Improvements in the Manufacture of Electrical Resistances.
Year: 1896

page_text: Date of Application, 13th June, 1896-Accepted, 18th July, 1896
COMPLETE SPECIFICATION.
Improvements in the Manufacture of Electrical Resistances.
We, AUGUSTUS JOHN MARQUAND of Bute Docks, Cardiff, in the County of
Glamorgan, Engineer, and DAVID LOWDON, of Barry Graving Dock, Barry
Dock, near Cardiff, Electrical 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 :-
The object of our invention is the manufacture of electrical resistances to be
used as regulators; the resistance being variable at will by the variation of the
pressure to which the resisting material is exposed.
Such regulators are needed for dynamo electric machines, electric heaters,
electro plating baths, lamps and a variety of other uses.
We make use of resilient sheets of fibrous material carbonised by exposure to
heat in the presence of carbonaceous vapour and with the exclusion of air. We
use the word "carbonised" to mean the deposition of conducting carbon within the
pores and on the surfaces of the material.
Preferably we proceed as follows:-We take woven asbestos cloth and we coat
it on both surfaces with coal tar which we apply by means of a brush, having
thinned the tar to a suitable consistency by the addition of a solvent such as
benzine, and preferably we dust the coated surfaces with powdered sulphur.
The sheet of fabric so prepared we enclose in an iron or other box to prevent
access of air and we heat the box and its contents in a furnace to a full red heat;
we remove the box from the furnace and allow it to cool before opening it.
It will be found that the asbestos fibre has become carbonised, the vapour from
the tar being decomposed by the heat and its carbon deposited in the fibre probably
in a graphitic condition.
The fabric is now ready for use, it is cut up into pieces of size convenient for
the purpose for which the resistance is required and the pieces are arranged in
piles of the required depth to attain the resistance and to pass the quantity of
current which the electrician desires; these dimensions are regulated by well
known rules.
The degree of electrical resistance of the fabric will depend much on the
temperature to which it has been exposed, the higher the temperature the less the
resistance. The heat employed should however not be excessive as extreme
temperatures somewhat impair the resilience of the fabric which it is important to
preserve.
Pressure may be applied to the pile by means of a screw approximating two
metal plates between which the pile is held. In some cases it is convenient to
apply the pressure by means of levers and other means may be resorted to. Whilst
the pressure is gradually increased the resistance of the pile decreases with
regularity; and as the pressure is relieved the resistance of the pile is progressively
regained.
We can obtain like results in using asbestos cardboard and many organic
substances may replace tar in carbonising the fibre.
The process above described may be varied without departing from our
invention.

page_text: Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what
we claim is :-
1. An electrical resistance which can be varied at will and which consists of
sheets of a resilient material such as carbonised asbestos cloth arranged in a pile,
and appliances by which the pile can be submitted to a graduated pressure.
2. The preparation of a material to be used as a variable electrical resistance by
impregnating a nonconducting substance such as asbestos with carbonaceous matter
and exposing it to heat either with or without the presence of sulphur.
3. A variable electrical resistance consisting of woven asbestos cloth carbonised
or impregnated with deposited carbon.
T
1000
A. J. MARQUAND.
DAVID LOWDON.