Improvements in Electric Condensers.
Year: 1894

page_text: Improvements in Electric Condensers.
A communication to me from abroad by Professor RICHARD THRELFALL, of the
University of Sydney, New South Wales.
I, GEORGE THRELFALL, of 95 Finsbury Pavement, London, E.C. Engineer,
do hereby declare the nature of this invention to be as follows:-
With electric condensers constructed in the usual manner, it has been difficult
to keep down the production of heat which is principally due to imperfect insulation
of the dielectric, leading to small conduction currents with their accompanying
losses; to the presence of small air bubbles between the armatures across which
discharges may occur; and to the phenomena of "electric" absorption or "residual
charge," causes herein referred to and included under the term of phenomena of
dielectric hysteresis.
Now my correspondent has discovered that an essential condition of success in
making condensers is the exclusion of air and water especially if condensers are to
be used with alternating currents and this invention is based on this discovery and
is for the manufacture or construction of electric condensers specially adapted by
exclusion of air and water to prevent or mitigate dielectric hysteresis and to work
satisfactorily with alternating currents of electricity of high voltage.
To this end a condenser according to this invention is made with a dielectric
such as paper of a pure and uniform description impregnated with oil at a suitable
temperature such as 140° Centigrade.
For
In carrying out the invention that kind of grey paper known in commerce under
the name of Swedish filter paper may advantageously be used. It is important
that the paper shall be uniform in texture and free from size. A sheet of such
paper having a thickness of say about 0.3 m/m is able to withstand an electrical
pressure of about one thousand volts, applied in an alternating manner.
condensers designed for an alternating pressure of two thousand volts, as ordinarily
defined, a double thickness of paper may with advantage be used. Two thicknesses
may at least in all cases be used so as to minimise any danger of short circuiting
that may arise from the presence of holes in the paper.
The paper is cut into sheets of the required size, care being taken that the edges
are cut clean so that the sheets will lie uniformly upon one another. The paper
thus prepared is then impregnated with oil.
The conditions to be satisfied by the oil are the following. It must be a good
insulator, and must not permanently lose its insulating properties after being
exposed for some hours to a temperature of say 140 Centigrade, or thereabout, and
it must not be too liquid. Oil sold for lubricating gas engines has been used with
success, but a heavier oil would probably answer even better. The blue oil from
the Hartly Vale (N.S.W.) Shale Works has also been used with success. Subject
to the insulating properties being satisfactory and the viscosity sufficient, the
greater the specific inductive capacity of the oil the better.
In order to impregnate the paper with the oil a suitable vessel such as a flat
dish made from thin metallic plate and somewhat larger than the sheets of paper,
and two or three inches deep, is about half filled with oil which is then raised
by appropriate means to a temperature of say 140° Centigrade, or thereabout. The

page_text: bath being maintained at this temperature, the sheets of paper are introduced one
by one, in such a manner that at first the paper lies on the surface of the oil.
The hot oil soaking through the paper will drive the air before it so that when,
after say about ten seconds, the paper is thrust beneath the oil, the effervescence
will be so small as to cause no inconvenience. Usually it will not be advisable
to have more than about one hundred papers in the dish at once, for they tend
to float up to the top of the oil and thus prevent the dipping of fresh sheets.
When say a hundred sheets have been collected, a small weight is placed upon
them and they are left to soak say for about an hour or until no further evolution
of gas occurs, the temperature being maintained approximately constant.
dish and its contents are then allowed to cool slowly.
The
The condenser is then built up in a suitable metallic box in the usual manner.
The sheet metal used with the sheets of dielectric prepared as described, may
advantageously consist of the thin pewter foil commonly used for covering bottles
as it possesses the advantage of not taking up so much room as the thicker varieties
of sheet metal commonly employed in condensers, and of being less expensive than
tin foil. These however may be used if desired. The inside of the box may advan-
tageously be lined with asbestos though other insulating substances such as ebonite,
glass, fibre and wood have been found to give satisfactory results. In building
up the condenser it is necessary to avoid as far as possible-introducing air
between the successive sheets of paper and metal foil. If the oil is sufficiently
viscous, there is no difficulty in doing this, for each sheet of paper as it is lifted
'out of the dish carries with it a thick film of oil. With a very little care, the
metal foil can be rubbed down eo as to get rid of most of the air bubbles. When
the desired number of sheets have been introduced, and any deficiency in the oil
made up by the addition of more oil, which has been previously heated to a
temperature of say 140° Cent. or thereabout, the condenser is removed to a large
vessel heated to about the same temperature and capable of being exhausted of air.
It is neither necessary nor advantageous to attempt to obtain a very good
vacuum. A reduction of pressure to about one or two inches of mercury will be
found sufficient.
The vessel should be exhausted and air allowed to flow in again, several times
in succession during the process, each period of exhaustion lasting say for about an
hour.
The condenser should afterwards be allowed to cool slowly and then be tested
for insulation. The ends of the tin foil forming the armatures of the condenser,
should be smoothly rubbed together and doubled on themselves so as to insure
a perfect contact; want of the due observance of this precaution is a very common
source of heating.
The actual terminals of the finished condenser may conveniently be passed
through tubes soldered to the lid of the box. The terminals may consist of stout
wire armed at their lower ends with springs and so placed as to bear on the tin
foil ends of the armatures when the lid of the box is placed in position. The li
should be secured air tight, as by solder, and be provided with means such as
a tube, to allow of the box being completely filled with oil, which should be
previously heated.
Dated this 26th day of April 1894

