IMPROVEMENTS IN OR RELATING TO INCANDESCING ELECTRIC LAMPS
Year: 1882

page_text: LETTERS PATENT to Thomas John Handford of 42 Southampton Buildings
in the County of Middlesex Patent Agent for an Invention of IMPROVEMENTS
IN OR RELATING TO INCANDESCING ELECTRIC LAMPS Communicated to him
from abroad by Thomas Alva Edison of Menlo Park in the State of New
Jersey United States of America
PROVISIONAL SPECIFICATION left by the said Thomas John Handford
at the Office of the Commissioners of Patents on the 28th December 1882.
THOMAS JOHN HANDFORD of 42 Southampton Buildings in the County of
Middlesex, Patent Agent, "IMPROVEMENTS IN OR RELATING TO INCANDESCING
a communication to.me from abroad by Thomas Alva Edison of
Menlo Park in the State of New Jersey, United States of America,
ELECTRIC LAMPS
"
This Invention relates to incandescing electric lamps, wherein flexible filaments
of carbon are enclosed in hermetically sealed chambers made entirely of glass, with
leading-in wires passing through and sealed into the glass; and the object of the
invention is, principally to lengthen the life of electric lamps of this character, by
diminishing the electrical carrying between the flexible carbon filament and the
enclosing chamber, or the metallic terminals of the filament within the lamp, and
incidentally, to increase the resistance of the lamp, so as to reduce the investment
required for conductors
Heretofore, in electric lamps of this character, it has been the general practice
to produce high vacua in the glass globes, previous to hermetically sealing the
same in order to preserve the carbon filaments from oxidation, and to prevent the
loss caused by convection of heat, which takes place when the globes are filled with
an inert gas.
The improvements made in the past upon the standard incandescing electric
lamps, have been principally by increasing the resistance of the flexible carbon
filaments, without a corresponding increase of the radiating surface, and without

page_text: diminishing in the least the degree of vacuum; but it has been found that advance
in this direction is limited, by reason of the increased deposit of the carbon upon
the walls of the glass chamber, obscuring the light and diminishing considerably
the exterior candle-power of the lamp, and also consuming, and destroying the fine
filament, thereby reducing materially the length of life, of the lamp.
It has been found that the amount of deposit in any given period, depends
generally upon the degree of incandescence per unit surface to which the carbon
filament is raised, the higher the degree of incandescence the more rapid being the
deposit it also depends upon the state of the vacuum, the higher the vacuum the
greater being the deposit.
This deposit it has been discovered is due to electrical carrying of the carbon of
the filament, the phenomenon being similar to that which takes place in the well
known Geissler tubes, wherein at certain stages of the vacuum or pressure
electrical carrying of the platina, forming the terminals, takes place, blackening
the walls of the glass vacuum chamber, notwithstanding that the platina is scarcely
above the temperature of the atmosphere, while at other stages of the vacuum or
pressure this blackening does not take place.
It has also been discovered that the economy of the carbon filament lamp, (the
number of standard lamps per horse power) increases as the pressure within the
globe diminishes, up to a certain stage of the vacuum, when any higher exhaustion
does not practically increase the economy. If the vacuum is diminished below this
point of greatest economy to prevent electrical carrying, the economy of the lamp
is diminished, since the residual air acts as a carrier of heat to the walls of the
chamber, where it is rapidly dissipated.
But, owing to the greatly diminished electrical carrying of the carbon from the
filament when the vacuum is low, it is possible to diminish the radiating surface so
as to raise each unit of surface to a higher degree of incandescence than would be
practicable were the vacuum higher and the electrical carrying at its maximum :
and as an increase in the degree of incandescence, is an advance in the direction of
economy, it is possible to regain by the economy of higher incandescence, the
energy lost by the increased convection of heat from the filament to the glass walls
of the lamp, caused by the greater density of the residual gas due to a low
Vacuum.
Hence a carbon filament lamp embodying these conditions of low vacuum and
high incandescence, and resistance will have a longer life than if the vacuum were
higher and its candle power will not be diminished by the obscuration of the globe
by a deposit of carbon: it will also permit of a reduction in the size of conductors
by reason of its increased resistance, and the filament itself will be more flexible
and less liable to break.
In carrying out the invention, the flexible carbon filament is produced by the
carbonization under strain or pressure of any suitable organic material, reduced or
not reduced to an amorphous or semi-amorphous condition, in the way now well
understood.
