sum of scientific knowledge in regard to dynamo-electric machines was so
meagre that the experts of the period should settle upon such a dictum
as this, but such was the fact, as will presently appear. Mechanical
generators of electricity were comparatively new at that time; their
theory and practice were very imperfectly understood; indeed, it is
quite within the bounds of truth to say that the correct principles were
befogged by reason of the lack of practical knowledge of their actual
use. Electricians and scientists of the period had been accustomed for
many years past to look to the chemical battery as the source from which
to obtain electrical energy; and in the practical application of such
energy to telegraphy and kindred uses, much thought and ingenuity had
been expended in studying combinations of connecting such cells so as to
get the best results. In the text-books of the period it was stated as a
settled principle that, in order to obtain the maximum work out of a
set of batteries, the internal resistance must approximately equal the
resistance of the exterior circuit. This principle and its application
in practice were quite correct as regards chemical batteries, but not as
regards dynamo machines. Both were generators of electrical current, but
so different in construction and operation, that rules applicable to the
practical use of the one did not apply with proper commercial efficiency
to the other. At the period under consideration, which may be said to
have been just before dawn of the day of electric light, the philosophy
of the dynamo was seen only in mysterious, hazy outlines--just emerging
from the darkness of departing night. Perhaps it is not surprising,
then, that the dynamo was loosely regarded by electricians as
the practical equivalent of a chemical battery; that many of the
characteristics of performance of the chemical cell were also attributed
to it, and that if the maximum work could be gotten out of a set of
batteries when the internal and external resistances were equal (and
this was commercially the best thing to do), so must it be also with a
dynamo.
It was by no miracle that Edison was far and away ahead of his time
when he undertook to improve the dynamo. He was possessed of absolute
KNOWLEDGE far beyond that of his contemporaries. This he ad acquired by
the hardest kind of work and incessant experiment with magnets of all
kinds during several years preceding, particularly in connection
with his study of automatic telegraphy. His knowledge of magnets was
tremendous. He had studied and experimented with electromagnets in
enormous variety, and knew their peculiarities in charge and discharge,
lag, self-induction, static effects, condenser effects, and the various
other phenomena connected therewith. He had also made collateral studies
of iron, steel, and copper, insulation, winding, etc. Hence, by reason
of this extensive work and knowledge, Edison was naturally in a position
to realize the utter commercial impossibility of the then best dynamo
machine in existence, which had an efficiency of only about 40 per
cent., and was constructed on the "cut-and-try" principle.
He was also naturally in a position to assume the task he set out to
accomplish, of undertaking to plan and-build an improved type of machine
that should be commercial in having an efficiency of at least 90 per
cent. Truly a prodigious undertaking in those dark days, when from the
standpoint of Edison's large experience the most practical and correct
electrical treatise was contained in the Encyclopaedia Britannica, and
in a German publication which Mr. Upton had brought with him after he
had finished his studies with the illustrious Helmholtz. It was at this
period that Mr. Upton commenced his association with Edison, bringing
to the great work the very latest scientific views and the assistance
of the higher mathematics, to which he had devoted his attention for
several years previously.
As some account of Edison's investigations in this connection has
already been given in Chapter XII of the narrative, we shall not enlarge
upon them here, but quote from An Historical Review, by Charles L.
Clarke, Laboratory Assistant at Menlo Park, 1880-81; Chief Engineer of
the Edison Electric Light Company, 1881-84:
"In June, 1879, was published the account of the Edison dynamo-electric
machine that survived in the art. This machine went into extensive
commercial use, and was notable for its very massive and powerful
field-magnets and armature of extremely low resistance as compared with
the combined external resistance of the supply-mains and lamps. By means
of the large masses of iron in the field-magnets, 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.]
Elementary as these principles will appear to the modern student or
engineer, they were denounced as nothing short of absurdity at the time
of their promulgation--especially so with regard to Edison's proposal
to upset the then settled dictum that the armature resistance should
be equal to the external resistance. His proposition was derided in
the technical press of the period, both at home and abroad. As public
opinion can be best illustrated by actual quotation, we shall present a
characteristic instance.
In the Scientific American of October 18, 1879, there appeared an
illustrated article by Mr. Upton on Edison's dynamo machine, in which
Edison's views and claims were set forth. A subsequent issue contained a
somewhat acrimonious letter of criticism by a well-known maker of dynamo
machines. At the risk of being lengthy, we must quote nearly all this
letter: "I can scarcely conceive it as possible that the article on the
above subject '(Edison's Electric Generator)' in last week's Scientific
American could have been written from statements derived from Mr. Edison
himself, inasmuch as so many of the advantages claimed for the machine
described and statements of the results obtained are so manifestly
absurd as to indicate on the part of both writer and prompter a positive
want of knowledge of the electric circuit and the principles governing
the construction and operation of electric machines.
"It is not my intention to criticise the design or construction of the
machine (not because they are not open to criticism), as I am now
and have been for many years engaged in the manufacture of electric
machines, but rather to call attention to the impossibility of
obtaining the described results without destroying the doctrine of the
conservation and correlation of forces.
. . . . .
"It is stated that 'the internal resistance of the armature' of this
machine 'is only 1/2 ohm.' On this fact and the disproportion between
this resistance and that of the external circuit, the theory of the
alleged efficiency of the machine is stated to be based, for we are
informed that, 'while this generator in general principle is the same
as in the best well-known forms, still there is an all-important
difference, which is that it will convert and deliver for useful work
nearly double the number of foot-pounds that any other machine will
under like conditions.'" The writer of this critical letter then
proceeds to quote Mr. Upton's statement of this efficiency: "'Now the
energy converted is distributed over the whole resistance, hence if the
resistance of the machine be represented by 1 and the exterior circuit
by 9, then of the total energy converted nine-tenths will be useful, as
it is outside of the machine, and one-tenth is lost in the resistance of
the machine.'"
After this the critic goes on to say:
"How any one acquainted with the laws of the electric circuit can make
such statements is what I cannot understand. The statement last quoted
is mathematically absurd. It implies either that the machine is
CAPABLE OF INCREASING ITS OWN ELECTROMOTIVE FORCE NINE TIMES WITHOUT
AN INCREASED EXPENDITURE OF POWER, or that external resistance is NOT
resistance to the current induced in the Edison machine.
"Does Mr. Edison, or any one for him, mean to say that r/n enables him
to obtain nE, and that C IS NOT = E / (r/n + R)? If so Mr. Edison has
discovered something MORE than perpetual motion, and Mr. Keely had
better retire from the field.
"Further on the writer (Mr. Upton) gives us another example of this mode
of reasoning when, emboldened and satisfied with the absurd theory above
exposed, he endeavors to prove the cause of the inefficiency of the
Siemens and other machines. Couldn't the writer of the article see that
since C = E/(r + R) that by R/n or by making R = r, the machine would,
according to his theory, have returned more useful current to the
circuit than could be due to the power employed (and in the ratio
indicated), so that there would actually be a creation of force! . . . .
"In conclusion allow me to say that if Mr Edison thinks he has
accomplished so much by the REDUCTION OF THE INTERNAL RESISTANCE of
his machine, that he has much more to do in this direction before his
machine will equal IN THIS RESPECT others already in the market."
Another participant in the controversy on Edison's generator was a
scientific gentleman, who in a long article published in the Scientific
American, in November, 1879, gravely undertook to instruct Edison in
the A B C of electrical principles, and then proceeded to demonstrate
mathematically the IMPOSSIBILITY of doing WHAT EDISON HAD ACTUALLY DONE.
This critic concludes with a gentle rebuke to the inventor for ill-timed
jesting, and a suggestion to furnish AUTHENTIC information!
In the light of facts, as they were and are, this article is so full of
humor that we shall indulge in a few quotations It commences in A B
C fashion as follows: "Electric machines convert mechanical into
electrical energy.... The ratio of yield to consumption is the
expression of the efficiency of the machine.... How many foot-pounds
of electricity can be got out of 100 foot-pounds of mechanical energy?
Certainly not more than 100: certainly less.... The facts and laws
of physics, with the assistance of mathematical logic, never fail to
furnish precious answers to such questions."
The would-be critic then goes on to tabulate tests of certain other
dynamo machines by a committee of the Franklin Institute in 1879, the
results of which showed that these machines returned about 50 per cent.
of the applied mechanical energy, ingenuously remarking: "Why is it that
when we have produced the electricity, half of it must slip away? Some
persons will be content if they are told simply that it is a way which
electricity has of behaving. But there is a satisfactory rational
explanation which I believe can be made plain to persons of ordinary
intelligence. It ought to be known to all those who are making or using
machines. I am grieved to observe that many persons who talk and write
glibly about electricity do not understand it; some even ignore or deny
the fact to be explained."
