Trunk-line Signalling Apparatus and Circuit for Telephone- exchanges.
Year: 1893

page_text: Date of Application (in United Kingdom), 13th May, 1893
Complete Specification Left, 12th Feb., 1894-Accepted, 28th Apr., 1894
Trunk-line Signalling Apparatus and Circuit for Telephone-
exchanges.
I, EDWARD JULIUS HALL, of Morris, in the County of Morris, and State of
New Jersey, United States of America, do hereby declare the nature of this
invention to be as follows:-
This invention relates to the construction and operation of telephone circuits,
and especially of trunk lines extending between central stations for the purpose
of affording facilities whereby telephone substation circuits converging to any two
central stations respectively, may be interconnected with one another.
The main objects of the present invention are to provide for divided switchboard
and other central office systems of apparatus, a double conductor trunk circuit or
line with an efficient and economical reciprocally operating arrangement of signals
whereby a connection or disconnection made therewith by any other circuit or
conductor at either end thereof shall be caused to automatically announce itself at
the other end, and to accomplish this provision without necessitating the construc-
tion of additional main line conductors for such signal service.
In pursuance of this object the invention consists in combining systems of divided
switchboards, including their necessary connection devices and office trunk conductors
with inter-station trunk conductors fitted with a reciprocal signalling system, whereby
connections and discounections at either end of said trunk may be indicated at the
other, without the necessity of constructing special conductors for such signalling
system.
It also consists in so arranging the two main line conductors of a trunk line, that
the said conductors respectively shall also be the main line conductors of inde-
pendent signalling circuits terminating at the trunk line stations.
It also consists in providing that the conversation circuit extensions of such
main line conductors shall connect therewith through the intermediation of
condensers, tending thereby to form inductively complete double conductor
telephone circuits, while the signal extensions of the two main line conductors
respectively shall directly and conductively connect with the ends thereof, and
shall extend to earth or to a suitable return conductor, including in their circuit
the signal devices and also electromagnetic resistances or self induction coils.
Also in so arranging the signalling apparatus of the two circuits whose independent
line sections are respectively the direct and return conductors of the main circuit
that the signal receiving apparatus of one of them shall be at the same station as
the signal transmitting apparatus of the other, the transmitting branch of each
being normally open at the two terminal stations respectively, the two signalling
circuits respectively being thus normally open at opposite ends of the trunk line,
and each being permanently closed at its remaining end. Thus one of the said
signalling circuits is closed through its signalling receiving appliances at A and

page_text: normally open at the branch including its transmitting appliances at B, while the
other is closed at B through its signal receiving appliances, its transmitting end
being open at A.
It further consists in providing each signal circuit with a suitable source of
electricity, and with means for its automatic closure by the act of making a con-
versation circuit trunk connection at its transmitting end; and for automatically
breaking again the same circuit on the disconnection of said conversation circuit,
whereby a suitable signal is caused to be set at either end of the trunk line, by the
act of making a connection at the other; the said signal remaining set during the
continuance of such connection; and being caused to disappear when the said
connection is discontinued.
It also comprises certain details of construction and connection set forth in the
Specification.
In a separate application for Letters Patent filed of even date herewith, I have
described a system of divided switchboards with which is associated an arrange-
ment of visual signals and circuits the whole being entitled an improvement in
"Central Office Apparatus and Circuits for Telephone Exchanges," and in the present
invention the same specific form of signalling device and mode of operation and
many of the same forms of apparatus may be employed or adopted, but I make no
claim herein to such appliances, my present invention concerning itself solely with
trunk line signalling, and with means and modes of accomplishing the same by
circuits independent of the telephone circuit, which independent circuits however
utilize the conductors of the said telephone circuit.
My present invention may however be regarded as being auxiliary to that
described in said Specification, since it is conveniently applicable to the said
system of divided switchboards, and since I contemplate its operation in such an
association.
In the drawings which form a part of this Specification,
Figure 1 is a general diagram, showing the course of a compound telephone
circuit through two central stations fitted with divided switchboards and united by
a trunk line.