page_text: A communication to me from abroad by Professor RICHARD THRELFALL, of the
University of Sydney, New South Wales.
I, GEORGE THRELFALL, of 95 Finsbury Pavement, London E.C. 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 :-
With electric condensers constructed in the usual manner it has been difficult to
keep down the production of heat which is principally due to imperfect insulation
of the dielectric, leading to small conduction currents with their accompanying
losses; to the presence of small air bubbles between the armatures across which
discharges may occur; and to the phenomena of "electric absorption" or "residual
charge."
Now my correspondent has discovered that an essential condition of success in
making condensers, is the exclusion of air and water, especially if condensers are to
be used with alternating currents, and this invention is based on this discovery and
is for the manufacture or construction of electric condensers specially adapted, by
exclusion as completely as possible of air and water, to prevent or mitigate dielectric
losses and to work satisfactorily with alternating currents of electricity of high
voltage.
To this end a condenser according to this invention is made with a dielectric
such as paper of a pure and uniform description impregnated with oil at a suitable
temperature such as 140° Centigrade.
In carrying out the invention that kind of grey paper known in commerce under
the name of Swedish filter paper may advantageously be used, though this invention
is not limited to the use of this paper only. It is important that the paper used
shall be uniform in texture and free from size. A sheet of paper of the kind
mentioned having a thickness of say about 0.3 m/m is able to withstand an
electrical pressure of about one thousand volts applied in an alternating manner.
For condensers designed for an alternating pressure of two thousand volts, as
ordinarily defined, a double thickness of paper may with advantage be used. Two
thicknesses at least may in all cases be used so as to minimise any danger of short
circuiting that may arise from the accidental presence of holes in the paper.
The paper is cut into sheets of the required size, care being taken that the edges
are cut clean so that the sheets will lie uniformly upon one another. The paper
thus prepared is then impregnated with oil.
The conditions to be satisfied by the oil are the following. It must be a good
insulator, and must not permanently lose its insulating properties after being.
exposed for some hours to a temperature of say 140° Centigrade, or thereabout, and
it must not be too liquid. The blue oil from the Hartley Vale (N.S.W.) Shale
Works has been used with success but care must be taken not to use an oil that has
been cooled excessively as heavy oil obtained from shale distillation now commonly
are in order to extract the paraffin. Subject to the insulating properties being
satisfactory and the viscosity sufficient, the greater the specific inductive capacity
of the oil the better.
In order to impregnate the paper with the oil a suitable vessel such as a flat dish
made from thin metallic plate and somewhat larger than the sheets of paper, and
two or three inches deep, is about half filled with oil which is then raised by
appropriate means to a temperature of say 140° Centigrade or thereabout. The
bath being maintained at this temperature, the sheets of paper are introduced one
by one, in such a manner that at first the paper lies on the surface of the oil. The