Here follows HIS explanation, after which he goes on to say: "At this
point plausibly comes in a suggestion that the internal part of the
circuit be made very small and the external part very large. Why
not (say) make the internal part 1 and the external 9, thus saving
nine-tenths and losing only one-tenth? Unfortunately, the suggestion is
not practical; a fallacy is concealed in it."
He then goes on to prove his case mathematically, to his own
satisfaction, following it sadly by condoling with and a warning to
Edison: "But about Edison's electric generator! . . . No one capable of
making the improvements in the telegraph and telephone, for which we are
indebted to Mr. Edison, could be other than an accomplished electrician.
His reputation as a scientist, indeed, is smirched by the newspaper
exaggerations, and no doubt he will be more careful in future. But there
is a danger nearer home, indeed, among his own friends and in his very
household.
". . . The writer of page 242" (the original article) "is probably a
friend of Mr. Edison, but possibly, alas! a wicked partner. Why does
he say such things as these? 'Mr. Edison claims that he realizes 90
per cent. of the power applied to this machine in external work.' . . .
Perhaps the writer is a humorist, and had in his mind Colonel Sellers,
etc., which he could not keep out of a serious discussion; but such
jests are not good.
"Mr. Edison has built a very interesting machine, and he has the
opportunity of making a valuable contribution to the electrical arts by
furnishing authentic accounts of its capabilities."
The foregoing extracts are unavoidably lengthy, but, viewed in the light
of facts, serve to illustrate most clearly that Edison's conceptions and
work were far and away ahead of the comprehension of his contemporaries
in the art, and that his achievements in the line of efficient dynamo
design and construction were indeed truly fundamental and revolutionary
in character. Much more of similar nature to the above could be quoted
from other articles published elsewhere, but the foregoing will serve as
instances generally representing all. In the controversy which
appeared in the columns of the Scientific American, Mr. Upton, Edison's
mathematician, took up the question on his side, and answered the
critics by further elucidations of the principles on which Edison had
founded such remarkable and radical improvements in the art. The type
of Edison's first dynamo-electric machine, the description of which gave
rise to the above controversy, is shown in Fig. 1.
Any account of Edison's work on the dynamo would be incomplete did
it omit to relate his conception and construction of the great
direct-connected steam-driven generator that was the prototype of the
colossal units which are used throughout the world to-day.
In the demonstrating plant installed and operated by him at Menlo
Park in 1880 ten dynamos of eight horse-power each were driven by a
slow-speed engine through a complicated system of counter-shafting,
and, to quote from Mr. Clarke's Historical Review, "it was found that
a considerable percentage of the power of the engine was necessarily
wasted in friction by this method of driving, and to prevent this waste
and thus increase the economy of his system, Mr. Edison conceived
the idea of substituting a single large dynamo for the several small
dynamos, and directly coupling it with the driving engine, and at the
same time preserve the requisite high armature speed by using an engine
of the high-speed type. He also expected to realize still further gains
in economy from the use of a large dynamo in place of several small
machines by a more than correspondingly lower armature resistance, less
energy for magnetizing the field, and for other minor reasons. To the
same end, he intended to supply steam to the engine under a much higher
boiler pressure than was customary in stationary-engine driving at that
time."
The construction of the first one of these large machines was commenced
late in the year 1880. Early in 1881 it was completed and tested, but
some radical defects in armature construction were developed, and it was
also demonstrated that a rate of engine speed too high for continuously
safe and economical operation had been chosen. The machine was laid
aside. An accurate illustration of this machine, as it stood in the
engine-room at Menlo Park, is given in Van Nostrand's Engineering
Magazine, Vol. XXV, opposite page 439, and a brief description is given
on page 450.
With the experience thus gained, Edison began, in the spring of 1881, at
the Edison Machine Works, Goerck Street, New York City, the construction
of the first successful machine of this type. This was the great machine
known as "Jumbo No. 1," which is referred to in the narrative as having
been exhibited at the Paris International Electrical Exposition, where
it was regarded as the wonder of the electrical world. An intimation of
some of the tremendous difficulties encountered in the construction of
this machine has already been given in preceding pages, hence we shall
not now enlarge on the subject, except to note in passing that the
terribly destructive effects of the spark of self-induction and the
arcing following it were first manifested in this powerful machine, but
were finally overcome by Edison after a strenuous application of his
powers to the solution of the problem.
It may be of interest, however, to mention some of its dimensions
and electrical characteristics, quoting again from Mr. Clarke: "The
field-magnet had eight solid cylindrical cores, 8 inches in diameter
and 57 inches long, upon each of which was wound an exciting-coil of 3.2
ohms resistance, consisting of 2184 turns of No. 10 B. W. G. insulated
copper wire, disposed in six layers. The laminated iron core of the
armature, formed of thin iron disks, was 33 3/4 inches long, and had an
internal diameter of 12 1/2 inches, and an external diameter of 26 7/16
inches. It was mounted on a 6-inch shaft. The field-poles were 33 3/4
inches long, and 27 1/2 inches inside diameter The armature 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 invented 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 feeder system was that
relating to the equal distribution of current on a large scale over
extended areas, in order that a constant and uniform electrical pressure
could be maintained in every part of the distribution area without
prohibitory expenditure for copper for mains and conductors.
This problem had a twofold aspect, although each side was inseparably
bound up in the other. On the one hand it was obviously necessary in a
lighting system that each lamp should be of standard candle-power, and
capable of interchangeable use on any part of the system, giving the
same degree of illumination at every point, whether near to or remote
from the source of electrical energy. On the other hand, this must be
accomplished by means of a system of conductors so devised and arranged
that while they would insure the equal pressure thus demanded, their
mass and consequent cost would not exceed the bounds of practical and
commercially economical investment.
The great importance of this invention can be better understood and
appreciated by a brief glance at the state of the art in 1878-79,
when Edison was conducting the final series of investigations which
culminated in his invention of the incandescent lamp and SYSTEM of
lighting. At this time, and for some years previously, the scientific
world had been working on the "subdivision of the electric light," as
it was then termed. Some leading authorities pronounced it absolutely
impossible of achievement on any extended scale, while a very few
others, of more optimistic mind, could see no gleam of light through the
darkness, but confidently hoped for future developments by such workers
as Edison.
The earlier investigators, including those up to the period above named,
thought of the problem as involving the subdivision of a FIXED UNIT
of current, which, being sufficient to cause illumination by one large
lamp, might be divided into a number of small units whose aggregate
light would equal the candle-power of this large lamp. It was found,
however, in their experiments that the contrary effect was produced,
for with every additional lamp introduced in the circuit the total
candle-power decreased instead of increasing. If they were placed in
series the light varied inversely as the SQUARE of the number of lamps
in circuit; while if they were inserted in multiple arc, the light
diminished as the CUBE of the number in circuit. [29] The idea of
maintaining a constant potential and of PROPORTIONING THE CURRENT to
the number of lamps in circuit did not occur to most of these
early investigators as a feasible method of overcoming the supposed
difficulty.
[Footnote 29: M. Fontaine, in his book on Electric Lighting
(1877), showed that with the current of a battery composed
of sixteen elements, one lamp gave an illumination equal to
54 burners; whereas two similar lamps, if introduced in
parallel or multiple arc, gave the light of only 6 1/2
burners in all; three lamps of only 2 burners in all; four
lamps of only 3/4 of one burner, and five lamps of 1/4 of a
burner.]
It would also seem that although the general method of placing
experimental lamps in multiple arc was known at this period, the idea
of "drop" of electrical pressure was imperfectly understood, if, indeed,
realized at all, as a most important item to be considered in attempting
the solution of the problem. As a matter of fact, the investigators
preceding Edison do not seem to have conceived the idea of a "system" at
all; hence it is not surprising to find them far astray from the correct
theory of subdivision of the electric current. It may easily be
believed that the term "subdivision" was a misleading one to these early
experimenters. For a very short time Edison also was thus misled, but
as soon as he perceived that the problem was one involving the
MULTIPLICATION OF CURRENT UNITS, his broad conception of a "system" was
born.
Generally speaking, all conductors of electricity offer more or less
resistance to the passage of current through them and in the technical
terminology of electrical science the word "drop" (when used in
reference to a system of distribution) is used to indicate a fall or
loss of initial electrical pressure arising from the resistance offered
by the copper conductors leading from the source of energy to the lamps.
The result of this resistance is to convert or translate a portion of
the electrical energy into another form--namely, heat, which in the
conductors is USELESS and wasteful and to some extent inevitable in
practice, but is to be avoided and remedied as far as possible.
It is true that in an electric-lighting system there is also a fall or
loss of electrical pressure which occurs in overcoming the much greater
resistance of the filament in an incandescent lamp. In this case there
is also a translation of the energy, but here it accomplishes a USEFUL
purpose, as the energy is converted into the form of light through the
incandescence of the filament. Such a conversion is called "work"
as distinguished from "drop," although a fall of initial electrical
pressure is involved in each case.