Figure 2 is a diagram illustrative of the principles of my invention, and
Figures 3 and 4 are respectively, forms of signal receiving apparatus, either of
which is well adapted for use in connection with my invention.
Similar letters and figures of references in the several drawings refer to similar
features.
In Figures 1 and 2, O and O2 are central telephone stations, C and D form the
divided switchboard of O; and C and D the divided switchboard of 02.
D and D2 are answering or terminal switchboards, subdivided into operators
sections, each of which is provided as usual with call receiving and transmitting
appliances, and with a set of plug and cord connectors, in the circuit of which
by means of suitable keys and switches the operators' appliances may be looped
or bridged in a manner well understood. At these boards calls are answered,
initial connections and disconnections are made, and the substation required on
any call, may be rung up; and the answering spring jack of the substation lines
are distributed between them.
The connecting boards C and C³ need not be subdivided. On these are mounted
spring jacke or plug sockets for all of the entering substation lines, and by means
of the said jacks, complete connections resulting from calls received at the
terminal switchboard are effected with any desired circuit, as more fully specified
in my said separate application to which reference has been made.
To each central station in practice, a number of substation lines converge,
although to avoid complication I have shown but two.
L is a circuit leading from a substation A to central station O, and L² a similar
circuit leading from a station B to the central station 0%. L entering its central
station, passes first to its jack or socket j on the connecting board C, and thence to
its jack or socket J at some one section of the terminal board D where also it is

page_text: provided with a call annunciator a; and in like manner L' is led successively
through jacks and J' to its annunciator a².
A series of office trunks are provided in this system extending between each
section of the terminal board to different points on the connecting boards, and by
means of these, the plug receiving connection of a calling circuit on the terminal
switchboard may be electrically united with the plug receiving connection of a
called circuit at the connecting board. Again for the sake of an easy under-
standing, I have shown but one such office truuk at its each central station. At
station O, the said trunk is represented by the letter t and at 03 by t². Preferably
the said office trunks each have a spring-jack or socket terminal at their terminal
board ends, and a connecting plug adapted for insertion in the line jacks at their
connecting board ends; k and k represent such springjacks, while the plugs are
shown as being inserted in appropriate sockets K and respectively to form a
through connection.
At the several terminal switchboard sections d, are provided in the usual way
link connections, each composed of flexible conductors with plug terminals, which
when thrust respectively into springjacks, effect a thorough connection between the
conductors represented by the said jacks, at the same time severing any original
circuit formed through the jacks in question. Such link connections are shown
at e at sections d and d³ of terminal switchboard D, and at e² at section d' of
terminal switchboard D².
It will be observed that in the drawings, the link e is shown as uniting the jack J
of line L to the jack k which is the office trunk terminal; the said jacks are shown
as being not located on the same but on adjacent terminal sections, a fact which
however has no significance. The link e forms the initial union of a through
connection, and it is usually desirable that the 'switchboard section initiating such
a connection, shall also initiate the disconnection when due. For this cause a
clearing out or "disconnecting" signal a³ is associated with the link e.
"
The link connection e³ at station 02 is shown as uniting an office trunk jack to
a second jack K³, which as will hereinafter appear, forms one end of a trunk line
extending between the two central stations; since connections as well as disconnec-
tions made thereby are uniformly in pursuance of orders made over said trunk line
from the distant station, no disconnecting signal for the link connection e³ is required
or provided.
T is an inter-station trunk line extending between the two central stations O
and 02. It unites a springjack K on the connecting board at one of the said
stations with a springjack K on the terminal board at the other station 0%
Where such trunk service exists, it usually is convenient to place their connection
beginning ends at the connecting board, to be reached in the same way as any
other called for line through an office trunk; and to provide a special section for
incoming trunks, as at d³ on the terminal board, so that trunk calls for other stations
may be received in the same manner as other calls at the terminal board, and may
reach a connection with the wanted line at the connecting board through office
trunks also in the same way.
