IMPROVEMENTS IN ELECTRIC TELEPHONES
Year: 1881

page_text: LETTERS PATENT to William Main, of Brooklyn, in the County of Kings and
State of New York, one of the United States of America, Engineer, for an
Invention of "IMPROVEMENTS IN ELECTRIC TELEPHONES."
PROVISIONAL SPECIFICATION left by the said William Main at the Office
of the Commissioners of Patents on the 2nd May 1881.
WILLIAM MAIN, of Brooklyn, in the County of Kings and State of New York,
one of the United States of America, Engineer. "IMPROVEMENTS IN ELECTRIC
TELEPHONES."
This Invention relates chiefly to battery transmitting telephones, but it is also in
part applicable to receiving telephones. Its object is to secure the accurate and
faithful transmission and reproduction of vocal sounds, even those which are faint
and distant, without change of tone, and without transmitting also the extraneous
and inaudible vibration which frequently occur coincidently with speech.
One feature of my Invention consists in the employment in a telephone of what
I term an acoustic vibrator, which answers to the diaphragm of other telephones,
and which consists of a vibrating plate, preferably of thin resonant wood, which is
hinged, pivoted, suspended, or attached at one edge or portion, leaving its
remaining portion and its other edges free to vibrate. In my preferred con-
struction I hang the vibrator from its upper edge, leaving it free to swing back
and forth in a direction at right angles to the plane of its surface.
Owing to the natural tendency of every vibrating body to vibrate at one certain
pitch peculiar to itself, an acoustic vibrator constructed as thus far described
responds more loudly to sounds of one pitch than to others. To correct this defect
I compose the vibrator of two plates of slightly different pitch, which are separated
a short distance so as to confine a layer of air between them. One of these plates
is suspended by hinges at its upper edge, and the other is borne by the first, being
sustained thereon through the medium of supports arranged symmetrically with
reference to the outline of the latter plate, which is preferably in the form of a flat
Pris )]

page_text: disk connected to the suspended plate by a concentric annular support, and arranged
outwardly or toward the sound opening in the enclosing case. This second or
outer plate being symmetrically supported vibrates more freely than the inner one;
and as the two vibrate somewhat differently each modifies the vibration of the
other, thus destroying the tendency of both to vibrate at any one pitch. This
result is aided by the confined layer of air between them. I also in employing
wooden plates cross the grain of adjacent plates, which also aids in destroying their
natural pitch.
In lieu of using this extra or outer plate I may employ ribs of thin light wood,
paper, or similar material, which I affix edgewise to the surface of the vibrating
plate, so that they stand at right angles to the plane thereof.
In constructing my transmitting telephone I affix to the back of this vibrator a
resistance-varying contact or electrode, which may be a button of carbon, platina,
or other indifferent conductor, and I arrange a second such electrode to press lightly
against the first. This latter or rear electrode may be mounted elastically in any
of the many well-known ways, but I prefer to suspend it in the same manner and
from the same axis as the acoustic vibrator, and to so dispose it that its tendency
to hang with its center of gravity directly beneath its point of suspension will press
it forward lightly and uniformly against the front electrode, which will resist its
pressure by virtue of the same tendency. Thus I secure the requisite light but
firm contact between the electrodes, and one which requires no adjustment, and
which cannot be deranged or impaired by variations in temperature or humidity
or by a slight change in the position of the instrument.
By this means I make an instrument so sensitive that it will clearly transmit
speech uttered in an ordinary tone of voice some forty feet away. A transmitter
as sensitive as this is apt to transmit the rumblings of the building and various
vibrations which are inaudible or scarcely audible at the transmitting station, but
which at the receiver are magnified into roarings and confusing sounds. In order
to suppress these microphonic effects I suspend my acoustic vibrator and electrodes
from an inert weight, as a piece of lead or other heavy metal, and I support this
weight through the medium of an elastic or non-vibratory connection, as by rubber
straps, or a cushion of rubber or felt. The vibrations of the ultimate support are
taken up by the elastic connections through the action upon them of the inert
body or weight, and are not communicated to the acoustic vibrator.or electrodes.
My telephone requires no case, but. for convenience I provide it with one, having
an opening opposite the acoustic vibrator for admitting sound. This opening
covered with wire gauze or cloth, which serves as a guard to exclude meddling
fingers, but which has no acoustic effect.
