Conductors for Incandescent Electric Lamps.
Year: 1886

page_text: Date of Application, 31st May, 1886.
Complete Left, 28th Feb., 1887.
Complete Accepted, 15th Apr., 1887.
Conductors for Incandescent Electric Lamps.
WILLIAM MAXWELL 34 Claybrook Road Fulham S.W. Engineer do hereby declare
the nature of my invention for IMPROVEMENTS IN THE MANUFACTURE OF CARBON
FILAMENTS OR OTHER CONDUCTORS FOR INCANDESCENT ELECTRIC LAMPS to be as
follows:-
In manufacturing carbon filaments or other refractory conductors for the
illuminating part of incandescent electric lamps it is usual-after baking to heat
such conductors electrically while surrounded by a hydrocarbon gas or liquid in order
to obtain a deposit of refractory carbon within their pores and upon their surfaces.
The process is attended with serious disadvantages for whether a hydrocarbon gas
or liquid is used its decomposition or dissociation commences before the immersed
conductor attains a high temperature and consequently the separation and deposition
of carbon begins before all the impurities and occluded gases can be expelled from the
substance of the conductor, the nature of the deposit and the rapidity of the deposition
varies with the rise or fall of temperature while the products of the decomposition and
the impurities expelled from the filament intermixes with and progressively alters the
character of the surrounding gas or liquid during the process.
The nature of my invention is a system or process for the production of a pure
dense and homogeneous deposit of highly refractory carbon upon the surface of the
carbon filament or other conductor at high temperatures from an atmosphere which
will not be materially vitiated during the process and by which the rate of the
deposition may be easily controlled.
To attain these results I inclose or surround the carbon filament or other refractory
conductor within an atmosphere or current-which may be attenuated to any required
degree of nitrogen or any other suitable gas which may be without direct chemical
action on the substance of the conductor undergoing treatment. I then gradually heat
the conductor by the electric current until it attains a sufficiently high temperature to
expel organic impurities occluded gases etcetera which are immediately swept away in
the stream of neutral gas, by this means the substance of the conductor is purified and

page_text: .
prepared for the deposit of carbon. I then admit to the incandescent conductor a current
of nitrogen or other suitable neutral gas impregnated or charged with any suitable pro-
portion of hydrocarbon vapour, a portion of the hydrocarbon vapour is decomposed by
the heated conductor and a part of the carbon deposited thereon while the vitiated
gaseous products are withdrawn by the air pump. When the deposition required las
been completed the depositing atmosphere may be displaced by the neutral gas.
The dilution of the hydrocarbon vapour with a neutral gas and its further
attenuation by a vacuum pump prevents a too rapid deposition while the high
temperature of the conductor throughout the entire process of deposition and the
prompt withdrawal of vitiated products ensures the production of a dense homogeneous
and refractory deposit of carbon. The rate of deposition can be controlled by the
degree of vacuum and the relative proportions of the hydrocarbon vapour and the
neutral gas.
In carrying out the process I make use of a case or chamber-which can be
rendered airtight-of any suitable form or construction furnished with valves or taps
in communication with an air or vacuum pump and with supplies of the gases vapours
etcetera required during the process. The case or chamber may be provided with
any suitable electrical fittings and instruments and with any suitable guages for
measuring the gases and vapours in use and also with any suitable form of refrigerator
or other means for obtaining any required degree of temperature.
The case or chamber may be formed to receive one or more carbon filaments or
other conductors and the gases vapours etcetera may be introduced at any required
temperature.