The main line spring jacks at the two switchboards as well as the office trunk
connections are associated each with some suitable visual signal indicator, which
may be operated by the making of connection or disconnection at the associated
jack or trunk connecting appliance as fully described in my said separate application
for Patent. To be adapted for this purpose, it is necessary to provide an indicator
which will become set at any point of a circuit, when a change of condition is made
at a second point; which will stay set until a reversal of such change is made;
and which will then respond to such reversal; thus furnishing a signal denoting
the making, continuance and the dissolution, of the engagement of any circuit
conductor where such signal is desired. Such a visual indicator may be formed
of the self restoring portion of the annunciator described in Letters Patent of
the United States No. 481,120 granted to Frank A. Pickernell August 16, 1892
and shown in Figure 3 of this Specification. Figure 4 shows the conventional

page_text: incandescent or glow lamp which also is well adapted for the purpose in hand,
requiring of course to be properly proportioned to each individual case.
I am
not however restricted to the use of these devices since many other well known
forms of needle and shutter annunciators will answer equally well.
In the drawings V V2 V3 V and V³ represent the main circuit signal indicators
and the office trunk signal indicators.
0
In the present invention, both termini of the inter-station trunk conductors T
are also provided with suitable automatically operated visual signal indicators
and these each being arranged likewise to be responsive to connections and
disconnections made at the other terminus, and to be connected and operated in a
manner presently to be described.
In Figure 1, the two substation circuits L and L2, the office trunks and t, the
link connections e and e³ and the inter-station trunk T are all shown as being
connected up to form a compound or through circuit extending between the
substations A and B through the central stations O and O².
Referring now more especially to Figure 2, L' is a metallic circuit, 17 and 18
being the direct and return conductors thereof, both of which have separable
contacts in the springjacks and J. In a central station where both metallic
and earth completed substation circuits centre, the circuits of the latter class
loop through the switchboards, so that in their springjacks they may offer two
conductors for connection with the two of the metallic circuits. L is such a
looped earth completed substation circuit, and 1 and 2 are its conductors which
pass through the spring jacks; and J, and to earth at G. This double conductor
construction makes it necessary of course, that the office and interstation trunk
circuits t t and T shall in like manner be formed with double conductors, and
accordingly the office trunk t has conductors 5 and 6; 2 has two conductors 15
and 16, and the two conductors of the inter-station trunk circuit are marked
respectively 9 and 10. The same form of spring jack may be used for all of the
circuits and conductors and any form of four contact jack will answer. Such a
form is indicated at k, K and K', where m n opay respectively are the main
contact springs which only are necessary so far as the main conductors are
concerned. The plug connectors employed must of course be of like but converse
construction to the jacks, and comprise four insulated conducting surfaces, two of
which n² and m³ taking the plug P2 for an example, register and make contact
with the jack spring contacts m and n.
Of the two plugs of the link connection e one in Figure 2 is supposed to be
inserted in the jack J, while the other P' is ready for insertion in the jack k.
The plug P³ of the office trunk t is ready to be inserted in the jack K of the
interstation trunk line T. One of the plugs P₁ of the link e² is ready to be
inserted in the other jack K' of said trunk, while its mate is in the jack of the
second office trunk; and finally the plug end of the said office trunk (2 is inserted
in the jack of substation circuit L³. When all of the plugs are so inserted, the
compound conversation circuit is completed and when both substation circuits are
metallic, is a double conductor circuit throughout; while when one of the sub-
station circuits is earth completed, the said compound circuit is still practically of
the double conductor type, since its trunk line is composed of two wires and since
one at least of its ends is looped.
