IMPROVEMENTS IN MEMBRANE TELEPHONES
Year: 1884

page_text: We, SILVANUS PHILLIPS THOMPSON of University College, Bristol, Doctor of
Science, and PHILIP JOLIN of 2 Elmdale Road, Redland, Bristol, Engineer do
hereby declare the nature of our invention for IMPROVEMENTS IN MEMBRANE
TELEPHONES to be as follows:-
Our improvements have for their object the perfection of telephonic instruments
such as are described in the English Mechanic of August 11th 1876 page 551
having armatures carried on membranes or drumheads, and our improvements
are designed to effect an increase in the sensitiveness and efficiency of the
apparatus.
We make our drumheads in the following manner. We unite together two or
more membranes, after placing between them an armature of clock-spring or of
thin iron and we also in some cases enclose between them a frame which may
be annular, square, hexagonal, or any other suitable shape cut out in metal, ebonite,
or other non-absorbent or stiff material to stretch them. To unite the membranes
we use glue, canada balsam or other suitable material. As membranes we employ
gold-beaters skin either crude or dressed, parchment, bladder, or animal-matter
paper. In some cases we apply a solution of chrome alum when we have united
the membranes. In some cases we unite two or more membranes, pressing them
whilst still plastic in a mould which raises or depresses a ridge in the membranes ;
this ridge being adapted to fit into a corresponding groove or rim in the case of
the instrument. We press the membranes before the cement is hardened in a
mould or die, and when hard the form so impressed remains. After hardening or
straining the drum-head may be impregnated from its under side with linseed
oil or with stearine, indiarubber, or other suitable material. When very thin
membranes are used we unite 4 or 5 of them with linseed oil, or other suitable
cement. The armature enclosed between the membranes may be of circular, oval,
rectangular or other convenient form, but we prefer that it should be slit into
two or more parts to minimise the circulation in the metal of internally-induced
currents. In those cases in which there is no framing or raised ridge employed,
the drum-head is mounted by being stretched over the end of a shallow truncated
cone of brass or other rigid material; and is preferably held thereon by being
bound on by a binding wire or cord tied over a groove or rim in or on the surface
of the cone.
The drum-head so mounted is placed in a case in front of an electro-

page_text: magnet and is held in its place by any suitable device: but we prefer the following.
A short tube or cylinder of metal, ebonite, or other rigid material having a screw
cut on the interior of one end, and having an inwardly projecting rim at the other
end, is placed over the cone and screws down to the body of the case so that the
inwardly-projecting rim bears on the cone and presses it and the drumhead against
its seat.
In some cases we attach an inner conical piece of metal or other material,
having a central hole, to the cone which carries the membrane; or we attach the
said inner conical piece to the external clamping tube or cylinder. As an electro-
magnet we employ one of the known forms of electromagnet, or some modification
thereof. For example we find it advantageous to construct the core of two parts
of different thicknesses, a thin part of iron, preferably slit to prevent internal induc-
tion-currents, which receives the coil, and a thicker part of iron or steel serving to
steady the magnetism of the thinner part. Another form of electromagnet which
we employ is formed in the following manner of three parts. There is, firstly, a
central pole-piece of iron, which receives the coil, and which may be slit to prevent
internally induced currents. There is, secondly, an external pole-piece of iron,
formed in a G-shape or Y-shape, surrounding the coil but not completely
enclosing it, and assisting in concentrating the magnetic effects of the coil upon
the armature. Thirdly, there is a curved body of iron or steel which is attached
at its ends to the two pole-pieces, and also serves as a handle for the instrument.
Dated this 22rd dose of November 1881