page_text: Conductors for Incandescent Electric Lamps.
I, WILLIAM MAXWELL of 34 Claybrook Road, Fulham, S.W., Engineer do hereby
declare the nature of this invention and in what manner the same is to be performed
to be particularly described and ascertained in and by the following statement :—
In manufacturing carbon filaments or other refractory conductors for lighting by
incandescence, it has hitherto been usual to heat them electrically whilst immersed in
hydrocarbon gas or liquid, in order to close interstices and to deposit carbon upon
it
them.
This process has certain disadvantages.
The dissociation of the hydrocarbon commences before the immersed conductor has
attained a high incandescence, and consequently carbon of an impure and variable
character is deposited thereon before the impurities and occluded gases have been
wholly expelled. The deposition proceeds very rapidly, and cannot be controlled with
sufficient nicety to yield exact results. Also the gas immediately surrounding the
incandescent conductor increases in temperature and changes in character, thus
altering the nature of the deposit. It is therefore impossible to produce a perfectly
pure or homogeneous carbon by this method, and it has therefore been customary
afterwards to cleanse the conductors by treating them with acids, alcohol, etc., or by
heating them in an inert gas. It has also been found that deposits of carbon so
produced upon conductors of dense structure and smooth surface, are only slightly
adherent, and may be separated therefrom.
It has been proposed to use a hydrocarbon vapour instead of a gas or liquid, but
owing to the very rapid deposition of its carbon at comparatively low temperatures and
the consequent quick deterioration of the vapour, it has been found impracticable to
heat the conductor to the extreme temperature that is required, to ensure a dense and
refractory deposit of carbon.
According to my invention I reverse the usual conditions, and by the use of an
atmosphere, which I maintain pure, and at a constant temperature, by a continual
change during the process of deposition, I am enabled to produce a pure, dense,
refractory, and perfectly homogeneous deposit of carbon, upon filaments, or con-
ductors, while by duly proportioning the constituents of this atmosphere, I am able to
control the rapidity of the deposition, and the amount of the deposit, with the utmost
exactness.
When the conductor contains impurities or occluded gases, I prepare it as a
preliminary for the deposit of carbon, by electrically heating it to a high temperature,
within a vessel through which a stream of a neutral gas that does not act chemically
on the conductor, is caused to flow, at a reduced pressure. Nitrogen or hydrogen is a
suitable gas for this purpose, but obviously other gases might be used. When by this
means the conductor has been thoroughly purified, I displace the neutral gas by the
atmosphere from which the carbon deposit is formed. This atmosphere consists of a
neutral gas, which may be the nitrogen or hydrogen previously employed, charged
with a definite proportion of vapour from a volatile hydrocarbon such as Benzine or
Pentane. The finely divided hydrocarbon vapour being carried into contact with the
heated conductor is partially decomposed, and a portion of its carbon is deposited
thereon, while the resultant gaseous products are immediately withdrawn, and this
action continues until the required deposit is formed. By thus diluting the hydro-

page_text: carbon vapour with neutral gas I am able to maintain the physical conditions
practically constant, while I so far moderate the decomposition of the vapour as to
allow the conductor to be heated to an extremely high temperature without any
inconvenience. The rate of the deposition can be easily controlled by regulating the
proportions of the gas and vapour, and the pressure and temperature at which they
are used.
In practically carrying out my invention I use a case or vessel that can be closed
air-tight, and is provided with pipes so that the gas, or gas and vapour employed, may
enter at one end, and pass out from the other, the entrance pipe being in connection
with the supplies of gas, etc., and the exit pipe with an air-pump or other means of
exhaust. The case is also furnished with electrical connections and may have guages
to ascertain the pressure and temperature of the fluid passing through it.
The intermixture of the neutral gas and the hydrocarbon vapour in definite pro-
portions may be effected-at known pressures and temperatures-by supplying them
through metersor through tubes of graduated area.
I place within the case, the filament or other conductors in holders that can be con-
nected to a source of electricty, and having closed the case, I withdraw the air and
allow a stream of gas such as nitrogen or hydrogen to pass through at a reduced
pressure one tenth of an atmosphere may be conveniently used-I then electrically
heat the conductors and having thus expelled their impurities and withdrawn thei
from the case by the exhaust, introduce the depositing atmosphere while the con-
ductors are at a high incandescence, and maintain the flow through, as before, until
the desired deposit is obtained. The depositing atmosphere may then be displaced by
the neutral gas.
Obviously where the filaments or other conductors are sufficiently pure and cleared
of occluded gases, the first part of the process may be dispensed with, the deposit
being effected as above described by the regulated flow of the mixture of neutral gas
and hydrocarbon vapour through the vessel.
Although the proportions of the neutral gas and the hydrocarbon vapour, and the
pressure and temperature at which they are used, may be varied within wide limits,
yet it is generally preferable, that the gas should not be saturated with the vapour,
that the pressure should be considerably under that of the atmosphere, and that the
temperature of the fluid mixture should be low.
Having thus particularly described and ascertained the nature of my said invention
and in what manner the same is to be performed, I hereby declare that I make no
general claim to the purifying of conductors for electric lamps by electrically heating
them in vacuo or in an inert atmosphere, nor do I claim generally the effecting of
deposition of carbon on conductors by electrically heating them in a hydrocarbon
atmosphere, but what I claim is :-
1. The herein described process for depositing carbon on the conductors of
incandescent electric lamps by electrically heating them in a vessel through which
continuously flows a mixture of hydrocarbon vapour with a gas that does not
chemically act upon the carbon.
2. In combination with the process stated in the preceding claim the herein
described preliminary process for purifying the conductors by electrically heating
them in a vessel through which continuously flows at reduced pressure a gas that does
not chemically act upon the conductor.