
real armatures were wound and connected to the commutator. To a
student of to-day all this seems simple, but in those days the art
of constructing dynamos was about as dark as air navigation is at
present.... Edison also improved the armature by dividing it and the
commutator into a far greater number of sections than up to that time
had been the practice. He was also the first to use mica in insulating
the commutator sections from each other."

In the mean time, during the progress of the investigations on the
dynamo, word had gone