Improvements in the Manufacture of Filaments for Incandescent Lamps.
Year: 1891

page_text: Date of Application, 8th Dec., 1891-Accepted, 9th Jan., 1892
Improvements in the Manufacture of Filaments for Incandescent
Lamps.
I, SYDNEY PITT of Sutton in the County of Surrey, Gentleman (a communica-
tion to me from abroad by Ludwig Karl Böhm, City, County and State of New
York, Electrician and Chemist) do hereby declare the nature of this invention and
in what manner the same is to be performed to be particularly described and ascer-
tained in and by the following statement :-
The invention relates to a process of making filaments for incandescent or glow
lamps of high or low resistance.
Heretofore the filaments used to emit light by incandesence when heating by the
passage of the current have consisted of carbon and it has always been the aim to
produce carbon of the utmost purity for such filaments.
same,
It has also been proposed to make filaments by soaking carbonaceous materials in
solutions of metal salts, such as chlorides and nitrates, and then carbonizing the
but such process is defective owing to the fact that in certain phases of the
process strong mineral acids, as for example hydrochloric and nitric acid, are set at
liberty, which are highly injurious to the filament and are likely to render the
filament worthless for practical use in glow lamps.
By other proposed processes filaments have been made consisting of carbon and a
metal as for example aluminium and tungsten.
In other processes a filament is first carbonized in the usual manner and is sub-
sequently coated by the heating action of an electric current passing through it in
which case the coating is likely to peel off or break away from the filament when
the current is passed through it, after the filament has been removed from the liquid
in which the coating is done.
The object of the present invention is to produce a filament which has a sub-
stantially different composition from those heretofore used, said filament being
formed from carbon and an oxide of the earth, alkali metals, preferably calcium or
magnesium oxide, and the invention consists in such filament and also in the process
of making such filament in which process no injurious decomposition products are
formed, such as the strong mineral oxides above mentioned, but on the other hand
the decomposition products are only such as are produced in the ordinary process of
carbonizing the materials commonly used in making filaments wholly of carbon.
The filament thus produced is a compound of carbon, a conductive material, and
calcium or magnesium oxide, which are substantially non-conducting and highly
refractory materials and when heated to incandesence by the passage of the current
constitute a substantial part of the light producing agent.
The conducting and non-conducting refractory substances are thoroughly com-
bined and united so as to form a compound having a sufficient conductivity to convey
the current, but of far higher resistance than the conductive material of the com-
pound alone, and also having far greater light emitting capacity.
The property of calcium oxide and magnesium oxide to emit light when heated
to incandesence is well known, this being the substance used in the well known
Drummond light, in which such substances are heated by the oxy-hydrogen blow
pipe flame, and plaster of paris has been employed in a form of arc light typified
by the well known Jablochkoff candle in which plaster of paris is employed as an
insulator to electrically separate the carbon electrodes between which the arc is
maintained, and enhances the light by its incandesence produced by the heat of the
current passing over it from one to the other carbon terminal of the candle.
In such use of plaster of paris it operates as a substantial non-conductor or

page_text: calcium carbonate as above described, this will be effected by the heat of the electric
current while the lamp is in the process of manufacture.
When desired a larger portion of the refractory material may be embodied in a
filament than can be introduced therein by soaking in soluble bicarbonate of
calcium.
This can be done by adding to the solution of bicarbonate of calcium some of the
freshly precipitated calcium carbonate which is kept in suspension in the solution
by adding gum-arabic to the solution and by passing a constant current of carbonic
acid gas through the solution.
It is especially desirable to do this in the manufacture of filaments for series
lamps which are of comparatively low resistance and are shorter and thicker than
high resistance filaments.
Gum arabic used as thus stated dries when the thread is dried and serves to retain
the calcium carbonate in the thread while being handled in the process of manu-
facture.
The products given off in the decomposition of the bicarbonate and carbonate,
namely, vapour of water and carbon oxides, obviously produce no injurious effect
upon the material of the filament, since they are substantially the same as the
products given off in the ordinary process of carbonizing filaments.
The herein described materials may be compounded in various proportions, it
being possible to make a homogeneous compound of carbon with a very large or very
small proportion of calcium or magnesium oxide and the proper proportions for any
given filament may be determined by experiment, it being understood, that, other
things being equal, the resistance is greater the larger the proportion of calcium
or magnesium oxide incorporated in the compound.
The effect of the current is to combine as above described the carbon with the
calcium or magnesium oxide to form a new body or series of bodies homogeneous
in texture and much superior to carbon.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed 1 declare that what I
claim is :-
1. The process of making filaments for incandescent lamps consisting of impreg-
nating carbonizable material with bicarbonate of calcium or magnesium and
carbonizing the same, whereby a compound filament is produced, substantially as
described.
2. The filament herein described, which consists of a homogeneous compound
formed from carbon and calcium or magnesium oxide, substantially as described.
Dated this 7th day of December 1891.
SYDNEY PITT.