The percentage of "drop" varies according to the quantity of copper
used in conductors, both as to cross-section and length. The smaller the
cross-sectional area, the greater the percentage of drop. The practical
effect of this drop would be a loss of illumination in the lamps as we
go farther away from the source of energy. This may be illustrated by
a simple diagram in which G is a generator, or source of energy,
furnishing current at a potential or electrical pressure of 110 volts;
1 and 2 are main conductors, from which 110-volt lamps, L, are taken in
derived circuits. It will be understood that the circuits represented in
Fig. 1 are theoretically supposed to extend over a large area. The main
conductors are sufficiently large in cross-section to offer but little
resistance in those parts which are comparatively near the generator,
but as the current traverses their extended length there is a gradual
increase of resistance to overcome, and consequently the drop increases,
as shown by the figures. The result of the drop in such a case would
be that while the two lamps, or groups, nearest the generator would be
burning at their proper degree of illumination, those beyond would give
lower and lower candle-power, successively, until the last lamp, or
group, would be giving only about two-thirds the light of the first two.
In other words, a very slight drop in voltage means a disproportionately
great loss in illumination. Hence, by using a primitive system of
distribution, such as that shown by Fig. 1, the initial voltage would
have to be so high, in order to obtain the proper candle-power at
the end of the circuit, that the lamps nearest the generator would be
dangerously overheated. It might be suggested as a solution of this
problem that lamps of different voltages could be used. But, as we are
considering systems of extended distribution employing vast numbers of
lamps (as in New York City, where millions are in use), it will be seen
that such a method would lead to inextricable confusion, and therefore
be absolutely out of the question. Inasmuch as the percentage of
drop decreases in proportion to the increased cross-section of the
conductors, the only feasible plan would seem to be to increase their
size to such dimensions as to eliminate the drop altogether, beginning
with conductors of large cross-section and tapering off as necessary.
This would, indeed, obviate the trouble, but, on the other hand, would
give rise to a much more serious difficulty--namely, the enormous
outlay for copper; an outlay so great as to be absolutely prohibitory in
considering the electric lighting of large districts, as now practiced.
Another diagram will probably make this more clear. The reference
figures are used as before, except that the horizontal lines extending
from square marked G represent the main conductors. As each lamp
requires and takes its own proportion of the total current generated,
it is obvious that the size of the conductors to carry the current for
a number of lamps must be as large as the sum of ALL the separate
conductors which would be required to carry the necessary amount of
current to each lamp separately. Hence, in a primitive multiple-arc
system, it was found that the system must have conductors of a size
equal to the aggregate of the individual conductors necessary for every
lamp. Such conductors might either be separate, as shown above (Fig.
2), or be bunched together, or made into a solid tapering conductor, as
shown in the following figure:
The enormous mass of copper needed in such a system can be better
appreciated by a concrete example. Some years ago Mr. W. J. Jenks made
a comparative calculation which showed that such a system of conductors
(known as the "Tree" system), to supply 8640 lamps in a territory
extending over so small an area as nine city blocks, would require
803,250 pounds of copper, which at the then price of 25 cents per pound
would cost $200,812.50!
Such, in brief, was the state of the art, generally speaking, at the
period above named (1878-79). As early in the art as the latter end of
the year 1878, Edison had developed his ideas sufficiently to determine
that the problem of electric illumination by small units could be solved
by using incandescent lamps of high resistance and small radiating
surface, and by distributing currents of constant potential thereto in
multiple arc by means of a ramification of conductors, starting from a
central source and branching therefrom in every direction. This was
an equivalent of the method illustrated in Fig. 3, known as the "Tree"
system, and was, in fact, the system used by Edison in the first
and famous exhibition of his electric light at Menlo Park around the
Christmas period of 1879. He realized, however, that the enormous
investment for copper would militate against the commercial adoption of
electric lighting on an extended scale. His next inventive step
covered the division of a large city district into a number of small
sub-stations supplying current through an interconnected network of
conductors, thus reducing expenditure for copper to some extent, because
each distribution unit was small and limited the drop.
His next development was the radical advancement of the state of the art
to the feeder system, covered by the patent now under discussion.
This invention swept away the tree and other systems, and at one bound
brought into being the possibility of effectively distributing large
currents over extended areas with a commercially reasonable investment
for copper.
The fundamental principles of this invention were, first, to sever
entirely any direct connection of the main conductors with the source of
energy; and, second, to feed current at a constant potential to central
points in such main conductors by means of other conductors, called
"feeders," which were to be connected directly with the source of energy
at the central station. This idea will be made more clear by reference
to the following simple diagram, in which the same letters are used as
before, with additions:
In further elucidation of the diagram, it may be considered that the
mains are laid in the street along a city block, more or less distant
from the station, while the feeders are connected at one end with the
source of energy at the station, their other extremities being connected
to the mains at central points of distribution. Of course, this system
was intended to be applied in every part of a district to be supplied
with current, separate sets of feeders running out from the station to
the various centres. The distribution mains were to be of sufficiently
large size that between their most extreme points the loss would not
be more than 3 volts. Such a slight difference would not make an
appreciable variation in the candle-power of the lamps.
By the application of these principles, the inevitable but useless loss,
or "drop," required by economy might be incurred, but was LOCALIZED IN
THE FEEDERS, where it would not affect the uniformity of illumination
of the lamps in any of the circuits, whether near to or remote from the
station, because any variations of loss in the feeders would not give
rise to similar fluctuations in any lamp circuit. The feeders might be
operated at any desired percentage of loss that would realize economy in
copper, so long as they delivered current to the main conductors at the
potential represented by the average voltage of the lamps.
Thus the feeders could be made comparatively small in cross-section. It
will be at once appreciated that, inasmuch as the mains required to be
laid ONLY along the blocks to be lighted, and were not required to be
run all the way to the central station (which might be half a mile or
more away), the saving of copper by Edison's feeder system was enormous.
Indeed, the comparative calculation of Mr. Jenks, above referred to,
shows that to operate the same number of lights in the same extended
area of territory, the feeder system would require only 128,739 pounds
of copper, which, at the then price of 25 cents per pound, would cost
only $39,185, or A SAVING of $168,627.50 for copper in this very small
district of only nine blocks.
An additional illustration, appealing to the eye, is presented in the
following sketch, in which the comparative masses of copper of the tree
and feeder systems for carrying the same current are shown side by side:
XII. THE THREE-WIRE SYSTEM
THIS invention is covered by United States Patent No. 274,290, issued to
Edison on March 20, 1883. The object of the invention was to provide
for increased economy in the quantity of copper employed for the main
conductors in electric light and power installations of considerable
extent at the same time preserving separate and independent control
of each lamp, motor, or other translating device, upon any one of the
various distribution circuits.
Immediately prior to this invention the highest state of the art of
electrical distribution was represented by Edison's feeder system, which
has already been described as a straight parallel or multiple-arc
system wherein economy of copper was obtained by using separate sets
of conductors--minus load--feeding current at standard potential or
electrical pressure into the mains at centres of distribution.
It should be borne in mind that the incandescent lamp which was accepted
at the time as a standard (and has so remained to the present day) was
a lamp of 110 volts or thereabouts. In using the word "standard,"
therefore, it is intended that the same shall apply to lamps of about
that voltage, as well as to electrical circuits of the approximate
potential to operate them.
Briefly stated, the principle involved in the three-wire system is to
provide main circuits of double the standard potential, so as to operate
standard lamps, or other translating devices, in multiple series of two
to each series; and for the purpose of securing independent, individual
control of each unit, to divide each main circuit into any desired
number of derived circuits of standard potential (properly balanced)
by means of a central compensating conductor which would be normally
neutral, but designed to carry any minor excess of current that might
flow by reason of any temporary unbalancing of either side of the main
circuit.
Reference to the following diagrams will elucidate this principle more
clearly than words alone can do. For the purpose of increased lucidity
we will first show a plain multiple-series system.
In this diagram G<1S> and G<2S> represent two generators, each producing
current at a potential of 110 volts. By connecting them in series this
potential is doubled, thus providing a main circuit (P and N) of 220
volts. The figures marked L represent eight lamps of 110 volts each, in
multiple series of two, in four derived circuits. The arrows indicate
the flow of current. By this method each pair of lamps takes, together,
only the same quantity or volume of current required by a single lamp in
a simple multiple-arc system; and, as the cross-section of a conductor
depends upon the quantity of current carried, such an arrangement as
the above would allow the use of conductors of only one-fourth the
cross-section that would be otherwise required. From the standpoint of
economy of investment such an arrangement would be highly desirable,
but considered commercially it is impracticable because the principle of
independent control of each unit would be lost, as the turning out of a
lamp in any series would mean the extinguishment of its companion also.