To prevent a rattling sound in the receiver when a person is shouting close to
the transmitter, I provide the rear electrode with a light wing-like plate, which gives
it a hold on the air so that it may be influenced by the sound-waves, and which
prevents it from being thrown out or nearly out of contact with the front électrode
when the latter is thrown into violent vibration. This plate may be a sheet of
mica or light metal, but I prefer that it be a plate of wood, and that it be in all
essential respects the same as the acoustic vibrator to which the first electrode is
affixed, that both electrodes may be influenced alike by sound-waves. Thus con--
structed my telephone transmits a shout as clearly and with as little alteration as
the ordinary tone of voice. By making the case with a sound opening at both
front and back the speaker may stand at either side of the instrument.
A rear electrode thus mounted may be used in connection with the ordinary
peripherally-confined iron diaphragm with good effect.
I prefer to atfix the electrode to the outer or additional plate of the vibrator,
since the vibration of that plate is more free and uniform than that of the suspended
plate.. I fix the electrode to the center of the outer plate, and it projects inwardly
through a sound-opening in the suspended plate.
My Invention is well adapted to be used in connection with two sets of
electrodes, either when both sets are placed on one side of a vibrator or when one

page_text: is placed on each side thereof. In the latter case I arrange each set in a separate
circuit or branch circuit, whose currents flow in opposite directions relatively to the
induction coil or coils which re-act upon the line circuit. As the vibrator vibrates
to one side the current in one circuit is strengthened while that in the other is
correspondingly diminished, so that alternately-opposite secondary currents are
sent to line.
My acoustic vibrator may be substituted for the ordinary diaphragm in receiving
telephones to good effect by suspending it before the pole or poles of the magnet
and affixing an armature to it. The magnet may be made light, and be also
suspended to advantage. Or my vibrator may be mechanically connected with a
vibrating coil through which the current passes, or with a stretched wire surrounded
by a coil through which coil the current passes, or with the contact-strip of the
well-known electro-motograph receiver, also known as the chemical telephone.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said William Main in the Great Seal Patent Office on the 24th October
1881.
WILLIAM MAIN, of Brooklyn, in the County of Kings and State of New York,
one of the United States of America, Engineer. "IMPROVEMENTS IN ELECTRIC
TELEPHONES."
This Invention relates chiefly to battery transmitting telephones, but it is also
in part applicable to receiving telephones. Its object is to secure the accurate and
faithful transmission and reproduction of vocal sounds, even those which are faint
and distant, without change of tone, and without transmitting also the external
rumblings and inaudible vibrations which frequently occur coincidently with
speech.
In the accompanying Drawings Figures 1 to 23 show various forms of resistance-
varying transmitting telephones, and Figs. 24 and 25 show a magnetic receiving
telephone constructed according to my Invention.
The preferred form of my transmitter is shown in Figures 1 to 4. Figure 1 is a
side elevation of the telephone, its enclosing case being in section cut along the
lines 1, 1, in Figures 2 and 4; Figure 2 is a front elevation of the telephone, the
case being partly in vertical section cut along the lines 2-2 in Figs. 1 and 3;
Fig. 3 is a vertical transverse mid-section of the telephone and case cut along
the lines 3-3 in Figures 2 and 4; and Figure 4 is a vertical mid-section cut along
the line 4-4 in Figures 1 and 3.
A, A, are two "acoustic-vibrators," as I term them, which answer to the diaphragm
in other telephones, but differ therefrom in being freely suspended from above
leaving their other edges free to vibrate. Both are hinged to a rod or axis a whose
ends rest in non-vibratory cushions b, b,.of
or other heavy metal hung on the rod a.
of two thin light boards or plates and
felt or rubber; c, c, are weights of lead
Each acoustic-vibrator A is constructed
e of resonant wood, the plate & being

page_text: hinged at its upper edge to the rod a, and the plate e being borne by the plate d
through the medium of a thin wooden ring f which separates them slightly, so that
a thin layer of air is confined between them. The plate d has a central sound-
opening g communicating with the air-space.
B, B, are the resistance-varying contacts or electrodes, one being fastened to each
plate e, so that the two project toward each other through the openings g,g. These
electrodes are long enough to separate the acoustic vibrator slightly so that the
tendency of the vibrators to hang directly beneath the axis a causes them to press
the electrodes lightly but firmly together. This pressure remains always uniform,
requiring no adjustment. I prefer that the electrodes be made of platina and
carbon respectively, but other substances may be used. They are connected by
wires i and to the local battery and induction-coil in the usual-way.
