IMPROVEMENTS RELATING TO THE PRODUCTION OF ELECTRIC LIGHT
Year: 1881

page_text: LETTERS PATENT to William Crookes, F.R.S., of Boy Court, Ludgate Hill, in
the City of London, for an Invention of IMPROVEMENTS RELATING TO THE
PRODUCTION OF ELECTRIC LIGHT."
"
PROVISIONAL SPECIFICATION left by the said William Crookes at the Office
of the Commissioners of Patents on the 31st March 1881.
WILLIAM CROOKES, F.R.S., of Boy Court, Ludgate Hill, in the City of London.
IMPROVEMENTS RELATING TO THE PRODUCTION OF ELECTRIC LIGHT."
This invention has for its object improvements relating to the production of
electric light.
In the production of electric light carbon is employed in various forms and
frequently in vacuous enclosures of glass into which conducting wires are
sealed:
My juvention relates to the preparation of the carbon and also of these glass
enclosures..
In the preparation of carbon in the form of filaments and thin rods for the
incandescent light or in the form of rods and bars for the are light. it is important
that-the carbon should be as pure as possible.
The chief source of impurity is the inorganic constituents or ash."
In the incandescent light of Swan, Edison, Maxim, Lane-Fox and others a
filament of vegetable fibre or cellulose either natural (as bamboo. fibres) or in a
manufactured state (as thread, paper or parchment-paper) is carbonised in a space
free from oxygen by which the oxygen and hydrogen of the cellulose are driven off
and the carbon is left behind.
Carbon is infusible and unaltered at very high temperatures.
The inorganic constituents of the cellulose consisting amongst other things of
silica iron lime and other bases are not driven off by heat and remain with the
carbon.
These impurities are injurious in various ways; they cause the glass to become
dim and the carbon to disintegrate and finally to break.

page_text: If the temperature does not rise to the reducing point of the oxides, the silicates
fuse and coat the particles of carbon interfering with the conductivity and
with the deposition of fresh carbon from the decomposition of hydrocarbons.
In the are light the carbon burns away and the silicates and other constituents
of the ash-fuse and accumulate till they fall off causing flickering or temporary
stoppage of the light. Or the constituents of the ash decompose and volatilise
colouring the electric light irregularly and in an unpleasant manner.
All these inconveniences I obviate by previously removing the constituents of
the ash.
This may be done either from the undecomposed vegetable fibre or from the
filament after carbonization or from the carbon sticks and rods for the are light.
I soak the fibre or carbon in hydrofluoric acid or in some chemical mixture
equivalent thereto or expose it to the vapour of hydrofluoric acid.
Hydrofluoric acid has strong solvent powers on silica as well as on the other
constituents of the ash of plants. If necessary a little hydrochloric or other acid
may be added as well.
The material should be soaked for some time and then well washed in pure
water till free from acid.
Vegetable fibre, cotton, paper, thread and the carbon made therefrom treated in
this manner leave little or no ash when burnt, and incandescent and are lights
made therefrom are free from the inconveniences described above.
Another method of removing the ash after carbonisation is to ignite the carbon
electrically or otherwise in an atmosphere of chlorine. The carbon removes the
oxygen and the chlorine unites with the silicon, calcium, iron & forming chlorides
which volatilise. This plan is equally effectual with the hydrofluoric plan but is
more troublesome.
In the preparation of the vacuous enclosures for the display of electric light it is
usual to insert conductors of platinum through the walls of the enclosure and to
fuse the glass around these to make a sound and air tight closing.
The metal platinum is selected because in its rate of expansion by heat it more
nearly corresponds with glass than do the other available metals and consequently
breakage or leakage at the joint is less liable to occur.
According to my invention I use cheaper and more suitable metals such as
copper or iron and I avoid difficulty from the difference between their rates
of expansion and that of glass by coating them with an enamel having an
intermediate rate of expansion or two or more coatings with different enamels
can with advantage be employed.
In exhausting the glass vessel I introduce an earthy metallic or other body
having great affinity for aqueous vapour or other gas and capable of absorbing and
holding it in a high vacuum and evolving it when heated. I exhaust to a moderate
degree (say the 1/10000th of an atmosphere) then heat the earthy or other body
and drive off the occluded gas or vapour this reduces the exhaustion very
considerably. I then reexhaust. As the earthy or other body cools it absorbs the
gas which it formerly gave out and produces an exhaustion higher than can be
obtained in the ordinary method of pumping by the mercury pump.
In this way it is easy to obtain a vacuum which will not conduct an induction
current capable of striking across several inches of air.
Incandescent lights in so high a vacuum as this last longer than in ordinary
vacua as obtained by the mercury pump.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said William Crookes in the Great Seal Patent Office on the
30th September 1881.
WILLIAM CROOKES, F.R.S., of Boy Court, Ludgate Hill, in the City of London.
IMPROVEMENTS RELATING TO THE PRODUCTION OF ELECTRIC LIGHT."
My invention has for its object improvements relating to the production of
electric light.
In the production of electric light carbon is employed in various forms and
frequently in vacuous enclosures of glass into which conducting wires are sealed.
My invention relates to the preparation of the carbon and also of these glass
enclosures.
In the preparation of carbon in the form of filaments and thin rods for the
incandescent light or in the form of rods and bars for the are light, it is important
that the carbon should be as pure as possible.
The chief source of impurity is the inorganic constituents or
ash."
In the incandescent lamp of Swan, Edison, Maxim, Laue-Fox and others, a
filament of vegetable fibre or cellulose either natural (as bamboo fibres) or in a
manufactured state (as thread, paper or parchment-paper) is carbonized in a space
free from oxygen by which the oxygen and hydrogen of the cellulose are driven
off and the carbon is left behind.
Carbon is infusible and unaltered at very high temperatures.
The inorganic constituents of the cellulose consisting amongst other things of
silica, iron lime and other bases are not driven off by heat during the process of
carbonization and remain with the carbon.
These impurities are injurious in various ways. They cause the glass to become
dim and the carbon to disintegrate and finally to break.
If the temperature does not rise to the reducing point of the oxides the silicates
fuse and coat the particles of carbon interfering with its conductivity, with its
radiating and emissive power for light, and with the deposition of fresh carbon
from the decomposition of hydrocarbons.
In the are light the carbon burns away and the silicates and other constituents
of the ash fuse and accumulate till they fall off causing flickering or temporary
stoppage of the light. Or the constituents of the ash decompose and volatilize
coloring the electric light irregularly and in an unpleasant manner.
All these inconveniences I obviate by previously removing the constituents of
the ash.
This may be done either while the material is in the undecomposed state or
after carbonization.
In the former case I take the fibre of bamboo, hemp, linen, paper, thread
(whether parchmentised or not) or other form of cellulose or other body containing
carbon from which it is intended to manufacture the carbon, and allow it to soak
in hydrofluoric acid until a sample removed and washed free from the acid leaves
very little or no inorganic ash upon being burnt. A little hydrochloric or other
acid may be added to increase the solvent power upon the constituents of the ash
other than silica, care being taken that it is not sufficiently strong to attack the
cellulose itself.
Instead of using hydrofluoric acid a mixture of a fluoride, such as fluoride of
calcium or fluoride of sodium and an acid capable of libérating hydrofluoric acid
from it (but not of attacking the cellulose) may be used, or the substance to be