By referring to the diagram it will be seen that each series of two
forms one continuous path between the main conductors, and if this path
be broken at any one point current will immediately cease to flow in
that particular series.
Edison, by his invention of the three-wire system, overcame this
difficulty entirely, and at the same time conserved approximately, the
saving of copper, as will be apparent from the following illustration of
that system, in its simplest form.
The reference figures are similar to those in the preceding diagram,
and all conditions are also alike except that a central compensating, or
balancing, conductor, PN, is here introduced. This is technically termed
the "neutral" wire, and in the discharge of its functions lies the
solution of the problem of economical distribution. Theoretically, a
three-wire installation is evenly balanced by wiring for an equal number
of lamps on both sides. If all these lamps were always lighted, burned,
and extinguished simultaneously the central conductor would, in fact,
remain neutral, as there would be no current passing through it, except
from lamp to lamp. In practice, however, no such perfect conditions can
obtain, hence the necessity of the provision for balancing in order to
maintain the principle of independent control of each unit.
It will be apparent that the arrangement shown in Fig. 2 comprises
practically two circuits combined in one system, in which the central
conductor, PN, in case of emergency, serves in two capacities--namely,
as negative to generator G<1S> or as positive to generator G<2S>,
although normally neutral. There are two sides to the system, the
positive side being represented by the conductors P and PN, and the
negative side by the conductors PN and N. Each side, if considered
separately, has a potential of about 110 volts, yet the potential of the
two outside conductors, P and N, is 220 volts. The lamps are 110 volts.
In practical use the operation of the system is as follows: If all the
lamps were lighted the current would flow along P and through each pair
of lamps to N, and so back to the source of energy. In this case the
balance is preserved and the central wire remains neutral, as no return
current flows through it to the source of energy. But let us suppose
that one lamp on the positive side is extinguished. None of the other
lamps is affected thereby, but the system is immediately thrown out of
balance, and on the positive side there is an excess of current to this
extent which flows along or through the central conductor and returns to
the generator, the central conductor thus becoming the negative of that
side of the system for the time being. If the lamp extinguished had been
one of those on the negative side of the system results of a similar
nature would obtain, except that the central conductor would for the
time being become the positive of that side, and the excess of current
would flow through the negative, N, back to the source of energy. Thus
it will be seen that a three-wire system, considered as a whole, is
elastic in that it may operate as one when in balance and as two when
unbalanced, but in either event giving independent control of each unit.
For simplicity of illustration a limited number of circuits, shown in
Fig. 2, has been employed. In practice, however, where great numbers
of lamps are in use (as, for instance, in New York City, where about
7,000,000 lamps are operated from various central stations), there is
constantly occurring more or less change in the balance of many circuits
extending over considerable distances, but of course there is a net
result which is always on one side of the system or the other for the
time being, and this is met by proper adjustment at the appropriate
generator in the station.
In order to make the explanation complete, there is presented another
diagram showing a three-wire system unbalanced:
The reference figures are used as before, but in this case the vertical
lines represent branches taken from the main conductors into buildings
or other spaces to be lighted, and the loops between these branch wires
represent lamps in operation. It will be seen from this sketch that
there are ten lamps on the positive side and twelve on the negative
side. Hence, the net result is an excess of current equal to that
required by two lamps flowing through the central or compensating
conductor, which is now acting as positive to generator G<2S> The arrows
show the assumed direction of flow of current throughout the system,
and the small figures at the arrow-heads the volume of that current
expressed in the number of lamps which it supplies.
The commercial value of this invention may be appreciated from the fact
that by the application of its principles there is effected a saving
of 62 1/2 per cent. of the amount of copper over that which would
be required for conductors in any previously devised two-wire system
carrying the same load. This arises from the fact that by the doubling
of potential the two outside mains are reduced to one-quarter the
cross-section otherwise necessary. A saving of 75 per cent. would thus
be assured, but the addition of a third, or compensating, conductor of
the same cross-section as one of the outside mains reduces the total
saving to 62 1/2 per cent.
The three-wire system is in universal use throughout the world at the
present day.
XIII. EDISON'S ELECTRIC RAILWAY
AS narrated in Chapter XVIII, there were two electric railroads
installed by Edison at Menlo Park--one in 1880, originally a third of a
mile long, but subsequently increased to about a mile in length, and the
other in 1882, about three miles long. As the 1880 road was built very
soon after Edison's notable improvements in dynamo machines, and as the
art of operating them to the best advantage was then being developed,
this early road was somewhat crude as compared with the railroad of
1882; but both were practicable and serviceable for the purpose of
hauling passengers and freight. The scope of the present article will
be confined to a description of the technical details of these two
installations.
The illustration opposite page 454 of the preceding narrative shows the
first Edison locomotive and train of 1880 at Menlo Park.
For the locomotive a four-wheel iron truck was used, and upon it
was mounted one of the long "Z" type 110-volt Edison dynamos, with a
capacity of 75 amperes, which was to be used as a motor. This machine
was laid on its side, its armature being horizontal and located toward
the front of the locomotive.
We now quote from an article by Mr. E. W. Hammer, published in the
Electrical World, New York, June 10, 1899, and afterward elaborated and
reprinted in a volume entitled Edisonia, compiled and published under
the auspices of a committee of the Association of Edison Illuminating
Companies, in 1904: "The gearing originally employed consisted of a
friction-pulley upon the armature shaft, another friction-pulley upon
the driven axle, and a third friction-pulley which could be brought
in contact with the other two by a suitable lever. Each wheel of the
locomotive was made with metallic rim and a centre portion made of wood
or papier-mache. A three-legged spider connected the metal rim of each
front wheel to a brass hub, upon which rested a collecting brush.
The other wheels were subsequently so equipped. It was the intention,
therefore, that the current should enter the locomotive wheels at one
side, and after passing through the metal spiders, collecting brushes
and motor, would pass out through the corresponding brushes, spiders,
and wheels to the other rail."
As to the road: "The rails were light and were spiked to ordinary
sleepers, with a gauge of about three and one-half feet. The sleepers
were laid upon the natural grade, and there was comparatively no effort
made to ballast the road. . . . No special precautions were taken to
insulate the rails from the earth or from each other."
The road started about fifty feet away from the generating station,
which in this case was the machine shop. Two of the "Z" type dynamos
were used for generating the current, which was conveyed to the two
rails of the road by underground conductors.
On Thursday, May 13, 1880, at 4 o'clock in the afternoon, this historic
locomotive made its first trip, packed with as many of the "boys" as
could possibly find a place to hang on. "Everything worked to a charm,
until, in starting up at one end of the road, the friction gearing
was brought into action too suddenly and it was wrecked. This accident
demonstrated that some other method of connecting the armature with the
driven axle should be arranged.
"As thus originally operated, the motor had its field circuit in
permanent connection as a shunt across the rails, and this field circuit
was protected by a safety-catch made by turning up two bare ends of the
wire in its circuit and winding a piece of fine copper wire across from
one bare end to the other. The armature circuit had a switch in it which
permitted the locomotive to be reversed by reversing the direction of
current flow through the armature.
"After some consideration of the gearing question, it was decided to
employ belts instead of the friction-pulleys." Accordingly, Edison
installed on the locomotive a system of belting, including an
idler-pulley which was used by means of a lever to tighten the main
driving-belt, and thus power was applied to the driven axle. This
involved some slipping and consequent burning of belts; also, if the
belt were prematurely tightened, the burning-out of the armature.
This latter event happened a number of times, "and proved to be such
a serious annoyance that resistance-boxes were brought out from the
laboratory and placed upon the locomotive in series with the armature.
This solved the difficulty. The locomotive would be started with these
resistance-boxes in circuit, and after reaching full speed the operator
could plug the various boxes out of circuit, and in that way increase
the speed." To stop, the armature circuit was opened by the main switch
and the brake applied.
This arrangement was generally satisfactory, but the resistance-boxes
scattered about the platform and foot-rests being in the way, Edison
directed that some No. 8 B. & S. copper wire be wound on the lower leg
of the motor field-magnet. "By doing this the resistance was put
where it would take up the least room, and where it would serve as an
additional field-coil when starting the motor, and it replaced all the
resistance-boxes which had heretofore been in plain sight. The boxes
under the seat were still retained in service. The coil of coarse wire
was in series with the armature, just as the resistance-boxes had been,
and could be plugged in or out of circuit at the will of the locomotive
driver. The general arrangement thus secured was operated as long as
this road was in commission."
On this short stretch of road there were many sharp curves and steep
grades, and in consequence of the high speed attained (as high as
forty-two miles an hour) several derailments took place, but fortunately
without serious results. Three cars were in service during the entire
time of operating this 1880 railroad: one a flat-car for freight; one an
open car with two benches placed back to back; and the third a box-car,
familiarly known as the "Pullman." This latter car had an interesting
adjunct in an electric braking system (covered by Edison's Patent No.
