Improvements in Telephonic Switching Apparatus.
Year: 1891

page_text: Date of Application, 4th Nov., 1891
Complete Specification Left, 3rd Sept., 1892-Accepted, 8th Oct., 1892
PROVISIONAL SPECIFICATION.
Improvements in Telephonic Switching Apparatus.
I, ALFRED ROSLING BENNETT, of 22, St. Albans Road, Harlesden, London,
N.W., Electrical Engineer, do hereby declare the nature of this invention to be
as follows:-
:-
The object of my invention is to provide an efficient system of telephonic
switching of the kind known as direct-call or double-call, the purpose of which is
to enable one subscriber to ring the bell of another without attracting che attention
of the operator at the exchange and, when required, to signal the operator without
disturbing the other subscriber. Such systems have heretofore usually involved
the employment of voltaic batteries at the subscribers' stations or polarized electro-
magnets at the exchange. My invention renders the use of these appliances
unnecessary. It is specially applicable to telephonic systems constructed on the
metallic circuit plan.
At the subscriber's station I place a magneto generator (although a voltaic
battery may be used instead) and call bell of any suitable pattern, which, when
the receiver is hung on the automatic switch-arm, can be brought into circuit with
the subscriber's metallic loop in the usual way. An earth wire is brought to the
subscriber's instrument and arranged so that by the depression of a button or lever
attached to the instrument it becomes connected to the metallic loop during the
continuance of such depression. Instead of the required temporary connection
between the metallic loop and the earth being effected by means of a button or
lever, an additional contact may be furnished on the automatic switch and arranged
so that the depression of the switch caused by the hanging up of the receiver will
establish a temporary connection between the loop and the earth, the said temporary
connection ceasing as soon as the automatic switch has been depressed to its full
extent by the weight of the receiver.
At the exchange the metallic loop is connected to a special form of spring-jack
or contact-device having three contacts. One wire of the loop is attached to one
of these contacts, which may be the upper one, and which I shall call contact No. 1,
while the second wire is attached to the other two contacts which may be placed
below or opposite to contact No. 1 and which I shall call contacts No. 2 and
No. 3. The second wire is attached to contact No. 2 directly, and an electro-
magnet or other suitable signalling device, is connected between contacts Nos. 2
and 3, so that the second wire is in connection with contact No. 3 only through
contact No. 2 and the coils of the signalling device. When the metallic loop is
out of use contacts Nos. 1 and 3 are in permanent connection with each other. In
that position a current from the subscriber's station will find a circuit through the
first wire of the loop; contact No. 1; contact No. 3; coils of signalling device;
contact No. 2; second wire of loop.
For connecting two metallic loops together a switch plug is used comprising two
conducting portions insulated from each other, which conducting portions are
joined, by a flexible cord containing two insulated conductors, to corresponding
conducting parts of a second and similar switch-plug. The plug must be so
arranged that when inserted oue conducting part makes contact with contact No. 1
and the second conducting part simultaneously with contacts Nos. 2 and 3,
establishing a short circuit between them and short-circuiting the coils of the
signalling device. When two metallic loops are so joined a current from one
subscriber's instrument will find a circuit through the other subscriber's instrument
without traversing any coils at the exchange, and this applies equally to ringing
and speaking currents.

page_text: To enable the subscribers to intimate to the exchange the termination of
a conversation and their desire to be disconnected I make the following
arrangement.
From each of the two conductors of each connecting cord used in the exchange,
preferably from its centre, I lead a branch wire or conductor to earth through a
resistance which consists preferably of a coil of wire. The resistance may or may
not be combined with cores, sheaths or condensers. Between the two resistances
and earth I put a source of electricity, such as a voltaic battery, so that when the
plugs of a connecting cord are in the spring-jacks the battery is charging the two
metallic loops, that are thereby connected, through the resistances. But normally
no current results and no leakage of ringing or speaking currents takes place since
although one pole of the source of electricity is to earth the other is in communication
with metallic loops that are everywhere insulated from it.
Now if a relay or other suitable apparatus is included between the resistances
and the source of electricity or between the source of electricity and earth the tem-
porary connection between the metallic loop and earth which is effected at the
subscriber's station by depressing the button or lever already described, or by the
act of hanging the receiver on the automatic switch-arm, will complete the circuit
of the source of electricity through the earth and a current will flow which will
actuate the relay and yield a signal that will advise the operator that disconnection
is desired. Instead of employing a separate relay the two resistances may be
furnished with iron cores and an armature and wound go that the armature may be
attracted and give a signal whenever the circuit of the battery or other source of
electricity is completed.