page_text: operated on may be exposed to the vapour of hydrofluoric acid as usually generated
by acting on fluor spar by oil of vitriol; or other equivalent method of exposing
the substance to the action of hydro-fluoric acid may be employed.
After being acted upon for a sufficient length of time the material is then to be
well washed in pure water so as to be free from acid.
If it is desired to operate on the material after carbonization it is necessary
that the carbon should be in the form of a fine powder or in extremely thin
filaments or sheets such as those produced by carbonizing thread or paper. The
process is then the same as that above described and the carbon may afterwards
he made up into dises, sticks or rods, for use in are lamps or other lamps in
which carbon dises, sticks or rods are used. It is not practicable to act upon the
sticks themselves as their thickness and density prevents the success of the
process.
The inorganic ash may also be removed from the carbon by heating it to redness
or whiteness in a suitable vessel containing free chlorine, or through which a
current of free chlorine is passed. If free oxygen, or an oxidizing atmosphere, be
present at the commencement of the operation the carbon will remove the oxygen
by combining with it in the process of combustion, but it is preferable to remove
it before the operation commences. Hydrochloric acid will act in a similar way to
free chlorine but less energetically. The carbon must be in the same state as
previously described, in order to ensure its being completely purified by the
chlorine or hydrochloric acid.
In making the vacuum in the vessel containing the carbon filament of an
incandescent lamp, great difficulty is experienced in getting rid of the residual gas.
This gas may be ordinary atmospheric oxygen or nitrogen, or it may be carbonic
oxide or carbonic acid derived from the combustion of portions of the carbon, or
it may be aqueous vapour or any other gas occluded by the carbon filament or the
metal conductors within the glass. When the exhaustion is carried to a high
degree, it is extremely difficult to predict what will be the nature of this residual
gas. The bulk of the occluded gas is got rid of by the ordinary process of heating
the filament electrically during the progress of the exhaustion, but this method is
not capable of removing the minute traces of residual gas above referred to.
To accomplish this I enclose in the bulb of the lamp, or in a chamber forming
part of the lamp and connected with the bulb, a small quantity of some substance
having a more or less general power of absorption of gases. Thorina heated to a
degree below redness, possesses this property in a remarkable degree, and alumina
and many other earths have a similar property. If the substance used has a
marked preference for some one gas, as some forms of alumina have for aqueous
vapour. I expose the substance to be used to this gas before using it, so that it may
be charged with it when first put into the lamp. After introducing the absorbing
substance into the lamp, I exhaust the bulb in the ordinary way, and with the
usual expedient for expelling the occluded gas from the carbon filament. When
the exhaustion is tolerably complete-say to the one-tenthousandth part of an
atmosphere the absorbing substance is heated by applying the flame of a spirit
lamp or other source of heat to the part of the glass where it is situated, and the
gas occluded in it is driven off. The bulb is then reexhausted as completely as
possible, the heat being continued during the reexhaustion. The bulb is then
scaled off, and the absorbing substance on cooling absorbs the residual gas in the
bulb. By this means the exhaustion can be easily pushed to a degree that would
be impracticable by the use of a pump only.
Instead of putting the absorbent material into the bulb of the lamp itself, it may
be contained in a vessel permanently attached to the pump, so as to be in connection
with the bulb of the lamp during the process of exhaustion.
By the above process it will generally be found easy to produce a very perfect
vacuum such that it will not permit an induction current to pass which will
strike across several inches of air. But the process may be varied by admitting