For the purposes of initially signalling either central station from the other,
over the main trunk T; of indicating at either end thereof the occupied.condition
of said trunk line at the other; and for the prompt and automatic announcement
at either terminal station of connections and disconnections made at the other, I
associate with the said trunk a system of reciprocal visual signs whereby the
simple act of making a connection with the trunk line by inserting a plug in its
spring-jack, or of making a disconnection therewith by withdrawing a plug from
its spring jack at either end, is enabled to operate a visual indicator of the class to
which I have herein before referred, such an indicator being responsive both to said
connections and disconnections and remaining set as long as the use of the line

page_text: continues. When both ends of such a system are located in the same station, it is
sufficiently economical to provide special conductors for the said system; but
when the trunk line extends between two stations, it is desirable on account of
considerations both of economy and simplicity, to dispense with all conductors
which are not absolutely essential; and in view of such considerations I have
devised means for operating the necessary automatic visual signals reciprocally
using no other main conductors than the direct and return conductors 9 and 10 of
the conversation circuit trunk line itself, these constituting respectively and
severally the main conductors also of the two independent signalling circuits
required. To accomplish this condensers M M M and M* are interposed between
each end of both conductors 9 and 10 and their respective springjack contacts.
I have found condensers with a capacity of three microfarads or thereabout to
answer perfectly. These form no bar to the practical continuity of the circuit
with respect to conversation currents, while they render it impassable for steady
currents such as are used in operating the visual signals.
The trunk line spring jack K at O, has two local contact springs r and s for
signalling circuits; and the spring jack K' at O' also has two local signalling circuit
spring contacts h and u. These last may if desired be electrically united. A
branch conductor 23 is led to an earth connection at station O from any point fon
the main trunk conductor 10 outside of the condenser, and in its circuit is included
the visual signal indicator v and an electro-magnetic or inductive resistance R³.
At the same station a second branch 24 is extended from the point f' on the main
trunk conductor 9 to the local spring jack contact r of the jack K. At station O³
the main conductor 9 which is thus left normally open in the jack K at O, is
provided with an earth branch 25 joining it at the point f, and included in the
circuit of said branch is the visual signal indicator and the electro magnetic
resistance R3; while the main conductor 10 which has a closed circuit branch to
earth at station O, is at O' provided with a branched conductor 31 connecting
directly to the normally insulated contact springs h and u of the jack K². At
station O, the office trunk plug P has a conducting contact surface wadapted when
inserted in jack K to register and make contact with the spring contact r. This
surface is united to an extra conductor 20 leading to an earth connection through
the visual signal indicator r, the electro magnetic resistance R, and an electrical
generator E which conveniently may be a primary or secondary battery. And in
like manner at station O' the plug P has conducting surface contacts to and to
adapted when the said plug is inserted in the jack K³ to register and make contact
with the spring contacts h and u. These surfaces are both united to a branch
conductor 29 leading through the inductive resistance R* and battery E' to earth.
Thus the two conductors 9 and 10 of the conversation circuit trunk line, each have
a closed earth branch including a signal receiving instrument at one end, and a
normally open earth branch at the other, the open branch of each being located at
the station where is located the closed branch of the other. Each also has an
inductive resistance in each earth branch; and a battery in one of them; and each
also has a suitable circuit closer in the open branch, operating to close such branch
by the insertion of the conversation circuit connecting plug, to maintain the battery
current flowing therein, and through the indicator at the other station, as long as
said plug remains inserted; and to open such branch and thereby the circuit of said
battery by the withdrawal of the said plug. The battery of course may be placed
at any point in the branch at either end.
The inductive resistance is made in a manner well understood, by winding
insulated copper wire over a properly shaped iron core or cores, the iron preferably
having a closed magnetic circuit, and may be wound to a real resistance of about
fifty ohms, which alone it practically offers to a steady current such as that
employed in the signalling circuit; but adapted in virtue of the arrangement and
construction of its winding and iron core to offer an apparent resistance of thirty
thousand ohms or thereabout to electrical changes occurring with a velocity of
six hundred per second. These appliances are well known in the art, and are

page_text: variously termed electro magnetic or-inductive resistances, and retardation, choking
or self induction coils. Their presence prevents the diversion of the conversation
-currents through the earth branches while permitting the free passage of the steady
signalling current; and on the other hand the condensers prevent the passage of
the signalling currents, but present no obstacle to the transmission of speech. It
will therefore be seen that a connection made with the double conductor trunk line
at station O to form a through telephone circuit, automatically closes the circuit
of the battery E through one of the conductors 9 of such circuit and the visual
signal at station 0%, which thereupon is set; that the withdrawal of the said
plug opens the said circuit, and thereupon effects the disappearance of such signal;
and that in consequence of these operations, no connection or disconnection can be
made at O with said trunk without notifying station O' of such action. And in
the same way a connection made at O with the said trunk line closes the circuit
of the battery E' through the visual signal o' and gives thereby one signal indicating
that the connection called for is made, while a disconnection produces a reversal
signal and indicates that such trunk line is free at 02.