The filament before carbonization however may be reduced to a smaller cross
section than usual heretofore in order to produce the reduction of radiating surface
and increase of resistance per unit of radiating surface necessary. to compensate for
the loss caused by the reduced vacuum.
For illustration it may be stated that the radiating surface can be reduced in
size two tenths: but the sizes of filaments used at present may be retained
the loss in economy being more than counterbalanced by the increased length
of life.
The flexible carbon filament is secured to the leading-in wires, which are sealed
in one glass part of the lamp, and this part is fused to the glass globe of the lamps
in the usual or any suitable way. The lamp is then connected with a Sprengel
pump and the globe exhausted until a high vacuum is obtained, so as to remove all
oxygen from the globe.
During the latter part of the operation of exhausting the lamp, the flexible

page_text: carbon filament may be gradually raised to an incandescence higher than that at
which it is intended afterwards to be used: -but this heating of the filament may
be omitted.
•
After the exhaustion of the lamp globe is completed, an inert gas is allowed to
pass into the globe, gradually reducing the vacuum, and increasing tlie pressure
within the globe.
This is preferably done by providing the pump with a tubé containing a solid
substance which when heated will evolve an inert gas. The heat is applied at the
proper time to the exterior of the tube containing the substance. The flexible
carbon filament is raised to incandescence during the time that the inert gas is
being admitted into the lamp globe and certain phenomena will be noticed during
this period. As the pressure gradually increases a light blue halo, very much
spread out, will appear upon the positive clamp of the filament. As more gas
passes in, the halo will increase in density and closely embrace the clamp: at this
pressure carbon from the filament is deposited on the clamp in considerable
quantity, which is due to the increase of the electrical resistance of the vacuum
and the consequent prevention of deposit upon the globe.
If now the pressure be further increased, the blue halo leaves the metallic
portions of the clamp and appears on the carbon at the junction of the latter, and
the metal of the clamp.
If now the pressure is carried beyond this point, the blue halo will disappear
entirely, and the resistance of the residual gas will be so great as to nearly or quite
extinguish the electrical carrying. The proper stage being reached, the lamp is
sealed off from the pump while incandescent
The particular pressure at which the lamp should be sealed off is dependent upon
the nature of the residual gas. With nitrogen for the inert gas, this pressure may
be when a mercurial column connected with the lamp stands at a height of about
twenty inches, but with hydrochloric acid gas, on account of its greater electrical
resistance, the pressure may be somewhat reduced.
At twenty-nine inches with nitrogen, and equivalent pressures with other gases,
the electrical carrying is greatly diminished. This vacuum of twenty nine inchies
or below that height for nitrogen, and equivalent pressures with other gases is
what is hereinafter termed a "low vacuum
כי
Since the blue halo in the lamp disappears altogether to the eye when a certain
pressure is reached, on account of the incandescence of the filament, and since the
operation should be carried beyond this point, a Geissler spark gauge may be used
to determine the exact point to seal off the lamp, the terminals of the Geissler
spark gauge being connected to an induction coil worked by a constant battery.
A mercurial column may be used for the purpose, but the Geissler spark gauge is
preferred for the reason that the electrical carrying depends, where the vacuum is
low, both upon the nature of the residual gas and the pressure, which conditions
will also affect the Geissler spark-gauge, in which the phenomena due to electrical
carrying can be observed after their disappearance to the eye in the lamp, while
the mercurial column is only affected by the pressure.
The different degrees of exhaustion at which certain phenomena will appear in
the spark gauge depend upon the size and distance apart of the electrodes, as well
as on the chamber of the gauge, and on the electromotive force of the coil. Hence
it is necessary to determine, in the first instance by the disappearance of the blue
halo from the clamps of the filament, due to increased pressure, the appearance of
the spark gauge at the exact moment when the lamp is to be sealed off, which is an
increased pressure of several inches of a column of mercury, after the disappearance
of the halo from the metallic terminals of the filament.
The residual gas may be allowed to flow in until the gas within the globe is at
atmospheric pressure, and good results will be obtained, as far as the electrical
carrying is concerned but the economy will be considerably diminished, without a
corresponding increase of the life of the filament. Hence it is best to diminish the

page_text: pressure for the sake of economy, but not to the point where the blue halo begins
to appear on the metallic terminals of the filament.
Instead of exhausting to a high vacuum with a mercury pump, and then
gradually reducing the vacuum to the proper point by means of an inert gas, the
lamp may be first exhausted to a high vacuum, and the inert gas then allowed to
flow into the lamp until the vacuum is reduced to atmospheric pressure, when the
inert gas may be pumped out until the desired pressure is obtained, or any other
way of displacing the oxygen by an inert gas and obtaining the desired pressure
may be employed.