page_text: into the lamp, after the first exhaustion has been effected (either with or without
the aid of the process above described), some gas for which the absorbing substance
has great absorbing power, and then heating and re-exhausting as before. In this
way many absorbing substances which have not the power of general absorption to
any great degree, but have a marked affinity for some special gas or vapour, such
as charcoal, spongy palladium, or phosphoric anhydride, may be used with very
satisfactory results.
Incandescent lights in so high a vacuum as this last longer than in ordinary
vacua as obtained by the mercury pump.
I am aware that it has been proposed that carbons for are lamps should be
manufactured by soaking rods of gas retort carbon in hydrofluoric and other acids..
in order to extract the impurities. I do not believe that it would he practicable so
to purify the carbon, on account of the difficulty of causing the acids thoroughly
to penetrate the dense material, and also on account of the difficulty of thoroughly
cleansing the carbon after the operation. I am also aware that it has been proposed
in some cases to purify the completed carbon filaments of incandescent lamps by
boiling them in hydrofluoric and other acids. I do not claim such processes as part
of my Invention. I am also aware that it has been proposed to introduce into the
glass bulbs of incandescent lamps, a small quantity of freshly burned lime or some
other substances which chemically combine with carbonic acid gas, and also a
small quantity of sodium or potassium or some other substance which readily
forms a nonvolatile oxide when in the presence of free oxygen, in order that
there may not remain in the residual gas within the bulb any trace of oxygen.
either in a separate form or combined with carbon in the form of carbonic acid
gas, inasmuch as the presence of oxygen in either of these forms would speedily
lead to the combustion of the carbon, and the destruction of the lamp. To
this also I make no claim. But what I do claim as my Invention is as
follows:
First. The
use of hydrofluoric acid with or without other acids in the
manufacture of carbous for electric lamps, and more especially the carbon
filaments of incandescent lamps, by exposing cellulose to the action of the
acid or acids and then carbonizing the same substantially as herein described.
Second. The use of hydrofluoric acid with or without other acids in the
manufacture of the various forms of carbon discs, sticks, or rods employed in
electric lamps, by exposing carbon after carbonization to the action of the acid or
acids, such carbon being in the shape of thin sheets or filaments, or fine powder,
substantially as herein described.
Third. The use of free chlorine or hydrochloric acid gas in the manufacture of
the various forms of carbons employed in electric lamps to purify the carbon
substantially as herein described.
Fourth. The introduction into the bulbs of incandescent electric lamps, or into
chambers in connection therewith, and forming part of the lamps, of substances
having a general power of absorbing gases, for the purpose of getting rid of the
residual gas, after the exhaustion by the pump is completed, substantially as herein
described.
Fifth. The method of obtaining a good exhaustion by the introduction into the
bulb of incandescent electric lamps, or into chambers connected therewith forming
part of the lamps, of substances having a selective power of absorbing gases whilst
ensuring by the means herein above described, or equivalent means, that the
residual gas after the exhaustion by the pump is completed, is of a nature to be
readily absorbed by such substances, substantially as herein described.
Sixth. The methods of getting rid of the residual gas herein above described, by
the use of substances having a general or selective power of absorbing gases, the
absorbing substances being placed in a chamber permanently connected with the
pump used for exhausting the bulbs, and not in the bulbs themselves or in chambers
connected therewith and forming part of the lamps, and thus getting rid of the

page_text: residual gas before sealing off the bulb of the lamp, substantially as herein
described.
In witness whereof I the said William Crookes have hereunto set my
hand and seal this twenty eighth day of September in the year of our
Lord One thousand eight hundred and eighty one.
WILLIAM CROOKES. (L.S.)