I have not shown or described any call key or telephone loopkey system in
association with the link connections e and eat the stations O and O, as no such
apparatus is required for a clear understanding of my invention, but it will of course
be understood that any of the well known systems of apparatus in which a cam key
is supplied with each link, controlling the terminals of a telephone and a generator
loop may be employed.
Although I have described the reciprocal signalling feature of my invention as
being associated with my system of divided switch boards, I desire it to be under-
stood that it is by no means restricted to such association, but is adapted for use in
-connection with double conductor trunk lines in any system of switching apparatus.
Dated this 13th day of May 1893
COMPLETE SPECIFICATION.
Trunk-line Signalling Apparatus and Circuit for Telephone-
exchanges.
I, EDWARD JULIUS HALL, of Morris, in the County of Morris, and State of
New Jersey, United States of America, 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 :-
This invention relates to the construction and operation of telephone circuits,
and especially of trunk lines extending between central stations for the purpose of
affording facilities whereby telephone substation circuits converging to any two
central stations respectively, may be interconnected with one another.
The main objects of the present invention are to provide for divided switchboard
and other central office systems of apparatus, a double conductor trunk circuit or
line with an efficient and economical reciprocally operating arrangement of signals
whereby a connection or disconnection made therewith by any other circuit or
conductor at either end thereof shall be caused to automatically announce itself at
the other end, and to accomplish this provision without necessitating the
-construction of additional main line conductors for such signal service.
In pursuance of this object the invention consists in combining systems of divided
switchboards including their necessary connection devices and office trunk
conductors with inter-station trunk conductors fitted with a reciprocal signalling
system, whereby connections and disconnections at either end of sajd trunk may
be indicated at the other, without the necessity of constructing special conductors
for such signalling system.
It also consists in so arranging the two main line conductors of a trunk line, that

page_text: the said conductors respectively shall also be the main line conductors of
independent signalling circuits terminating at the trunk line stations.
It also consists in providing that the conversation circuit extensions of such main
line conductors shall connect therewith through the intermediation of condensers,
tending thereby to form inductively complete double conductor telephone circuits,
while the signal extensions of the two main line conductors respectively shall.
directly and conductively connect with the ends thereof, and shall extend to earth
or to a suitable return conductor, including in their circuit the signal devices and
also electromagnetic resistances or self induction coils. Also in so arranging the
signalling apparatus of the two circuits whose independent line sections are
respectively the direct and return conductors of the main circuit that the signal
receiving apparatus of one of them shall be at the same station as the signal
transmitting apparatus of the other, the transmitting branch of each being normally
open at the two terminal stations respectively, the two signalling circuits.
respectively being thus normally open at opposite ends of the trunk line, and each.
being permanently closed at its remaining end. Thus one of the said signalling
circuits is closed through its signalling receiving appliances at A and normally
open at the branch including its transmitting appliances at B, while the other is.
closed at B through its signal receiving appliances, its transmitting end being
open at A.
It further consists in providing each signal circuit with a suitable source of
electricity, and with means for its automatic closure by the act of making a.
conversation circuit trunk connection at its transmitting end; and for automatically
breaking again the same circuit on the disconnection of said conversation circuit.
whereby a suitable signal is caused to be set at either end of the trunk line, by the
act of inaking a connection at the other; the said signal remaining set during the
continuance of such connection; and being caused to disappear when the said
connection is discontinued.