As a further example of the inert gases which may be used, cyanogen may be
mentioned the same being produced from solid cyanide of mercury, which evolves
cyanogen when heated.
Other inert gases may however be used being evolved
from the decomposition of different salts by heat.
The making of the enclosing chamber entirely of glass, through which the
leading-in wires are passed, and in which they are sealed, and the hermetical
closing of such glass enclosing chamber, assure the retention of the same condition
of pressure that it is found desirable to give the lamp when manufactured, which
is a feature of essential importance in lamps with a low vacuum as well as with
lamps having a high vacuum. Since electrical carrying takes place also with
incandescing conductors made of other material than carbon, the invention is not
limited to carbon, but is intended to include all flexible filamentary incandescing
conductors, having like carbon a high specific resistance.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Thomas John Handford in the Great Seal Patent Office on the
27th June 1883.
THOMAS JOHN HANDFORD of 42 Southampton Buildings in the County of
Middlesex Patent Agent "IMPROVEMENTS IN OR RELATING TO INCANDESCING
ELECTRIC LAMPS" a communication to me from abroad by Thomas Alva Edison of
Menlo Park in the State of New Jersey, United States of America
This Invention relates to incandescing electric lamps, wherein flexible filaments
of carbon are inclosed in hermetically sealed chambers made entirely of glass, with
leading-in wires passing through and sealed into the glass, and the object of the
Invention is principally to lengthen the life of electric lamps of this character by
diminishing the electrical carrying between the flexible carbon filament and the
enclosing chamber, or the metallic terminals of the filament within the lamp, and
incidentally to increase the resistance of the lamp so as to reduce the investment
required for conductors.
Heretofore in electric lamps of this character, it has been the general practice to
produce high vacua in the glass globes previous to bermetically sealing the
same in order to preserve the carbon filaments from oxidation, and to prevent the
loss caused by convection of heat, which takes place when the globes are filled
with an inert gas.
The improvements made in the past upon the standard incandescing electric
lamps have been principally by increasing the resistance of the flexible carbon
filaments without a corresponding increase of the radiating surface, and without
diminishing in the least the degree of vacuum; but it has been found that advance
in this direction is limited by reason of the increased deposit of the carbon upon
the walls of the glass chamber, obscuring the light and diminishing considerably
the exterior candle power of the lamp and also consuming and destroying the fine
filament, thereby reducing materially the length of life of the lamp.
It has been found that the amount of deposit in any given period depends
generally upon the degree of incandescence per unit surface to which the carbon
filament is raised, the higher the degree of incandescence the more rapid being the
deposit; it also depends upon the state of the vacuum, the higher the vacuum the
greater being the deposit.
This deposit it has been discovered is due to electrical carrying of the
carbon of the filament, the phenomenon being similar to that which takes place in
the well-known Geissler tubes wherein at certain stages of the vacuum or pressure
electrical carrying of the platina forming the terminals takes place blackening the
walls of the glass vacuum chamber, notwithstanding that the platina is scarcely
above the temperature of the atmosphere, while at other stages of the vacuum or
pressure this blackening does not take place.
It has also been discovered that the economy of the carbon filament lamp (the
number of standard lamps per horse power) increases as the pressure within the
globe diminishes, up to a certain stage of the vacuum, when any higher exhaustion
does not practically increase the economy. If the vacuum is diminished below this
point of greatest economy, to prevent electrical carrying, the economy of the lamp
is diminished since the residual air acts as a carrier of heat to the walls of the
chamber where it is rapidly dissipated. But owing to the greatly diminished
electrical carrying of the carbon from the filament when the vacuum is low, it is
possible to diminish the radiating surface so as to raise each unit of surface to a
higher degree of incandescence than would be practicable were the vacuum, higher

page_text: and the electrical carrying at its maximum; and as an increase in the degree of
incandescence, is an advance in the direction of economy it is possible to regain by
the economy of higher incandescence, the energy lost by the increased convection
of heat from the filament to the glass walls of the lamp, caused by the greater
density of the residual gas due to a low vacuum.
Hence a carbon filament lamp embodying these conditions of low vacuum and
high incandescence, and resistance, will have a longer life than if the vacuum were
higher, and its candle power will not be diminished by the obscuration of the
globe by a deposit of carbon, it will also permit of a reduction in the size of
conductors for carrying the current to and from the lamps by reason of its increased
resistance and the filament itself will be more flexible and less liable to break.
