winding
consisted of 146 copper bars on the face of the core, connected into a
closed-coil winding by means of 73 copper disks at each end of the core.
The cross-sectional area of each bar was 0.2 square inch their average
length was 42.7 inches, and the copper end-disks were 0.065 inch thick.
The commutator had 73 sections. The armature resistance was 0.0092
ohm, [28] of which 0.0055 ohm was in the armature bars and 0.0037 ohm in
the end-disks." An illustration of the next latest type of this machine
is presented in Fig. 2.

[Footnote 28: Had Edison in Upton's Scientific American
article in 1879 proposed such an exceedingly low armature
resistance for this immense generator (although its ratio
was proportionate to the original machine), his critics
might probably have been sufficiently indignant as to be
unable to express themselves coherently.]

The student may find it interesting to look up Edison's United States
Patents Nos. 242,898, 263,133, 263,146, and 246,647, bearing upon the
construction of the "Jumbo"; also illustrated articles in the technical
journals of the time, among which may be mentioned: Scientific American,
Vol. XLV, page 367; Engineering, London, Vol. XXXII, pages 409 and 419,
The Telegraphic Journal and Electrical Review, London, Vol. IX, pages
431-433, 436-446; La Nature, Paris, 9th year, Part II, pages 408-409;
Zeitschrift fur Angewandte Elektricitaatslehre, Munich and Leipsic, Vol.
IV, pages 4-14; and Dredge's Electric Illumination, 1882, Vol. I, page
261.

The further development of these great machines later on, and their
extensive practical use, are well known and need no further comment,
except in passing it may be noted that subsequent machines had each
a capacity of 1200 lamps of 16 candle-power, and that the armature
resistance was still further reduced to 0.0039 ohm.

Edison's clear insight into the future, as illustrated by his persistent
advocacy of large direct-connected generating units, is abundantly
vindicated by present-day practice. His Jumbo machines, of 175
horse-power, so enormous for their time, have served as prototypes, and
have been succeeded by generators which have constantly grown in size
and capacity until at this time (1910) it is not uncommon to employ
such generating units of a capacity of 14,000 kilowatts, or about 18,666
horse-power.

We have not entered into specific descriptions of the many other forms
of dynamo machines inve