The connection of the battery to the metallic circuits as described constitutes a
standing test of the electrical condition and fitness for work of the metallic circuits
since if either of them is faulty by reason of a leakage to earth at any point the
circuit of the battery will be completed through the fault as soon as the plug is
inserted and the deflection of the relay will show the operator that the circuit is not
proper order.
in
In lieu of using the earth to complete the circuit of the battery a special wire
to each subscriber's station may be employed for the purpose.
I have herein described the broad principles of my invention and indicated one
effective method of carrying it out, but it must be understood that the construction,
connections and arrangement of the resistances, sources of electricity and other
details, may be varied considerably.
Subject to certain modifications to be particularly ascertained in my Complete
Specification the invention is likewise applicable to telephonic exchanges worked
with single wires.
The invention may be adapted to multiple switch-boards constructed either with
the spring-jacks joined in series according to the plan of the Western Electric
Company's multiple switch-boards, and others, or in parallel substantially according
to the plans developed in my Specification No. 19486 of 1890.
may
The details of such adaptations, which will be fully set forth in my Complete
Specification, include the provision of signalling devices of special design to be
looped between contacts Nos. 2 and 3. These special signalling devices may be,
broadly speaking, of three different types. Firstly, contacts Nos. 2 and 3
have looped between them an electro-magnet of high inductance and magnetic
inertia; or, secondly, one consisting of an armature or needle influenced by two
coils of fine wire insulated from each other like the coils of a phonophore so that
the continuity of the metallic loop is interrupted at that point; or, thirdly, the
continuity of the metallic loop is broken between contacts Nos. 2 and 3 by the
interposition of a condenser of sufficient capacity to permit the currents from a
magneto to actuate a relay or other signalling device which is also joined between
the said contacts. All such signalling devices are of course short-circuited by the
insertion of a switch-plug and the consequent establishment of a direct connection
between contacts Nos. 2 and 3.

page_text: When the spring-jacks are connected in series the provision of a third contact
at each multiple jack is desirable so that the insertion of a plug may short-
circuit the metallic loop at that point and render it unnecessary for the currents
passing between subscribers to suffer in efficiency by traversing the rest of the
loop and the coils of the signalling device, which they would otherwise be forced
to do. All the spring-jacks would thus resemble that already described, having
one contact, No. 1, on one side of the loop, and two, Nos. 2 and 3, which are
short-circuited by the insertion of a plug, on the other.
When the jacks are connected in parallel, only the one controlling the signalling
device requires three contacts. All the multiple or repetition jacks may be
simple sockets without springs like those described in my Specification No. 19486
of 1890 aforesaid.
COMPLETE SPECIFICATION.
Improvements in Telephonic Switching Apparatus.
I, ALFRED ROSLING BENNETT, of 22 St. Albans Road, Harlesden, in the
County of Middlesex, Electrician, 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 :-
My invention relates to telephonic switching apparatus.
The object of my invention is to provide an efficient system of telephonic
switching of the kind known as "direct" or "double-call," the purpose of which
is to enable one subscriber after being connected to another subscriber to ring the
bell of the said second subscriber without attracting the attention of the operator at
the exclange, and, further, if he wishes, to signal the operator without disturbing
the said second subscriber with whom he is in connection. Such systems have
heretofore usually involved the employment of voltaic batteries or specially con-
structed apparatus at the subscribers' stations or at the exchange. My invention
renders the use of the ordinary appliances practicable. My invention is only
applicable to telephonic systems in which complete metallic circuits are used for the
subscribers' line wires.
At the subscribers' stations I prefer to use a magneto generator and call bell of
any of the usual types or of any suitable pattern, which, when the receiver is hung
on the automatic switch-arm, can be brought into circuit with the subscriber's
metallic loop in a manner well understood. I may however employ voltaic
batteries for calling purposes at the subscribers' stations in lieu of magneto
generators.
In carrying out my invention I employ a wire connected to the earth and brought
to the subscriber's instrument so arranged that by the depression of a special
button or lever attached thereto or fixed in the vicinity thereof, the said earth
wire becomes connected to one or both wires of the metallic loop or subscriber's
line and the said metallic loop thereby connected to earth during the continuance of
such depression. Instead of the required connection between the metallic loop
and the earth being effected by means of a button or lever an additional contact
may be furnished to the automatic switch-arm with which the instrument is usually
provided, and arranged so that the depression of the switch caused by the hanging
up of the telephone receiver will establish a temporary connection between the loop
and the earth, the said temporary connection ceasing as soon as the automatic
switch has been depressed to its full extent by the weight of the receiver.