A transmitting telephone thus constructed is extremely sensitive to sound, and
will clearly transmit speech uttered in an ordinary tone of voice at a distance of
40 to 60 feet from. it. The rumblings and jarrings of the building are not
transmitted. The vocal and other sounds are transmitted without change of tone,
so that speech sounds naturally at the receiver. Sounds of different pitch are
transmitted impartially, and loud sounds and shouting are transmitted as clearly as
low sounds.
My telephone does not require a case, but I prefer to protect it and conceal its
parts from sight by enclosing it in a case C. This case has a large sound-opening D
at each side opposite the plates e, e, and this opening is covered by a guard h of
wire gauze or wire cloth, or a grating of light rods, to keep out the fingers of
meddlesome people. The speaker may stand at either side of the instrument
opposite either opening.
The operation in transmitting is as follows:-A sound-wave coming in either
direction causes the front acoustic-vibrator (the one it first encounters) to vibrate
in sympathy with it, and then passing onward to the rear vibrator it also sets that'
into vibration. As the front vibrator vibrates backwardly it presses its electrode
against the rear electrode, which yields somewhat, and on the return or forward
vibration the front electrode tends to move away from the rear electrode which
follows it, and thus the pressure between the electrodes is alternately increased and
decreased, and the intensity of the current through the electrodes is correspondingly
varied.
In consequence of the acoustic-vibrators being left entirely free to vibrate, and
of the electrodes being made self-adjusting, these variations in the current exactly
correspond with the sound-waves in the air, whatever be the amplitude and
frequency of the sound-waves.
Each plate & and e tends to vibrate at a pitch of its own, the pitch of the plate d
differing slightly from that of the plate e, so that by their attachment each modifics
the other's vibration, so that the compound plate vibrates impartially to sounds of
.any pitch.
The felt cushions b, b, absorb the inaudible vibrations and rumblings of the
building, so that they are not communicated to the acoustic vibrators, and the
weights c, c, serve to keep the rod a pressed closely upon the cushions, and prevent
its being affected by such vibrations as may pass through the cushions.
Figs. 5 and 6 show a modified construction. Figure 5 is a vertical section of
Fig. 6, cut along the line 5-5 therein; and Fig. 6 is a front elevation of Figure 5,
partly broken away and the case C in section on the line 6-6 in Fig. 5; here the
rod a is hung by rubber bands b, b, instead of resting on cushions, and the part f
connecting the plate e to the plate d is a small central washer placed behind the
electrode B.
Figs. 7 and 8 correspond to Figs. 5 and 6 with the case and supports removed.
Here the part f is a ring which connects the discs d and e together at their
peripheries. Figure 9 shows the same construction with the sound-opening g
omitted so as to enclose the air between the plates, and the electrode is fixed to the
plate d.

page_text: Figures 10, 11, and 12 show plates of different shape and differently mounted.
In Figure 10 the plate e is square and connected to the plate d by two straight.
strips ff. In Figure 11 eis octagonal and connected by 4 posts f, f. Figure 12
is a side or edge view of Figures 10 and 11.
Figures 13 and 14 show the plates e, e, omitted.
Figure 15 is a vertical section similar to Fig. 7, and shows the front electrode B.
provided with the acoustic-vibrator A, shown in Figures 1 to 4, and the rear
electrode hung by a metal arm k, and having a plate e' of mica or thin metal
fixed to it.
Figure 16 is a side view of another form, both electrodes being hung by arms k; k,
and each having a thin wooden disk e fixed to it.
In Figures 13, 14, and 16, if the two opposite plates are close enough together,.
each will somewhat modify the vibration of the other, though not as thoroughly as
when they are fixed together, as in Figures 1 to 12.
Figure 17 is a side elevation of another form of telephone, the case C being in
section on the line 17-17 in Figure 18, which is a front elevation with the case C
in section on the line 18-18 in Figure 17. The rear electrode B is hung on an
arm k. The rod a is hung below the weight c, which is suspended by rubber-
bands b, b. The acoustic-vibrator consists of a single flat plate having thin wooden
walls l, l, of veneer or cardboard fixed edgewise to it, so as to project from its
surface at about right angles thereto. These walls serve to modify the vibration of
the plate, and prevent its vibrating in any one pitch, and they also serve to confine
the sound waves and direct them against the plate. The case C is lined with felt,
rubber, or other muffling material m to suppress echoes.
