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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.
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:
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