At the exchange the metallic loop or subscriber's line wire is connected to a
form of spring-jack or contact device having three contacts. One wire of the loop
is attached to one of these contacts, which may be the upper one, and which I shall
call contact No. 1, while the second wire is attached to the other two contacts

page_text: which may be placed below or opposite to contact No. 1, and which I shall call
contacts No. 2 and No. 3. The second wire of the loop is attached to contact No. 2
directly, and an electro-magnet, or other suitable signalling device, is connected
between contact No. 2 and contact No. 3, so that the said second wire of the loop
is in connection with contact No. 3 only through contact No. 2 and the coils of the
signalling device. I also arrange that when the line wires are out of use contacts
Nos. 1 and 3 are in permanent connection with each other. In that position a
current from the subscriber's station will find a circuit as follows; through the
first wire of the loop; contact No. 1; contact No. 3; coils of signalling device;
contact No. 2; second wire of loop.
The switch-plug used in connecting two metallic loops together comprises two
conducting portions insulated from each other, which conducting portions are
joined by a flexible cord containing two insulated conductors to corresponding con-
ducting parts of a second and similar switch plug. The conducting parts of the
plugs are so arranged that when inserted in the contact blocks one conducting part
makes contact with contact No. 1, and the second conducting part either simul-
taneously with contacts Nos. 2 and 3 or with contact No. 2 alone. In the first
case a short circuit is established between contacts 2 and 3, in the second case
contact No. 3 is insulated and cut out of circuit altogether. Either of these plans
may be adopted with equal success. When two metallic circuits are so joined a
current from one subscriber's instrument will be able to flow through the other
subscriber's instrument without traversing any coils at the exchange and this
applies equally to currents for ringing and speaking. An efficient arrangement of
switch-plug will be hereinafter described in detail.
To enable a subscriber who has been placed in communication with another
subscriber through the exchange, to intimate to the operator the termination of a
conversation and desire for disconnection, I proceed as follows, that is to say:--
From each of the two conductors of each connecting cord used in the exchange,
preferably from their centres, I lead a branch wire or conductor to earth through
& resistance which consists preferably of a coil of wire. The resistance may or
may not be combined with iron cores or sheaths or with condensers. Between the
two resistances and earth I put a source of electricity such as a voltaic battery, so
that when the plugs of a connecting cord are in the spring-jacks the battery is
charging the two metallic loops which are thereby connected. But ordinarily no
current flows through the earth branch and no leakage of ringing or speaking
currents takes place, since, although one pole of the battery is put to earth, the
other is in communication with metallic loops that are everywhere insulated from it.
I moreover include an indicator or a relay or other suitable apparatus between the
resistances and the battery or between the battery and the earth.
If a temporary connection between the metallic loop and earth be effected at the
subscriber's station by depressing the special lever or button already described, or
by the act of hanging up the receiver on the automatic switch-arm, the circuit of
the battery through the earth will be completed and a current will flow which will
actuate the indicator or relay at the exchange and give a signal that will advise
the operator that disconnection is desired. Instead of employing a separate relay
the two resistances may be furnished with iron cores and an armature, and wound
so that the armature may be attracted and yield a signal when the circuit of the
battery through the earth is completed.
In order that my invention may be more clearly understood I will now proceed
to describe the same with reference to the accompanying drawings, in which
Figure 1 is a diagrammatic representation of the apparatus I use at the subscriber's
station.
Figure 2 is a similar view showing a modification of the same.
Figure 3 is a diagram showing a triple-contact spring-jack adapted to meet the
requirements of my invention.
Figure 4 is a sectional view of a plug for use with the spring-jack shown in
Figure 3.

page_text: Showing a singly
prug.
Figure 6 is a diagram showing one arrangement of the signal apparatus I use at
the exchange.
Figure 7 illustrates a modification of the apparatus shown in Figure 6.
Figure 8 is a diagram showing part of a multiple switchboard furnished with
triple-contact spring-jacks according to my invention.
Figure 9 is a diagram showing how my apparatus is applied to a multiple switch-
board telephone system in which the subscriber's contact blocks are arranged in
parallel on the switchboard.