Figures 19, 20, 21, and 22 show another form. Figure 19 is a vertical mid-
section; Figure 20 is a vertical section cut on the line 20-20 in Figure 19, and
looking from the rear; and Figures 21 and 22 are horizontal sections cut on the
line 21-22, the case C being omitted. The acoustic vibrator A and arms k, k, are
hung from the weight c by flexible strips instead of hinges. The walls are fixed
to the case C. There are two pairs of electrodes B, B, which may be connected
with the wires i, j, in either of the ways shown in Figures 21 and 22.
Figure 23. is a diagram showing two pairs of electrodes B, B, arranged on
opposite sides of the vibrator A; E is the induction-coil; F, the line circuit; and
G, the local circuit divided into two branches n¹ and n², each of which branches
includes a battery p and one pair of electrodes B. As the vibrator A vibrates to
one side it increases the resistance in one branch, and decreases the resistance in the
other branch, and as these branches are reversed relatively to the induction-coil E,
alternately-reversed currents circulate through said coil, so that alternately reversed
secondary currents are sent to line. Instead of this one battery and two induction
coils may be used, one coil in each branch of the circuit, and the line circuit
passing through both coils.
Figure 24 is a vertical section of my magnetic receiving-telephone, and
Figure 25 is a rear elevation thereof. The acoustic vibrator A is the same as
in Figures 1 to 4, and to its center is fixed an armature q; H is a permanent
magnet suspended from the rod a, and having a soft-iron core 7 fixed to each of its
poles facing the armature q and wound with a coil s; t, t, are adjusting screws
to keep the cores from touching the armature. As the magnetic attraction acting
on the armature q varies, it causes the vibrator A to vibrate, thereby producing a
sound.
In using wooden plates for my vibrator A, I cross the grain of adjacent plates, as
indicated by the shading in the Drawing, to improve the tone.
Having thus fully described and ascertained the nature of the said Invention, I
declare that I claim,-
First. In a telephone an acoustic-vibrator A suspended from its upper edge so
as to be free to swing, as and for the purposes described.
Second. An acoustic vibrator A consisting of a plate d hung or supported at one

page_text: -
edge or side with a plate e arranged adjacent to it, and supported by it through the
medium of a part f, substantially as described.
Third. An acoustic-vibrator A consisting of a plate & hung or supported at one
edge or side, having an opening g and a plate e supported by it through the medium
of a ring-shaped piece f, substantially as described with reference to Figures 1 to 4
and 7 to 9.
Fourth. An acoustic vibrator A consisting of two adjacent plates d and e of
light resonant wood arranged with their grain crossed, substantially as shown.
Fifth. A transmitting telephone consisting of a suspended acoustic-vibrator A
bearing an electrode B with another electrode B hung from above, so that the force
of gravity shall press the two lightly together, substantially as described and
shown.
Sixth. A transmitting telephone consisting of two acoustic vibrators A, both
suspended freely from above and each bearing an electrode B, which electrodes
project toward each other and are pressed lightly together, by means substantially
as described.
Seventh. An electrode B hung from above and having a light vibrating plate e¹
fixed to it, as shown in Figures 15 and 16, and for the purpose described.
Eighth. A telephone case having a muffling lining arranged out of contact with
the acoustic vibrator and electrodes, as shown in Figures 17 to 20, and as herein-
before described.
Ninth. The acoustic vibrator or vibrators A and electrodes B hung from an
axis a which is supported by an elastic body b, substantially as set forth and
shown.
Tenth. An acoustic vibrator A freely suspended and bearing two opposite
electrodes with a suspended electrode arranged to press lightly against each, and
the opposite pairs of electrodes arranged in oppositely-polarized circuits or branch
circuits, as shown in Figure 23 and as set forth.
Eleventh. A receiving-telephone consisting of a suspended acoustic vibrator A
in combination with a suspended electrically variable magnet H, as shown in
Figures 24 and 25, and as herein before described.
In witness whereof, I, the said William Main, have hereunto set my hand
and seal, this Thirtieth day of September, in the year of our Lord
1881.