page_text: hot oil soaking through the paper will drive the air before it so that when, after
say about ten seconds, the paper is thrust beneath the oil, the effervescence will be
so small as to cause no inconvenience. Usually it will not be advisable to have
more than about one hundred papers in the dish at once, for they tend to float up
to the top of the oil and thus prevent the dipping of fresh sheets.
When say a hundred sheets have been collected, a small weight is placed upon
them and they are left to soak say for about an hour or until no further evolution
of gas occurs, the temperature being maintained approximately constant. The
dish and its contents are then allowed to cool slowly.
The condenser is then built up in a suitable metallic box in the usual manner.
The sheet metal used with the sheets of dielectric prepared as described, may
advantageously consist of the thin pewter foil commonly used for covering bottles
as it possesses the advantage of not taking up so much room as the thicker varieties
of sheet metal commonly employed in condensers, and of being less expensive than
real tin foil. These however may be used if desired. The inside of the box may
advantageously be lined with mica, though other insulating substances such as glass,
and asbestos sheet have been found to give satisfactory results. In building up the
condenser it is necessary to avoid-as far as possible--introducing air between the
successive sheets of paper and metal foil. If the oil is sufficiently viscous, there is
no difficulty in doing this, for each sheet of paper as it is lifted out of the dish
carries with it a thick film of oil. With a very little care, each sheet of metal foil
can be rubbed down after being placed on an impregnated sheet of paper so as to
get rid of most of the air bubbles that might otherwise be left between the two.
When the desired number of sheets of prepared paper and metal foil have been
introduced into the box and any deficiency in the oil made up by the addition of
more oil, which bas been previously heated to a temperature of say 140° Cent.
or thereabout, the condenser-consisting of the box with alternately arranged
sheets of prepared paper and metal foil and oil-is removed to a large vessel heated
to about the same temperature and capable of being exhausted of air.
It is neither necessary nor advantageous to attempt to obtain a very good
vacuum. A reduction of pressure to about one or two inches of mercury will be
found sufficient.
The vessel should be exhausted and air allowed to flow in again, several times in
succession during the process, each period of exhaustion lasting say for about three
hours and the whole process 10 hours.
The condenser should afterwards be allowed to cool slowly and then be tested
for insulation. The ends of each of the two series of sheets of tin foil forming the
two armatures of the condenser, should be smoothly rubbed together i.e. the com-
ponent sheets of each armature and doubled on themselves so as to insure a perfect
contact; want of the due observance of this precaution is a very common source of
heating.
The actual terminals of the finished condenser may conveniently be passed
through tubes soldered or screwed to the lid of the box. The terminals may
consist of stout wire armed at their lower ends with springs and so placed as to
bear on the tin foil ends of the armatures when the lid of the box is placed in
position and thus compress the sheets forming the condenser. The compression
may be increased by insulated screws passing through the lid. The lid may be
secured in place by solder, or by nuts and bolts, and be provided with means such
as a tube, to allow of the box being completely filled with oil, which should be
previously heated say to 130° C.
I am aware that it has been proposed to construct condensers suitable for high
electrical pressures by building up a condenser of thin layers of paper and tin foil
and after compressing it and baking it at a temperature of about 100° C., immersing
it in paraffin oil in order to fill up the interstices within it, but this method of using
an insulating oil to saturate the paper, which I do not broadly claim, is obviously
quite different from the method hereinbefore described as constituting the present
invention. Having since the date of the present Application had communicated to me

page_text: by my foreign correspondent that an electric condenser can with advantage be
constructed practically free from air and water by using a dielectric consisting of
material such as paper, silk, cellulose, or other fibrous material in the form of sheets
of a pure and uniform description impregnated at a suitable temperature, say
from 130° C. to 150° C. with an insulating material that is capable of being readily
liquefied by heat to facilitate the impregnation of the sheet dielectric and displace-
ment therefromn of air and moisture but which is solid or semi-solid at ordinary
atmospheric temperature, the process of manufacture in other respects resembling
that hereinabove described, I have applied for Letters Patent No. 1906 of 1895,
and in the Specification thereof have described the manufacture of electric
condensers with such materials.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed I declare that what
I claim is :-
1. The manufacture of electric condensers practically free from air and water
and suitable for use with alternating currents of high potential, by impregnating
sheets of material such as paper of a pure and uniform description with an insulating
material having the properties herein referred to by floating the sheets upon such
an insulating material that is liquid and heated, subsequently immersing the sheets
in the liquid heated insulating material and allowing them to soak therein, then
building up the condenser with the prepared sheets of dielectric material and sheet
metal in a box and heating and exhausting the condenser substantially as herein
described whether or not the finished condenser be finally subject to pressure.
2. In an electric condenser a dielectric consisting of sheets of material such as
paper of a pure and uniform description impregnated with insulating material
having the properties herein referred to by Hoating the sheets upon the liquid
heated insulating material, immersing the sheets therein and allowing them to soak
therein substantially as herein described for the purpose specified.
3. An electric condenser constructed substantially in the manner hereinbefore
described.
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