248,430). "Each car axle had a large iron disk mounted on and revolving
with it between the poles of a powerful horseshoe electromagnet. The
pole-pieces of the magnet were movable, and would be attracted to the
revolving disk when the magnet was energized, grasping the same and
acting to retard the revolution of the car axle."
Interesting articles on Edison's first electric railroad were published
in the technical and other papers, among which may be mentioned the New
York Herald, May 15 and July 23, 1880; the New York Graphic, July 27,
1880; and the Scientific American, June 6, 1880.
Edison's second electric railroad of 1882 was more pretentious as
regards length, construction, and equipment. It was about three miles
long, of nearly standard gauge, and substantially constructed. Curves
were modified, and grades eliminated where possible by the erection
of numerous trestles. This road also had some features of conventional
railroads, such as sidings, turn-tables, freight platform, and
car-house. "Current was supplied to the road by underground feeder
cables from the dynamo-room of the laboratory. The rails were insulated
from the ties by giving them two coats of japan, baking them in the
oven, and then placing them on pads of tar-impregnated muslin laid
on the ties. The ends of the rails were not japanned, but were
electroplated, to give good contact surfaces for fish-plates and copper
bonds."
The following notes of Mr. Frederick A. Scheffler, who designed the
passenger locomotive for the 1882 road, throw an interesting light on
its technical details:
"In May, 1881, I was engaged by Mr. M. F. Moore, who was the first
General Manager of the Edison Company for Isolated Lighting, as a
draftsman to undertake the work of designing and building Edison's
electric locomotive No. 2.
"Previous to that time I had been employed in the engineering department
of Grant Locomotive Works, Paterson, New Jersey, and the Rhode Island
Locomotive Works, Providence, Rhode Island....
"It was Mr. Edison's idea, as I understood it at that time, to build a
locomotive along the general lines of steam locomotives (at least,
in outward appearance), and to combine in that respect the framework,
truck, and other parts known to be satisfactory in steam locomotives at
the same time.
"This naturally required the services of a draftsman accustomed to
steam-locomotive practice.... Mr. Moore was a man of great railroad and
locomotive experience, and his knowledge in that direction was of great
assistance in the designing and building of this locomotive.
"At that time I had no knowledge of electricity.... One could count
so-called electrical engineers on his fingers then, and have some
fingers left over.
"Consequently, the ELECTRICAL equipment was designed by Mr. Edison and
his assistants. The data and parts, such as motor, rheostat, switches,
etc., were given to me, and my work was to design the supporting frame,
axles, countershafts, driving mechanism, speed control, wheels and
boxes, cab, running board, pilot (or 'cow-catcher'), buffers, and
even supports for the headlight. I believe I also designed a bell and
supports. From this it will be seen that the locomotive had all the
essential paraphernalia to make it LOOK like a steam locomotive.
"The principal part of the outfit was the electric motor. At that
time motors were curiosities. There were no electric motors even for
stationary purposes, except freaks built for experimental uses. This
motor was made from the parts--such as fields, armature, commutator,
shaft and bearings, etc., of an Edison 'Z,' or 60-light dynamo. It was
the only size of dynamo that the Edison Company had marketed at that
time.... As a motor, it was wound to run at maximum speed to develop
a torque equal to about fifteen horse-power with 220 volts. At the
generating station at Menlo Park four Z dynamos of 110 volts were used,
connected two in series, in multiple arc, giving a line voltage of 220.
"The motor was located in the front part of the locomotive, on its side,
with the armature shaft across the frames, or parallel with the driving
axles.
"On account of the high speed of the armature shaft it was not possible
to connect with driving-axles direct, but this was an advantage in one
way, as by introducing an intermediate counter-shaft (corresponding to
the well-known type of double-reduction motor used on trolley-cars since
1885), a fairly good arrangement was obtained to regulate the speed of
the locomotive, exclusive of resistance in the electric circuit.
"Endless leather belting was used to transmit the power from the motor
to the counter-shaft, and from the latter to the driving-wheels, which
were the front pair. A vertical idler-pulley was mounted in a frame over
the belt from motor to counter-shaft, terminating in a vertical screw
and hand-wheel for tightening the belt to increase speed, or the reverse
to lower speed. This hand-wheel was located in the cab, where it was
easily accessible....
"The rough outline sketched below shows the location of motor in
relation to counter-shaft, belting, driving-wheels, idler, etc.:
"On account of both rails being used for circuits, . . . the
driving-wheels had to be split circumferentially and completely
insulated from the axles. This was accomplished by means of heavy wood
blocks well shellacked or otherwise treated to make them water and
weather proof, placed radially on the inside of the wheels, and then
substantially bolted to the hubs and rims of the latter.
"The weight of the locomotive was distributed over the driving-wheels in
the usual locomotive practice by means of springs and equalizers.
"The current was taken from the rims of the driving-wheels by a
three-pronged collector of brass, against which flexible copper brushes
were pressed--a simple manner of overcoming any inequalities of the
road-bed.
"The late Mr. Charles T. Hughes was in charge of the track construction
at Menlo Park.... His work was excellent throughout, and the results
were highly satisfactory so far as they could possibly be with the
arrangement originally planned by Mr. Edison and his assistants.
"Mr. Charles L. Clarke, one of the earliest electrical engineers
employed by Mr. Edison, made a number of tests on this 1882 railroad. I
believe that the engine driving the four Z generators at the power-house
indicated as high as seventy horse-power at the time the locomotive was
actually in service."
The electrical features of the 1882 locomotive were very similar
to those of the earlier one, already described. Shunt and series
field-windings were added to the motor, and the series windings could
be plugged in and out of circuit as desired. The series winding was
supplemented by resistance-boxes, also capable of being plugged in or
out of circuit. These various electrical features are diagrammatically
shown in Fig. 2, which also illustrates the connection with the
generating plant.
We quote again from Mr. Hammer, who says: "The freight-locomotive had
single reduction gears, as is the modern practice, but the power was
applied through a friction-clutch The passenger-locomotive was very
speedy, and ninety passengers have been carried at a time by it; the
freight-locomotive was not so fast, but could pull heavy trains at a
good speed. Many thousand people were carried on this road during 1882."
The general appearance of Edison's electric locomotive of 1882 is shown
in the illustration opposite page 462 of the preceding narrative. In the
picture Mr. Edison may be seen in the cab, and Mr. Insull on the front
platform of the passenger-car.
XIV. TRAIN TELEGRAPHY
WHILE the one-time art of telegraphing to and from moving trains was
essentially a wireless system, and allied in some of its principles to
the art of modern wireless telegraphy through space, the two systems
cannot, strictly speaking be regarded as identical, as the practice of
the former was based entirely on the phenomenon of induction.
Briefly described in outline, the train telegraph system consisted of
an induction circuit obtained by laying strips of metal along the top or
roof of a railway-car, and the installation of a special telegraph
line running parallel with the track and strung on poles of only medium
height. The train, and also each signalling station, was equipped
with regulation telegraph apparatus, such as battery, key, relay, and
sounder, together with induction-coil and condenser. In addition, there
was a special transmitting device in the shape of a musical reed, or
"buzzer." In practice, this buzzer was continuously operated at a speed
of about five hundred vibrations per second by an auxiliary battery. Its
vibrations were broken by means of a telegraph key into long and
short periods, representing Morse characters, which were transmitted
inductively from the train circuit to the pole line or vice versa, and
received by the operator at the other end through a high-resistance
telephone receiver inserted in the secondary circuit of the
induction-coil.
The accompanying diagrammatic sketch of a simple form of the system, as
installed on a car, will probably serve to make this more clear.
An insulated wire runs from the metallic layers on the roof of the car
to switch S, which is shown open in the sketch. When a message is to be
received on the car from a station more or less remote, the switch
is thrown to the left to connect with a wire running to the telephone
receiver, T. The other wire from this receiver is run down to one of
the axles and there permanently connected, thus making a ground. The
operator puts the receiver to his ear and listens for the message, which
the telephone renders audible in the Morse characters.
If a message is to be transmitted from the car to a receiving station,
near or distant, the switch, S, is thrown to the other side, thus
connecting with a wire leading to one end of the secondary of
induction-coil C. The other end of the secondary is connected with the
grounding wire. The primary of the induction-coil is connected as shown,
one end going to key K and the other to the buzzer circuit. The other
side of the key is connected to the transmitting battery, while the
opposite pole of this battery is connected in the buzzer circuit. The
buzzer, R, is maintained in rapid vibration by its independent auxiliary
battery, B<1S>.
When the key is pressed down the circuit is closed, and current from
the transmitting battery, B, passes through primary of the coil, C, and
induces a current of greatly increased potential in the secondary.