It also comprises certain details of construction and connection more specifically
set forth in the several clauses of claims recited at the end of this Specification.
In a separate application for Letters Patent filed of even date herewith, I have
described a system of divided switchboards with which is associated an arrangement.
of visual signals and circuits the whole being entitled an improvement in "Central
Office Apparatus and Circuits for Telephone Exchanges," and in the present invention
the same specific form of signalling device and mode of operation and many of the
same forms of apparatus may be employed or adopted, but I make no claim herein
to such appliances, my present invention concerning itself solely with trunk line
signalling, and with means and modes of accomplishing the same by circuits
independent of the telephone circuit, which independent circuits however utilize-
the conductors of the said telephone circuit.
My present invention may however be regarded as being auxiliary to that.
described in said Specification, since it is conveniently applicable to the said
system of divided switchboards, and since I contemplate its operation in such an
association.
In the drawings (left with the Provisional Specification) which form a part of
this Specification,
Figure 1 is a general diagram, showing the course of a compound telephone
circuit through two central station fitted with divided switchboards and united by
a trunk line.
Figure 2 is a diagram illustrative of the principles of my invention, and
Figures 3 and 4 are respectively, forms of signal receiving apparatus, either of
which is well adapted for use in connection with my invention.
Similar letters and figures of references in the several drawings refer to similar
features.
In Figures 1 and 2, O and Os are central telephone stations, C and D form the:
divided switchboard of O; and C and D' the divided switchboard of O³.
D and D³ are answering or terminal switchboards, subdivided into operators.

page_text: sections, each of which is provided as usual with call receiving and transmitting
appliances, and with a set of plug and cord connectors, in the circuit of which by
means of suitable keys and switches the operators' appliances may be looped or
bridged in a manner well understood. At these boards calls are answered, initial
connections and disconnections are made, and the substation required on any call,
may be rung up; and the answering spring-jack of the substation lines are
distributed between them.
The connecting boards C and C* need not be subdivided. On these are mounted
spring jacks or plug sockets for all of the entering substation lines, and by means
of the said jacks, complete connections resulting from calls received at the terminal
switchboard are effected with any desired circuit, as more fully specified in my said
separate application to which reference has been made.
To each central station in practice, a number of substation lines converge,
although to avoid complication I have shown but two.
L is a circuit leading from a substation A to central station O, and L³ a similar
circuit leading from a station B to the central station 0%. L entering its central
station, passes first to its jack or socket j on the connecting board C, and thence to
its jack or socket J at some one section of the terminal board D where also it is
provided with a call annunciator a; and in like manner L³ is led successively
through jacks and J' to its annunciator a².
At
A series of office trunks are provided in this system extending between each
section of the terminal board to different points on the connecting boards, and by
means of these, the plug receiving connection of a calling circuit on the terminal
switchboard may be electrically united with the plug receiving connection of a
called circuit at the connecting board. Again for the sake of an easy under-
standing, I have shown but one such office trunk at its each central station.
station O, the said trunk is represented by the letter t and at O' by t². Preferably
the said office trunks each have a spring-jack or socket terminal at their terminal
board ends, and a connecting plug adapted for insertion in the line jacks at their
connecting board ends; k and k² represent such spring-jacks, while the plugs are
shown as being inserted in appropriate sockets K and respectively to form a
through connection.
At the several terminal switchboard sections d, are provided in the usual way
link connections, each composed of flexible conductors with plug terminals, which
when thrust respectively into spring-jacks, effect a thorough connection between
the conductors represented by the said jacks, at the same time severing any original
circuit formed through the jacks in question. Such link connections are shown
at e at sections d and d³ of terminal switchboard D, and at e² at section d³ of
terminal switchboard D².
It will be observed that in the drawings, the link e is shown as uniting the jack J
of line L to the jack k which is the office trunk terminal; the said jacks are shown
as being not located on the same but on adjacent terminal sections, a fact which
however has no significance. The link e forms the initial union of a through con-
nection, and it is usually desirable that the switchboard section_initiating such a
connection, shall also initiate the disconnection when due. For this cause a
"clearing out" or "disconnecting" signal a³ is associated with the link e.
