Memories Of Menlo Park - 11
Undaunted by the dicta of contemporaneous science, Mr. Edison attacked the dynamo
problem with his accustomed vigor and thoroughness. He chose the drum form for his
armature, and experimented with different kinds of iron. Cores were made of cast iron,
others of forged iron; and still others of sheets of iron of various thicknesses separated
from each other by paper or paint. These cores were then allowed to run in an excited
field, and after a given time their temperature was measured and noted. By such practical
methods Edison found that the thin, laminated cores of sheet iron gave the least heat, and
had the least amount of wasteful eddy currents. His experiments and ideas on magnetism
at that period were far in advance of the time. His work and tests regarding magnetism
were repeated later on by Hopkinson and Kapp, who then elucidated the whole theory
mathematically by means of formulae and constants. Before this, however, Edison had
attained these results by pioneer work, founded on his original reasoning, and utilized
them in the construction of his dynamo, thus revolutionizing the art of building such
machines.
After thorough investigation of the magnetic qualities of different kinds of iron, Edison
began to make a study of winding the cores, first determining the electromotive force
generated per turn of wire at various speeds in fields of different intensities. He also
considered various forms and shapes for the armature, and by methodical and systematic
research obtained the data and best conditions upon which he could build his generator.
In the field magnets of his dynamo he constructed the cores and yoke of forged iron
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."
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