Referring now to Figure 1, K is a special lever or button in permanent connec-
tion with the earth by the wire E and normally kept out of contact with the
metallic stop C by a spring S. The stop C being permanently connected to
the subscriber's metallic loop M¹ M or to one wire thereof by the wire W,
the depression of the lever or button K by the finger brings it in contact with C,
and puts the metallic loop to earth during the continuance of the depression. In
Figure 2, K represents the automatic switch-arm of the subscriber's instrument,
which when the line is out of use is kept in the position indicated by dotted lines,
by the weight of the telephone receiver T suspended from it, and which when
relieved of the weight of the said telephone receiver rises to its upper position
under the pull of the spring S. The lever carries a metallic spring or arm A,
which is connected permanently to the metallic circuit M₁ M², or to one wire
thereof by the wire W, and which rubs against and comes in contact with a metal
piece C secured to an insulated part D, every time the lever K is depressed or
raised by the hanging on or taking off of the telephone T. Since the metallic
piece C is connected to earth by the wire E it follows that the subscriber's
metallic circuit is momentarily earthed every time such a movement of the lever K
occurs. At other times the arm or spring A rests on the insulated part D and is
therefore not in circuit.
In Figure 3 is shown the general arrangement of a spring-jack or contact device
with three contacts. MM represent the two wires of the metallic loop or
subscriber's line, 1 the spring contact which normally rests on the fixed contact 3,
and 2 a second fixed contact which is only brought into play on the insertion of a
suitable switch-plug. In constructing the spring-jack the details may obviously be
considerably varied but the type shown in the drawing is an efficient form.
I represents an ordinary indicator or annunciator drop of any suitable pattern
connected between contacts 2 and 3.
A form of plug which may be used with the contact device shown in Figure 3 is
shown in Figure 4. D' is the body of the plug formed of insulating material and
C' is the conducting part intended for connecting with contact 1 of the spring-jack,
and C is the second conducting part designed to connect simultaneously with
contacts 2 and 3. In Figure 5 the same plug is represented but with the part C²
cut short so as to connect, when inserted, with contact 2 only.
Figures 6 and 7 show diagrammatically two methods of arranging the signalling
devices at the exchange as hereinbefore described. In Figure 6, C¹ C³ are the
two conductors of the flexible connecting cord of which A are the switch-plug
contacts, R1, R2, are two resistances joined as a bridge between the two con-
ductors CC, V the source of electricity one pole of which is joined through the
relay Z to earth by the wire E, the other pole being joined at 13 to the bridge
containing the resistances. The relay Z completes the circuit of the electro-
magnetic indicator I and the local battery V. In some cases the relay Z may be
replaced by an indicator or annunciator drop of any efficient type and the local
circuit and battery V' dispensed with, but the coils of such indicator or annunciator
drop should be wound with fine wire to a resistance of at least 400 ohms.
In Figure 7 the resistances R¹ R2 are converted into electro-magnets by the
addition of soft iron cores and a soft-iron armature Z. The closing of the circuit
of the battery V causes a current to flow through one or both coils R' R', the
armature Z is thereby attracted and closes, the local circuit.containing the indicator I

page_text: and the battery V'. In some cases this local circuit may be dispensed with and the
armature Z adapted to release the necessary indicating-shutter.
Instead of using the earth to complete the circuit of the battery a special wire
to each subscriber's station or a common return wire may be employed for the
purpose although the use of the earth is advantageous inasmuch as the connection
of the earthed battery to the metallic circuits as described constitutes a test of the
electrical condition of the metallic circuits, since if either of them is faulty by
reason of any leak to earth at any point a circuit will be completed through the
fault as soon as the switch plugs are inserted and an indication by the relay will
reveal the fact.
The resistances R' R' may each consist of 3000 ohms, made of German silver or
other wire. The relays Z may have a resistance of 600 ohms and the batteries V
may each consist of ten ordinary Leclanché cells.
The invention may be used with multiple switch-boards constructed with the
contact-blocks or spring-jacks arranged either in series or parallel.
In adapting the invention to series multiple switch-boards it is desirable that the
spring-jacks used on the said switch-boards should be provided with a means for
short-circuiting or cutting-out the indicator, since otherwise the telephonic currents
would have to flow through its coils, which would be very undesirable. This may
be accomplished by employing the spring-jack with three contacts already
described, or an efficient equivalent, and making the connections as in Figure 8 in
which M M are two metallic circuits arranged in three sections S¹ S' and S³ of
a multiple board. It is obvious that the insertion of connecting cords furnished
with switch-plugs similar to those shown in Figures 4 and 5 at any section will
prevent the passage of telephonic currents, used for talking or ringing, through
the indicators I¹ I³.