The current as it passes into the primary, being broken up into short
impulses by the tremendously rapid vibrations of the buzzer, induces
similarly rapid waves of high potential in the secondary, and these
in turn pass to the roof and thence through the intervening air by
induction to the telegraph wire. By a continued lifting and depression
of the key in the regular manner, these waves are broken up into long
and short periods, and are thus transmitted to the station, via the
wire, in Morse characters, dots and dashes.
The receiving stations along the line of the railway were similarly
equipped as to apparatus, and, generally speaking the operations of
sending and receiving messages were substantially the same as above
described.
The equipment of an operator on a car was quite simple consisting merely
of a small lap-board, on which were mounted the key, coil, and buzzer,
leaving room for telegraph blanks. To this board were also attached
flexible conductors having spring clips, by means of which connections
could be made quickly with conveniently placed terminals of the ground,
roof, and battery wires. The telephone receiver was held on the head
with a spring, the flexible connecting wire being attached to the lap
board, thus leaving the operator with both hands free.
The system, as shown in the sketch and elucidated by the text,
represents the operation of train telegraphy in a simple form, but
combining the main essentials of the art as it was successfully and
commercially practiced for a number of years after Edison and Gilliland
entered the field. They elaborated the system in various ways, making it
more complete; but it has not been deemed necessary to enlarge further
upon the technical minutiae of the art for the purpose of this work.
XV. KINETOGRAPH AND PROJECTING KINETOSCOPE
ALTHOUGH many of the arts in which Edison has been a pioneer have been
enriched by his numerous inventions and patents, which were subsequent
to those of a fundamental nature, the (so-called) motion-picture art
is an exception, as the following, together with three other additional
patents [30] comprise all that he has taken out on this subject: United
States Patent No. 589,168, issued August 31, 1897, reissued in two
parts--namely, No. 12,037, under date of September 30,1902, and No.
12,192, under date of January 12, 1904. Application filed August 24,
1891.
[Footnote 30: Not 491,993, issued February 21, 1893; No.
493,426, issued March 14, 1893; No. 772,647, issued October
18, 1904.]
There is nothing surprising in this, however, as the possibility of
photographing and reproducing actual scenes of animate life are so
thoroughly exemplified and rendered practicable by the apparatus
and methods disclosed in the patents above cited, that these basic
inventions in themselves practically constitute the art--its development
proceeding mainly along the line of manufacturing details. That such
a view of his work is correct, the highest criterion--commercial
expediency--bears witness; for in spite of the fact that the courts have
somewhat narrowed the broad claims of Edison's patents by reason of the
investigations of earlier experimenters, practically all the immense
amount of commercial work that is done in the motion-picture field
to-day is accomplished through the use of apparatus and methods licensed
under the Edison patents.
The philosophy of this invention having already been described in
Chapter XXI, it will be unnecessary to repeat it here. Suffice it to say
by way of reminder that it is founded upon the physiological phenomenon
known as the persistence of vision, through which a series of sequential
photographic pictures of animate motion projected upon a screen in rapid
succession will reproduce to the eye all the appearance of the original
movements.
Edison's work in this direction comprised the invention not only of a
special form of camera for making original photographic exposures from a
single point of view with very great rapidity, and of a machine adapted
to effect the reproduction of such pictures in somewhat similar manner
but also of the conception and invention of a continuous uniform, and
evenly spaced tape-like film, so absolutely essential for both the above
objects.
The mechanism of such a camera, as now used, consists of many
parts assembled in such contiguous proximity to each other that an
illustration from an actual machine would not help to clearness of
explanation to the general reader. Hence a diagram showing a sectional
view of a simple form of such a camera is presented below.
In this diagram, A represents an outer light-tight box containing a
lens, C, and the other necessary mechanism for making the photographic
exposures, H<1S> and H<2S> being cases for holding reels of film before
and after exposure, F the long, tape-like film, G a sprocket whose teeth
engage in perforations on the edges of the film, such sprocket being
adapted to be revolved with an intermittent or step-by-step movement
by hand or by motor, and B a revolving shutter having an opening and
connected by gears with G, and arranged to expose the film during the
periods of rest. A full view of this shutter is also represented, with
its opening, D, in the small illustration to the right.
In practice, the operation would be somewhat as follows, generally
speaking: The lens would first be focussed on the animate scene to be
photographed. On turning the main shaft of the camera the sprocket, G,
is moved intermittently, and its teeth, catching in the holes in the
sensitized film, draws it downward, bringing a new portion of its length
in front of the lens, the film then remaining stationary for an instant.
In the mean time, through gearing connecting the main shaft with the
shutter, the latter is rotated, bringing its opening, D, coincident with
the lens, and therefore exposing the film while it is stationary, after
which the film again moves forward. So long as the action is continued
these movements are repeated, resulting in a succession of enormously
rapid exposures upon the film during its progress from reel H<1S> to its
automatic rewinding on reel H<2S>. While the film is passing through the
various parts of the machine it is guided and kept straight by various
sets of rollers between which it runs, as indicated in the diagram.
By an ingenious arrangement of the mechanism, the film moves
intermittently so that it may have a much longer period of rest than
of motion. As in practice the pictures are taken at a rate of twenty or
more per second, it will be quite obvious that each period of rest is
infinitesimally brief, being generally one-thirtieth of a second or
less. Still it is sufficient to bring the film to a momentary condition
of complete rest, and to allow for a maximum time of exposure,
comparatively speaking, thus providing means for taking clearly defined
pictures. The negatives so obtained are developed in the regular
way, and the positive prints subsequently made from them are used for
reproduction.
The reproducing machine, or, as it is called in practice, the Projecting
Kinetoscope, is quite similar so far as its general operations in
handling the film are concerned. In appearance it is somewhat different;
indeed, it is in two parts, the one containing the lighting arrangements
and condensing lens, and the other embracing the mechanism and objective
lens. The "taking" camera must have its parts enclosed in a light-tight
box, because of the undeveloped, sensitized film, but the projecting
kinetoscope, using only a fully developed positive film, may, and,
for purposes of convenient operation, must be accessibly open. The
illustration (Fig. 2) will show the projecting apparatus as used in
practice.
The philosophy of reproduction is very simple, and is illustrated
diagrammatically in Fig. 3, reference letters being the same as in Fig.
1. As to the additional reference letters, I is a condenser J the source
of light, and K a reflector.
The positive film is moved intermittently but swiftly throughout its
length between the objective lens and a beam of light coming through the
condenser, being exposed by the shutter during the periods of rest. This
results in a projection of the photographs upon a screen in such rapid
succession as to present an apparently continuous photograph of the
successive positions of the moving objects, which, therefore, appear to
the human eye to be in motion.
The first claim of Reissue Patent No. 12,192 describes the film. It
reads as follows:
"An unbroken transparent or translucent tape-like photographic film
having thereon uniform, sharply defined, equidistant photographs of
successive positions of an object in motion as observed from a single
point of view at rapidly recurring intervals of time, such photographs
being arranged in a continuous straight-line sequence, unlimited in
number save by the length of the film, and sufficient in number to
represent the movements of the object throughout an extended period of
time."
XVI. EDISON'S ORE-MILLING INVENTIONS
THE wide range of Edison's activities in this department of the arts is
well represented in the diversity of the numerous patents that have been
issued to him from time to time. These patents are between fifty and
sixty in number, and include magnetic ore separators of ten distinct
types; also breaking, crushing, and grinding rolls, conveyors,
dust-proof bearings, screens, driers, mixers, bricking apparatus and
machines, ovens, and processes of various kinds.
A description of the many devices in each of these divisions would
require more space than is available; hence, we shall confine ourselves
to a few items of predominating importance, already referred to in the
narrative, commencing with the fundamental magnetic ore separator, which
was covered by United States Patent No. 228,329, issued June 1, 1880.
The illustration here presented is copied from the drawing forming part
of this patent. A hopper with adjustable feed is supported several feet
above a bin having a central partition. Almost midway between the hopper
and the bin is placed an electromagnet whose polar extension is so
arranged as to be a little to one side of a stream of material falling
from the hopper. Normally, a stream of finely divided ore falling from
the hopper would fall into that portion of the bin lying to the left
of the partition. If, however, the magnet is energized from a source of
current, the magnetic particles in the falling stream are attracted
by and move toward the magnet, which is so placed with relation to
the falling material that the magnetic particles cannot be attracted
entirely to the magnet before gravity has carried them past. Hence,
their trajectory is altered, and they fall on the right-hand side of
the partition in the bin, while the non-magnetic portion of the stream
continues in a straight line and falls on the other side, thus effecting
a complete separation.