In order to accomplish these results the Invention is carried out in the following
manner, reference being had to the several figures on the sheet of drawings
hereunto annexed.
Figure 1, of the drawings is a view of the lamp.
Figure 2, an elevation showing the principal parts of a Sprengel pump and
devices connected therewith as hereinafter described.
Figures 3, 4, and 5 illustrate phenomena which appear at the positive clamp of
the filament; and
Figures 6, 7, 8, 9, and 10 illustrate phenomena which appear in a Geissler
spark gauge all as hereinafter fully described
In carrying the Invention into effect the flexible carbon filament is produced by
the carbonization under strain or pressure of any suitable organic material, reduced,
or not reduced to an amorphous or semi-amorphous condition in the way now well
understood
The filament before carbonization however may be reduced to a smaller cross-
section than usual heretofore in order to produce the reduction of radiating surface,
and increase of resistance per unit of radiating surface necessary to compensate for
the loss caused by the reduced vacuum.
For illustration it may be stated that the radiating surface of the carbon
filament can be reduced in size two tenths; but the sizes of filaments used at
present may be retained the loss in economy being more than counterbalanced by
the increased length of life.
The flexible carbon filament having been secured to the leading-in wires 1. 2,
Figure 1, the lamp globe, or bulb, A, is connected with a Sprengel pump D,
Figure 2. the mercury entering at b, and passing out at c, and the globe, or bulb, is
exhausted until a high vacuum is obtained so as to remove all oxygen from the
globe, or bulb. During the latter part of the operation of exhausting the lamp,
the flexible carbon filament may be gradually raised to an incandescence higher
than that at which it is intended afterwards to be used but this heating of the
filament may be omitted.
After the exhaustion of the lamp globe, or bulb is completed, an inert, or azotic
gas is allowed to pass into the globe, or bulb, gradually reducing the vacuum and
increasing the pressure within the globe or bulb.
The flexible carbon filament is raised to incandescence during the time that the
inert gas is being admitted into the lamp globe, or bulb, and certain phenomena
will be noticed during this period. As the pressure gradually increases a light
blue halo very much spread out will appear upon the positive clamp of the filament.
As more gas passes in, the halo will increase in density and closely embrace the
clamp at this pressure carbon from the filament is deposited on the clamp in con-
siderable quantity, which is due to the increase of the electrical resistance of the
vacuum and the consequent prevention of deposit upon the globe or bulb. If now
the pressure be further increased, the blue halo leaves the metallic portions of the
clamp and appears on the carbon at the junction of the latter and the metal of the
clamp.
If now the pressure is carried beyond this point the blue halo will disappear
entirely and the resistance of the residual gas will be so great as to nearly or quite

page_text: extinguish the electrical carrying. The proper stage being reached the lamp is
sealed off from the pump while incandescent.
The particular pressure at which the lamp should be sealed off is dependent
upon the nature of the residual gas. With nitrogen for the inert gas, this pressure
may be when a mercurial columu (for measuring the amount of rarefaction)
connected with the lamp stands at a height of about twenty inches but with
hydrochloric acid gas on account of its greater electrical resistance, the pressure
may be somewhat reduced.
At twenty nine inches with nitrogen and equivalent pressures with other gases
the electrical carrying is greatly diminished. This vacuum of twenty nine inches
or below that height for nitrogen, and equivalent pressures with other gases is what
is hereinafter termed a "low vacuum.'
"
Figures 3, 4, and 5, represent the appearance of the blue halo on. the positive
clamp at three stages; Figure 3, when it first appears which corresponds, with
nitrogen for the inert gas, to a pressure shown in the mercury column of
30 3/16 inches; Figure 4, when it becomes dense and closely embraces the clamp.
which occurs at a pressure of 29 % 18 inches; and Figure 5, when it is about to
disappear which occurs at a pressure of 28 15/16 inches.
16
Since the blue halo in the lamp disappears altogether to the eye when a certain
pressure is reached on account of the incandescence of the filament; and since the
operation should be carried beyond this point, a Geissler spark gauge E, Figure 2,
connected with the exhaust tube of the pump may be used to determine the exact
point to seal off the lamp, the terminals of the Geissler spark-gauge being con-
nected to an induction coil F worked by a constant battery G. A mercurial
column may be used for the purpose of determining the exact point to seal off the
lamp, but the Geissler spark-gauge is preferred, for the reason that the electrical
carrying depends, where the vacuum is low, both upon the nature of the residual
gas and the pressure, which conditions will also affect the Geissler spark-gauge in
which the phenomena due to electrical carrying can be observed after their
disappearance to the eye in the lamp, while the mercurial column is only affected
by the pressure.
