A New and Improved Method of Making High Electrical Resistances
Year: 1897

page_text: Date of Application, 6th Aug., 1897
Complete Specification Left, 21st Apr., 1898-Accepted, 20th May, 1898
PROVISIONAL SPECIFICATION.
A New and Improved Method of Making High Electrical Resistances
FREDERICK BENJAMIN FAWCETT, 1, Rockleaze Avenue, Sneyd Park, Bristol,
Gentleman, do hereby declare the nature of this invention to be as follows:-
In place of the thin covered wire or carbon used in the manufacture of electrical
resistances, I use a very thin film of metal deposited in vacuo by the well known
electrical method, on glass or some other suitable non-conductor.
The film when deposited is given the form of a narrow strip, by a suitable cutting
mechanism. Conductors are attached to convenient points on the strip thus pre-
pared, and the whole is then immersed in a non conducting medium from which
dissolved gases and other volatile impurities are expelled.
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Tel 1
1907
A New and Improved Method of Making High Electrical Resistances.
FREDERICK BENJAMIN FAWCETT, 1, Rockleaze Avenue, Sneyd Park, Bristol,
Gentleman, 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 place of the covered wire or carbon used in the manufacture of high electrical
resistances, I use a very thin film of metal deposited in vacuo, by the well known
electrical method, on glass or some other suitable non conductor.
Depositing the film. The glass strips upon which the film is to be deposited
(after being previously well cleaned) are slipped into metal holders, which serve the
double purpose of holding the strip, and also preventing its edges from being
deposited upon.
In front of the glass strip, and not less than half an inch from it the wire or wires
from which the deposit is to be made are fastened by suitable glass rods. I use
three wires of platinum and gold, the length of the glass strip, and separated from
each other by an eighth of an inch so as to form a grid. This makes the deposit
more even.
This grid must be connected to the negative terminal of an induction coil, thus
forming the cathode. The anode is provided by a suitable wire placed behind
the strip, and connected to the positive terminal of the coil.
This whole arrangement is then covered by a thick glass shade, which fits
into a groove in an ebonite base, on which the above mentioned strip-holders, and
depositing wires are fixed; and it is also provided with a tube which connects it

page_text: to a mercury pump. The glass shade having been put into position, and rendered
air tight by means of marine glue run into the groove in the base, a high vacuum
is produced by the pump, and the current being turned on the deposit of metallic
alloy takes place. When the glass strip is rendered partially opaque the film is
probably thick enough.
The strip is now taken out of the depositing vessel, and wires soldered on to
either end of the film (no flux being used). It is well after doing so to bind with
wire a piece of asbestos on to the solder, so as to prevent the connecting wires being
pulled off by accidental strain:
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Preparing the film.-In order to enable the film to arrive at a constant electrical
resistance I boil it in heavy mineral oil under reduced pressure for a considerable
time. This is done in the following way. Several strips of glass with their films
uppermost, are placed in an iron tank, the strips being, firmly held by clamps at
either end. This tank is fitted with an air-tight cover provided with a pipe con-
necting it with an air puseveral hours at a time, and their resistance, tested.
When it is found that the resistance has not altered since the previous boiling, the
film may be considered to have arrived at a stable condition with regard to its
electrical resistance, and is now ready for the cutting process.
The films are boiled for
#
The reason for cutting the film is twofold; firstly to increase its effective resist-
ance, and secondly to enable it to be adjusted to the required value.
The cutting of the film is carried out in the following way. The before
mentioned tank, in which the films are boiled, is mounted on wheels running in
grooves, and capable of movement in a direction along the length of the film, the
amount of movement being regulated by a micrometer screw. A hard tempered
steel style is further provided, fixed at the end of a rigid arm, the other end of
which is pivotted. The style is thus rendered capable of movement only in an
arc of a circle; this movement being at right angles to the direction of movement
of the tank.
Let now a scratch be made with the style on the film say from left to right
commencing the scratch an eighth of an inch or so 'from say, the left edge of the
film and traversing the rest of its width. Then let the film be moved forward a
small distance by means of the micrometer screw, and let a corresponding scratch
be made from right to left commencing an eighth of an inch from the right edge
of the film. It will be seen that if this process be carried on down the length of
the film the scratches commencing alternately on the right and on the left the
effect is to cut the film into a long and narrow zigzag strip. The scratches must
be such that they remove all the film from the glass where they occur, thus
making it, impossible for the current to get across a scratch, and therefore com-
pelling it to keep to the zigzag strip.
The electrical resistance of the film is tested frequently during cutting, and the
cutting process continued until its resistance has arrived at the required value.
The film is now taken out of the tank, in which it has been boiled and adjusted,
and placed in its permanent glass vessel. The two conducting wires are sealed
through the sides of the vessel in the usual way, and the other end of the vessel
is drawn down and a small size glass tube sealed into it. The vessel is now filled
with heavy mineral oil or some other suitable non conductor and connected to an
air pump by the aforesaid, small glass tube. The oil must be boiled under reduced
pressure, for a considerable time in order to remove all dissolved gases and other
volotile impurities when this has been done (while still hot and on the pump) the
small glass tube is drawn, off, thus hermetically sealing, the vessel.
In order to enable the resistance, of the film to settle down to a constant value,
it is well to heat the glass vessel containing the film in boiling water for several
days testing its resistance each day until it becomes constant (this has the effect
of ageing the film). If now, it is found that the film has settled down to too low
a resistance a small subsidiary coil, of wire or some other suitable resistance is put
in series with the film thus bringing its resistance to the required value..

page_text: The glass vessel containing the film and the auxiliary resistance. (if any) are
now fixed in a convenient box, which is fitted with the usual binding screws.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is:-
1. The application of a thin film of metallic alloy, deposited in the hereinbefore
mentioned manner to the purpose of electrical resistances.
2. The application of the herein before mentioned cutting process to the
metallic film for the purpose of increasing its effective resistance, and also
adjusting the same.
3. The application of the herein before mentioned process of boiling the film
in the insulating oil, to enable it to arrive more quickly at a constant resistance.
4. The combination of the metallic film with an hermetically sealed containing
vessel.