This simple but effective principle was the one employed by Edison
in his great concentrating plant already described. In practice, the
numerous hoppers, magnets, and bins were many feet in length; and they
were arranged in batteries of varied magnetic strength, in order
that the intermingled mass of crushed rock and iron ore might be
more thoroughly separated by being passed through magnetic fields of
successively increasing degrees of attracting power. Altogether there
were about four hundred and eighty of these immense magnets in the
plant, distributed in various buildings in batteries as above mentioned,
the crushed rock containing the iron ore being delivered to them by
conveyors, and the gangue and ore being taken away after separation by
two other conveyors and delivered elsewhere. The magnetic separators at
first used by Edison at this plant were of the same generality as the
ones employed some years previously in the separation of sea-shore sand,
but greatly enlarged and improved. The varied experiences gained in
the concentration of vast quantities of ore led naturally to a greater
development, and several new types and arrangements of magnetic
separators were evolved and elaborated by him from first to last, during
the progress of the work at the concentrating plant.
The magnetic separation of iron from its ore being the foundation idea
of the inventions now under discussion, a consideration of the separator
has naturally taken precedence over those of collateral but inseparable
interest. The ore-bearing rock, however, must first be ground to powder
before it can be separated; hence, we will now begin at the root of
this operation and consider the "giant rolls," which Edison devised
for breaking huge masses of rock. In his application for United States
Patent No. 672,616, issued April 23, 1901, applied for on July 16, 1897,
he says: "The object of my invention is to produce a method for the
breaking of rock which will be simple and effective, will not require
the hand-sledging or blasting of the rock down to pieces of moderate
size, and will involve the consumption of a small amount of power."
While this quotation refers to the method as "simple," the patent under
consideration covers one of the most bold and daring projects that
Edison has ever evolved. He proposed to eliminate the slow and expensive
method of breaking large boulders manually, and to substitute therefor
momentum and kinetic energy applied through the medium of massive
machinery, which, in a few seconds, would break into small pieces a rock
as big as an ordinary upright cottage piano, and weighing as much as six
tons. Engineers to whom Edison communicated his ideas were unanimous
in declaring the thing an impossibility; it was like driving two
express-trains into each other at full speed to crack a great rock
placed between them; that no practical machinery could be built to
stand the terrific impact and strains. Edison's convictions were
strong, however, and he persisted. The experiments were of heroic size,
physically and financially, but after a struggle of several years and
an expenditure of about $100,000, he realized the correctness and
practicability of his plans in the success of the giant rolls, which
were the outcome of his labors.
The giant rolls consist of a pair of iron cylinders of massive size and
weight, with removable wearing plates having irregular surfaces formed
by projecting knobs. These rolls are mounted side by side in a very
heavy frame (leaving a gap of about fourteen inches between them), and
are so belted up with the source of power that they run in opposite
directions. The giant rolls described by Edison in the above-named
patent as having been built and operated by him had a combined weight of
167,000 pounds, including all moving parts, which of themselves weighed
about seventy tons, each roll being six feet in diameter and five feet
long. A top view of the rolls is shown in the sketch, one roll and one
of its bearings being shown in section.
In Fig. 2 the rolls are illustrated diagrammatically. As a sketch of
this nature, even if given with a definite scale, does not always carry
an adequate idea of relative dimensions to a non-technical reader,
we present in Fig. 3 a perspective illustration of the giant rolls as
installed in the concentrating plant.
In practice, a small amount of power is applied to run the giant rolls
gradually up to a surface speed of several thousand feet a minute. When
this high speed is attained, masses of rock weighing several tons in one
or more pieces are dumped into a hopper which guides them into the gap
between the rapidly revolving rolls. The effect is to partially arrest
the swift motion of the rolls instantaneously, and thereby develop and
expend an enormous amount of kinetic energy, which with pile-driver
effect cracks the rocks and breaks them into pieces small enough to
pass through the fourteen-inch gap. As the power is applied to the rolls
through slipping friction-clutches, the speed of the driving-pulleys is
not materially reduced; hence the rolls may again be quickly speeded up
to their highest velocity while another load of rock is being hoisted
in position to be dumped into the hopper. It will be obvious from the
foregoing that if it were attempted to supply the great energy necessary
for this operation by direct application of steam-power, an engine of
enormous horse-power would be required, and even then it is doubtful
if one could be constructed of sufficient strength to withstand the
terrific strains that would ensue. But the work is done by the great
momentum and kinetic energy obtained by speeding up these tremendous
masses of metal, and then suddenly opposing their progress, the
engine being relieved of all strain through the medium of the slipping
friction-clutches. Thus, this cyclopean operation may be continuously
conducted with an amount of power prodigiously inferior, in proportion,
to the results accomplished.
The sketch (Fig. 4) showing a large boulder being dumped into the
hopper, or roll-pit, will serve to illustrate the method of feeding
these great masses of rock to the rolls, and will also enable the reader
to form an idea of the rapidity of the breaking operation, when it is
stated that a boulder of the size represented would be reduced by
the giant rolls to pieces a trifle larger than a man's head in a few
seconds.
After leaving the giant rolls the broken rock passed on through other
crushing-rolls of somewhat similar construction. These also were
invented by Edison, but antedated those previously described; being
covered by Patent No. 567,187, issued September 8, 1896. These rolls
were intended for the reducing of "one-man-size" rocks to small pieces,
which at the time of their original inception was about the standard
size of similar machines. At the Edison concentrating plant the broken
rock, after passing through these rolls, was further reduced in size by
other rolls, and was then ready to be crushed to a fine powder through
the medium of another remarkable machine devised by Edison to meet his
ever-recurring and well-defined ideas of the utmost economy and
efficiency.
NOTE.--Figs. 3 and 4 are reproduced from similar sketches on pages 84
and 85 of McClure's Magazine for November, 1897, by permission of S. S.
McClure Co.
The best fine grinding-machines that it was then possible to obtain were
so inefficient as to involve a loss of 82 per cent. of the power
applied. The thought of such an enormous loss was unbearable, and he did
not rest until he had invented and put into use an entirely new
grinding-machine, which was called the "three-high" rolls. The device
was covered by a patent issued to him on November 21, 1899, No. 637,327.
It was a most noteworthy invention, for it brought into the art not only
a greater efficiency of grinding than had ever been dreamed of before,
but also a tremendous economy by the saving of power; for whereas the
previous efficiency had been 18 per cent. and the loss 82 per cent.,
Edison reversed these figures, and in his three-high rolls produced a
working efficiency of 84 per cent., thus reducing the loss of power by
friction to 16 per cent. A diagrammatic sketch of this remarkable
machine is shown in Fig. 5, which shows a front elevation with the
casings, hopper, etc., removed, and also shows above the rolls the rope
and pulleys, the supports for which are also removed for the sake of
clearness in the illustration.
For the convenience of the reader, in referring to Fig. 5, we will
repeat the description of the three-high rolls, which is given on pages
487 and 488 of the preceding narrative.
In the two end-pieces of a heavy iron frame were set three rolls, or
cylinders--one in the centre, another below, and the other above--all
three being in a vertical line. These rolls were about three feet in
diameter, made of cast-iron, and had face-plates of chilled-iron. [31]
The lowest roll was set in a fixed bearing at the bottom of the frame,
and, therefore, could only turn around on its axis. The middle and top
rolls were free to move up or down from and toward the lower roll, and
the shafts of the middle and upper rolls were set in a loose bearing
which could slip up and down in the iron frame. It will be apparent,
therefore, that any material which passed in between the top and the
middle rolls, and the middle and bottom rolls, could be ground as fine
as might be desired, depending entirely upon the amount of pressure
applied to the loose rolls. In operation the material passed first
through the upper and middle rolls, and then between the middle and
lowest rolls.
[Footnote 31: The faces of these rolls were smooth, but as
three-high rolls came into use later in Edison's Portland
cement operations the faces were corrugated so as to fit
into each other, gear-fashion, to provide for a high rate of
feed]
This pressure was applied in a most ingenious manner. On the ends of the
shafts of the bottom and top rolls there were cylindrical sleeves, or
bearings, having seven sheaves in which was run a half-inch endless wire
rope. This rope was wound seven times over the sheaves as above, and led
upward and over a single-groove sheave, which was operated by the piston
of an air-cylinder, and in this manner the pressure was applied to the
rolls. It will be seen, therefore that the system consisted in a single
rope passed over sheaves and so arranged that it could be varied
in length, thus providing for elasticity in exerting pressure and
regulating it as desired. The efficiency of this system was incomparably
greater than that of any other known crusher or grinder, for while a
pressure of one hundred and twenty-five thousand pounds could be exerted
by these rolls, friction was almost entirely eliminated, because the
upper and lower roll bearings turned with the rolls and revolved in the
wire rope, which constituted the bearing proper.
Several other important patents have been issued to Edison for crushing
and grinding rolls, some of them being for elaborations and improvements
of those above described but all covering methods of greater economy and
effectiveness in rock-grinding.