Figures 6, 7, 8, 9, and 10, represent the phenomena of the spark-gauge, Figure 6,
shows the spark between the terminals at the atmospheric pressure; Figure 7,
shows a halo on the ends of the positive pole which occurs at 20 inches of the
mercury column; Figures 8, and 9 show it extending along the wire, which occurs
at about 29 inches; and Figure 10, shows the halo spread out so as to touch the
walls of the tube which occurs at 30 inches of the mercury column, all these
phenomena being observed with nitrogen for the inert gas
The different degrees of exhaustion at which certain phenomena will appear in
the spark-gauge, depend upon the size, and distance apart of the electrodes as well
as on the chamber of the gauge, and on the electro-motive force of the coil. Hence
it is necessary to determine in the first instance by the disappearance of the blue
halo from the clamp of the filament due to increased pressure, the appearance of
the spark-gauge at the exact moment when the lamp is to be sealed off, which is
when an increased pressure is reached equivalent to several inches of a column of
mercury after the disappearance of the halo from the metallic terminals of the
filament.
It will be understood that it is best to diminish the pressure for the sake of
economy but not to the point where the blue halo begins to appear on the metallic
terminals of the filament.
In introducing the inert gas the exhaust tube may be connected with a reservoir
of pure inert gas which can be allowed to pass as desired into the vacuum by means
of a stop cock, but by this method it is not practicable to properly manipulate the
gas or make it free from oxygen. This difficulty is not experienced when the gas
is evolved from a solid in a tube by the application of heat to the exterior of the
tube and the heat can be so applied as to set free the exact quantity of gas desired,
the quantity being regulated with a nicety and exactness which cannot be obtained

page_text: with a stop cock. The method last referred to is therefore by preference employed
and is represented in the drawing Figure 2.
His a chamber or tube connected with the exhaust tube of the pump. The
tube H, contains the solid material for producing the inert gas when the tube is
heated. This material may be for illustration solid cyanide of mercury which
evolves cyanogen when heated. Other inert gases may however be used being
evolved from the decomposition of different salts by heat. I, is a chamber or tube
containing a drying agent.
In lieu of exhausting to a high vacuum with a mercury pump and then gradually
reducing the vacuum to the proper point by means of an inert gas, the lamp may
be first exhausted to a high vacuum, and the inert gas, then allowed to flow into
the lamp until the vacuum is reduced to atmospheric pressure, when the inert gas
may be pumped out until the desired pressure is obtained, or any other method of
displacing the oxygen by an inert gas and obtaining the desired pressure may be
employed.
The making of the enclosing chamber entirely of glass through which the leading-
in wires are passed and in which they are seated, and the hermetical closing of
such glass enclosing chamber, assure the retention of the same conditions of pressure
that it is found desirable to give the lamp when manufactured, which is a feature
of essential importance in lamps with a low vacuum, as well as with lamps having
a high vacuum. Since electrical carrying takes place also with continuous incan-
descing conductors made of other material than carbon, the Invention is not limited
to carbon but is intended to include all flexible continuous filamentary incandescing
conductors having like carbon a high specific resistance.
Having now described and particularly ascertained the nature of the said Inven-
tion, and the manner in which the same is or may be used or carried into effect I
would observe in conclusion that what I consider to be novel and original and
therefore claim as the Invention secured to me by the hereinbefore in part recited
Letters Patent is :-
First. The incandescing electric lamp hereinbefore described, having a flexible
filamentary continuous conductor of carbon or other material having like carbon a
high specific resistance, a hermetically sealed enclosing chamber made entirely of
glass, leading-in wires passing through and sealed into the glass, and an inert, or
azotic gas having a pressure gauged to prevent electrical carrying, substantially in
the manner herein specified.
Second. The method of exhausting the globe of an incandescing lamp, of pro-
viding it with an inert gas free from oxygen, and of determining the definite
pressure at which the supply of gas shall be stopped, and the lamp sealed off,
combined in a continuous operation substantially as hereinbefore described.
In witness whereof I the said Thomas John Handford have hereunto set my
hand and seal this twenty sixth day of June in the year of our Lord one
thousand eight hundred and eighty three.