When indicators are used with multiple switch-boards it is usual to group the
subscribers into sets each set being under the sole care of one operator at one
section of the switch-board, and all the indicators of any such set of subscribers
are placed on the section in front of the operator who has charge of that set of
subscribers. I prefer to call that section of the switch-board the "indicator
section" for that set of subscribers. With multiple switch-boards with contacts
arranged in parallel it is desirable to employ special triple contacts similar to those
hereinabove described, on the indicator section only of each subscriber's line, the
contacts on all the other sections of the same line being simply sockets like those
described in my Specification of British Letters Patent No. 19,486 of 1890. The
indicator can then only be short-circuited or cut out if the spring-jack on the
indicator section is used, and it therefore becomes necessary to use indicators which
do not partially short-circuit the metallic loop and spoil the speaking when
connection is established on any other section than the indicator section. An
indicator of ordinary type but made of high resistance and inductance, looped
between contacts 2 and 3 of the spring-jack, would prevent such short-circuiting
although left in. Or the metallic continuity of the wire between contacts 2 and 3
may be interrupted by the interposition of a condenser of about half a microfarad
capacity through which the magneto currents from the subscribers ringing apparatus
would actuate a phonic or other delicate relay also joined between contacts 2 and 3.
According to another method I may employ between contacts 2 and 3 a phonoporic
coil with an iron core, the changes induced in which under the influence of magneto
currents would actuate an armature. The two coils, which are of course insulated
from each other, may each be wound with fine wire to a resistance of 1000 ohms.
When the spring-jack on the indicator section is being used all such signalling
devices are short-circuited or cut out by the insertion of the switch-plug; when
the contact on any other section is employed to establish connection the devices
remain in circuit but do not affect the speaking.
When my invention is applied to multiple switch boards the system of operator's
test described in my Specifications of British Letters Patent No. 12,484 and
.No. 18,451 of 1891 may advantageously be employed, and for this purpose the

page_text: same resistance coils R' R' may be utilized if the ringing-off and testing connections
are made as in Figure 9. In this drawing five metallic circuits M¹ to M are shown,
four being joined two and two by the usual connecting cords, and the fifth, M³,
being out of use. PI to P represent the subscribers' telephones, E¹ to E' the
derivations to earth used for signalling the termination of a conversation ;
S', S, S³, three sections of a multiple switch-board; C1, C2, the conductors of the
connecting cord; R1, R2, the resistances; 11, 12, 13, the signalling devices in the
testing circuit; T1, T2, T3, the testing cords; V the testing battery V₁ the ring-off
battery, 7, 7, polarized relays or indicators which are deflected only by a
current in one direction and left unaffected by a current in the other direction.
Obviously instead of using these separate relays the resistance coils may be fitted
with polarized cores and armatures and made to serve the purpose of relays as
hereinbefore mentioned. The two batteries V₁ and V' have their poles differently
placed in respect to the line and to the ring-off relays Z1, Z2, Z3, that is to say if
these relays are arranged to be actuated only by the passage of a negative current
to line then the zinc pole of V₁ must be put to line and the copper pole to earth,
while the copper pole V2 is to line. Although the testing current from V' will
always pass through the coils of the ring-off relays Z', 7, 73, they will remain
unaffected thereby, whereas the connection of a metallic circuit to earth at a
subscriber's station will immediately bring the battery V₁ into play and deflect the
ring-off relay in connection with that circuit. As the testing relays I', 1³, 1³, are
only traversed by currents from V' they need not be polarized.
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 combination with a telephone system having complete metallic circuits
and switch-boards with flexible connecting cords, of a switch at the subscriber's
station adapted to connect the subscriber's metallic loop to the earth or to a third
or return wire, and a circuit at the exchange for each connecting cord joined as a
branch or deviation from each conductor of each of the said flexible connecting
cords to earth or to the aforesaid return wire, and including a battery and indicator,
for the purpose specified.
2. In a telephone exchange worked with metallic circuits an indicator circuit
comprising a battery and indicator joined to the flexible connecting cords used for
coupling two subscribers' line wires, substantially as described with reference to
Figure 6 or Figure 7 of the accompanying drawings, for the purpose specified.
3. In a telephone exchange the combination with the subscribers' metallic circuit
line wires, of triple contact switch-plugs and indicators connected substantially as
described with reference to Figure 8 of the drawings, for the purpose specified.
4. The means whereby any subscriber may connect his metallic circuit line wire
to the earth or to a third or return wire, substantially as described with reference
to either Figure 1 or Figure 2 of the accompanying drawings, for the purpose
specified.
5. A telephone system arranged substantially as described with reference to
Figure 9 of the accompanying drawings, and operating as specified.
1.
1000
·
A. R. BENNETT.
Haseltine, Lake & Co.,
45, Southampton Buildings, London, Agents for the Applicant.