Edison's work on conveyors during the period of his ore-concentrating
labors was distinctively original, ingenious and far in advance of
the times. His conception of the concentrating problem was broad and
embraced an entire system, of which a principal item was the continuous
transfer of enormous quantities of material from place to place at
the lowest possible cost. As he contemplated the concentration of six
thousand tons daily, the expense of manual labor to move such an immense
quantity of rock, sand, and ore would be absolutely prohibitive. Hence,
it became necessary to invent a system of conveyors that would be
capable of transferring this mass of material from one place to another.
And not only must these conveyors be capable of carrying the material,
but they must also be devised so that they would automatically receive
and discharge their respective loads at appointed places. Edison's
ingenuity, engineering ability, and inventive skill were equal to the
task, however, and were displayed in a system and variety of conveyors
that in practice seemed to act with almost human discrimination. When
fully installed throughout the plant, they automatically transferred
daily a mass of material equal to about one hundred thousand cubic feet,
from mill to mill, covering about a mile in the transit. Up and down,
winding in and out, turning corners, delivering material from one to
another, making a number of loops in the drying-oven, filling up bins
and passing on to the next when they were full, these conveyors in
automatic action seemingly played their part with human intelligence,
which was in reality the reflection of the intelligence and ingenuity
that had originally devised them and set them in motion.
Six of Edison's patents on conveyors include a variety of devices that
have since came into broad general use for similar work, and have been
the means of effecting great economies in numerous industries of widely
varying kinds. Interesting as they are, however, we shall not attempt to
describe them in detail, as the space required would be too great. They
are specified in the list of patents following this Appendix, and may be
examined in detail by any interested student.
In the same list will also be found a large number of Edison's patents
on apparatus and methods of screening, drying, mixing, and briquetting,
as well as for dust-proof bearings, and various types and groupings
of separators, all of which were called forth by the exigencies and
magnitude of his great undertaking, and without which he could not
possibly have attained the successful physical results that crowned his
labors. Edison's persistence in reducing the cost of his operations is
noteworthy in connection with his screening and drying inventions, in
which the utmost advantage is taken of the law of gravitation. With
its assistance, which cost nothing, these operations were performed
perfectly. It was only necessary to deliver the material at the top of
the chambers, and during its natural descent it was screened or dried as
the case might be.
All these inventions and devices, as well as those described in detail
above (except magnetic separators and mixing and briquetting machines),
are being used by him to-day in the manufacture of Portland cement, as
that industry presents many of the identical problems which presented
themselves in relation to the concentration of iron ore.
XVII. THE LONG CEMENT KILN
IN this remarkable invention, which has brought about a striking
innovation in a long-established business, we see another characteristic
instance of Edison's incisive reasoning and boldness of conception
carried into practical effect in face of universal opinions to the
contrary.
For the information of those unacquainted with the process of
manufacturing Portland cement, it may be stated that the material
consists preliminarily of an intimate mixture of cement rock and
limestone, ground to a very fine powder. This powder is technically
known in the trade as "chalk," and is fed into rotary kilns and
"burned"; that is to say, it is subjected to a high degree of heat
obtained by the combustion of pulverized coal, which is injected
into the interior of the kiln. This combustion effects a chemical
decomposition of the chalk, and causes it to assume a plastic
consistency and to collect together in the form of small spherical
balls, which are known as "clinker." Kilns are usually arranged with
a slight incline, at the upper end of which the chalk is fed in and
gradually works its way down to the interior flame of burning fuel at
the other end. When it arrives at the lower end, the material has been
"burned," and the clinker drops out into a receiving chamber below. The
operation is continuous, a constant supply of chalk passing in at one
end of the kiln and a continuous dribble of clinker-balls dropping
out at the other. After cooling, the clinker is ground into very fine
powder, which is the Portland cement of commerce.
It is self-evident that an ideal kiln would be one that produced the
maximum quantity of thoroughly clinkered material with a minimum amount
of fuel, labor, and investment. When Edison was preparing to go into
the cement business, he looked the ground over thoroughly, and, after
considerable investigation and experiment, came to the conclusion that
prevailing conditions as to kilns were far from ideal.
The standard kilns then in use were about sixty feet in length, with an
internal diameter of about five feet. In all rotary kilns for burning
cement, the true clinkering operation takes place only within a limited
portion of their total length, where the heat is greatest; hence the
interior of the kiln may be considered as being divided longitudinally
into two parts or zones--namely, the combustion, or clinkering, zone,
and the zone of oncoming raw material. In the sixty-foot kiln the length
of the combustion zone was about ten feet, extending from a point six or
eight feet from the lower, or discharge, end to a point about eighteen
feet from that end. Consequently, beyond that point there was a zone of
only about forty feet, through which the heated gases passed and came
in contact with the oncoming material, which was in movement down toward
the clinkering zone. Since the bulk of oncoming material was small,
the gases were not called upon to part with much of their heat, and
therefore passed on up the stack at very high temperatures, ranging from
1500 degrees to 1800 degrees Fahr. Obviously, this heat was entirely
lost.
An additional loss of efficiency arose from the fact that the material
moved so rapidly toward the combustion zone that it had not given up
all its carbon dioxide on reaching there; and by the giving off of
large quantities of that gas within the combustion zone, perfect and
economical combustion of coal could not be effected.
The comparatively short length of the sixty-foot kiln not only limited
the amount of material that could be fed into it, but the limitation in
length of the combustion zone militated against a thorough clinkering of
the material, this operation being one in which the elements of time and
proper heat are prime considerations. Thus the quantity of good clinker
obtainable was unfavorably affected. By reason of these and other
limitations and losses, it had been possible, in practice, to obtain
only about two hundred and fifty barrels of clinker per day of
twenty-four hours; and that with an expenditure for coal proportionately
equal to about 29 to 33 per cent. of the quantity of clinker produced,
even assuming that all the clinker was of good quality.
Edison realized that the secret of greater commercial efficiency and
improvement of quality lay in the ability to handle larger quantities
of material within a given time, and to produce a more perfect product
without increasing cost or investment in proportion. His reasoning led
him to the conclusion that this result could only be obtained through
the use of a kiln of comparatively great length, and his investigations
and experiments enabled him to decide upon a length of one hundred and
fifty feet, but with an increase in diameter of only six inches to a
foot over that of the sixty-foot kiln.
The principal considerations that influenced Edison in making this
radical innovation may be briefly stated as follows:
First. The ability to maintain in the kiln a load from five to seven
times greater than ordinarily employed, thereby tending to a more
economical output.
Second. The combustion of a vastly increased bulk of pulverized coal
and a greatly enlarged combustion zone, extending about forty feet
longitudinally into the kiln--thus providing an area within which
the material might be maintained in a clinkering temperature for a
sufficiently long period to insure its being thoroughly clinkered from
periphery to centre.
Third. By reason of such a greatly extended length of the zone of
oncoming material (and consequently much greater bulk), the gases and
other products of combustion would be cooled sufficiently between the
combustion zone and the stack so as to leave the kiln at a comparatively
low temperature. Besides, the oncoming material would thus be gradually
raised in temperature instead of being heated abruptly, as in the
shorter kilns.
Fourth. The material having thus been greatly raised in temperature
before reaching the combustion zone would have parted with substantially
all its carbon dioxide, and therefore would not introduce into the
combustion zone sufficient of that gas to disturb the perfect character
of the combustion.
Fifth. On account of the great weight of the heavy load in a long kiln,
there would result the formation of a continuous plastic coating on that
portion of the inner surface of the kiln where temperatures are
highest. This would effectively protect the fire-brick lining from the
destructive effects of the heat.
Such, in brief, were the essential principles upon which Edison based
his conception and invention of the long kiln, which has since become so
well known in the cement business.
Many other considerations of a minor and mechanical nature, but which
were important factors in his solution of this difficult problem, are
worthy of study by those intimately associated with or interested in the
art. Not the least of the mechanical questions was settled by Edison's
decision to make this tremendously long kiln in sections of cast-iron,
with flanges, bolted together, and supported on rollers rotated by
electric motors. Longitudinal expansion and thrust were also important
factors to be provided for, as well as special devices to prevent the
packing of the mass of material as it passed in and out of the kiln.
Special provision was also made for injecting streams of pulverized coal
in such manner as to create the largely extended zone of combustion. As
to the details of these and many other ingenious devices, we must refer
the curious reader to the patents, as it is merely intended in these
pages to indicate in a brief manner the main principles of Edison's
notable inventions. The principal United States patent on the long kiln
was issued October 24, 1905, No. 802,631.
That his reasonings and deductions were correct in this case have been
indubitably proven by some years of experience with the long kiln in its
ability to produce from eight hundred to one thousand barrels of
good clinker every twenty-four hours, with an expenditure for coal
proportionately equal to about only 20 per cent. of the quantity of
clinker produced.
To illustrate the long cement kiln by diagram would convey but little
to the lay mind, and we therefore present an illustration (Fig. 1) of
actual kilns in perspective, from which sense of their proportions may
be gathered.
XVIII. EDISON'S NEW STORAGE BATTERY