page_text: COMPLETE SPECIFICATION.
We, SILVANUS PHILLIPS THOMPSON of University College, Bristol, Doctor of
Science and PHILIP JOLIN, of 2 Elmdale Road, Redland, Bristol, Engineer do
hereby declare the nature of our invention for "IMPROVEMENTS IN MEMBRANE
TELEPHONES and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
"
In the Specification of our Letters Patent dated A.D. 1884 Nº 9471 we have set
forth various improvements in telephonic instruments of the class described in the
English Mechanic of August 11th 1876 page 551 which instruments have
"
armatures carried on membranes or drumheads.
Our present Improvements are designed to further improve such instruments
and thereby increase their sensitiveness and efficiency.
We use drum heads made as described in our said former Specification by uniting
two or more membranes after placing between them a suitable armature and in
some cases a frame to stretch them the said membranes being united by glue
canada balsam or other suitable cement. Such membranes may be of gold beaters
skin, either crude or dressed, parchment, bladder, or animal matter, or paper, one
important feature of our improvements is the employment of a solution of chrome
alum applied to the membranes preferably after they are united, to protect the
same against atmospheric and other injurious effects The solution may be con-
veniently applied with a brush to the finished membranes. When very thin
membranes are used we unite a number say four or five of them with linseed oil
or other suitable Cement.
The armature enclosed between the membranes is circular oval rectangular or
of other convenient form, we prefer that it should be slit into two or more parts
to minimize the circulation in the metal of internally induced currents, as described
in our said former Specification.
Another feature of our present Invention is the employment of a separate shallow
truncated cone or ring of brass or other rigid material having a groove or depres-
sion at one end thereof around its outer surface the membrane being supported
upon the said cone or ring by being stretched over the end and secured by tying
around the groove. The drumhead so mounted is placed in a case in front of an
electro magnet and can be held in its place by any suitable device we have how-
ever devised special means for this purpose as hereinafter described. In addition
to the high degree of efficiency secured by this method of mounting, a drumhead
so mounted possesses the obvious advantage of being capable of removal or replace-
ment with very little trouble.
Whatever may be the form of electro magnet employed in our instruments we
find it advantageous to construct the core of two parts of different thicknesses
viz. A thin part of iron (preferably slit to prevent internal induction currents)
which receives the coil and a thicker part of iron or steel serving to steady the
magnetism of the thinner part.
In the accompanying Drawing
Figure 1 is a central section of an instrument constructed according to our
Invention and
Figure 2 is a similar section shewing a modified construction of parts of the

page_text: A is a box or case formed of ebonite or other suitable material, a is the drum-
head or diaphragm, b is the armature, c is the aforesaid truncated cone.
The membrane is mounted and stretched on the said cone and is secured thereon
by turning it over the edge of the cone into a groove or channel formed therein and
binding the said membrane by a wire or cord laid over the portion in the said
groove as shewn at a¹.
In
d is a short tube or cylinder of metal ebonite or other suitable material having
a screw cut on its interior as shewn, and an inwardly projecting rim at d'. The
said tube is placed over the cone and screwed down to the body of the case.
the instrument shewn in Figure 1 the conical funnel f is formed in one piece with
the cone c, and the inwardly projecting rim d¹ of the aforesaid tube d bears on the
cone and presses it down upon the drum head which is thereby kept securely
against its seat.
In figure 2 the cone c is separately formed, and the funnel f is a part of the tube
or mouth piece d the latter being screwed down upon the cone to hold the drum
head in place as above specified.
The electro magnet is preferably constructed with pole pieces substantially as
described in our said former Specification No 9471, g¹ is the core on which the coil
or bobbin h is placed as shewn, g² is the C shaped or Y shaped pole piece formed
to partially enclose the bobbin; both of these pole pieces being attached to the
part which as shewn in figure 1 forms a handle for the instrument but which
is sometimes arranged within the case or box as shewn in Figure 2.
Having now particularly described and ascertained the nature of our said
Invention and in what manner the same is to be performed we declare that what
we claim is
1st The treatment or preparation of the membrane or membranes of telephones
with a solution of chrome alum for the purpose described.
2nd In membrane telephones the employment of a truncated cone or ring separate
and capable of being removed from the instrument and supporting and stretching
the membrane or membranes substantially as described.
3rd The improved telephone instrument hereinbefore described and illustrated by
the drawings annexed.