The link connection e' at station O' is shewn as uniting an office trunk jack k³ to
a second jack K, which as will hereinafter appear, forms one end of a trunk line
extending between the two central stations; since connections as well as discon-
nections made thereby are uniformly in pursuance of orders made over said trunk
line from the distant station, no disconnecting signal for the link "connection e³ is
required or provided.
T is an inter-station trunk line extending between the two central stations O
and 0%. It unites a spring jack K on the connecting board at one of the said
stations with a spring jack K' on the terminal board at the other station O³.
Where such trunk service exists, it usually is convenient to place their connection
beginning ends at the connecting board, to be reached in the same way as any

page_text: other called for line through an office trunk; and to provide a special section for
incoming trunks, as at d² on the terminal board, so that trunk calls for other
stations may be received in the same manner as other calls at the terminal board,
and may reach a connection with the wanted line at the connecting board through
office trunks also in the same way.
The main line spring jacks at the two switchboards as well as the office trunk
connections are associated each with some suitable visual signal indicator, which.
may be operated by the making of connection or disconnection at the associated
jack or trunk connecting appliance as fully described in my said separate application
for Patent. To be adapted for this purpose, it is necessary to provide an indicator
which will become set at any point of a circuit, when a change of condition is made
at a second point; which will stay set until a reversal of such change is made,
and which will then respond to such reversal; thus furnishing a signal denoting the
making, continuance and the dissolution, of the engagement of any circuit con-
ductor where such sigual is desired. Such a visual indicator may be formed of
the self-restoring portion of the annunciator described in Letters Patent of the
United States No. 481,120 granted to Frank A. Pickernell, August 16, 1892 and
shown in Figure 3 of this Specification. Figure 4 shows the conventional incan-
descent or glow lamp which also is well adapted for the purpose in hand, requiring
of course to be properly proportioned to each individual case. I am not however
restricted to the use of these devices since many other well known forms of needle
and shutter annunciators will answer equally well.
In the drawings V. V2 V3 V4 and V represent the main circuit signal indicators
and the office trunk signal indicators.
ย
In the present invention, both termini of the inter-station trunk conductors T
are also provided with suitable automatically operated visual signal indicators vs.
and these each being arranged likewise to be responsive to connections and
disconnections made at the other terminus, and to be connected and operated in a
manner presently to be described.
In Figure 1 the two substation circuits L and L2 the office trunks t and t³ the
link connections e and and the inter-station trunk T are all shown as being
connected up to form a compound or through circuit extending between the
substations A and B through the central stations O and O*.
Referring now more specially to Figure 2, L2 is a metallic circuit, 17 and 18
being the direct and return conductors thereof, both of which have separable
contacts in the spring jacks j and J. In a central station where both metallic
and earth completed substation circuits centre, the circuits of the latter class loop
through the switchboards, so that in their spring jacks they may offer two con-
ductors for connection with the two of the metallic circuits. L is such a looped
earth completed substation circuit, and 1 and 2 are its conductors which pass
through the spring jacks ; and J, and to earth at G. This double conductor
construction makes it necessary of course that the office and interstation trunk
circuits t, t, and T shall in like manner be formed with double conductore, and
accordingly the office trunk t has conductors 5 and 6; t has two conductors 15
and 16, and the two conductors of the interstation trunk circuit are marked
respectively 9 and 10. The same form of spring jack may be used for all of the
circuits and conductors and any form of four contact jack will answer.
Such a
form is indicated at k, K, and K2, where m n o p ay respectively are the main
contact springs which only are necessary so far as the main conductors are
concerned. The plug connectors employed must of course be of like but converse
construction to the jacks, and comprise four insulated conducting surfaces, two of
which n² and m³ taking the plug P' for an example, register and make contact
with the jack spring contacts m and n.
Of the two plugs of the link connection e, one in Figure 2 is supposed to be
inserted in the jack J, while the other P' is ready for insertion in the jack k. The
plug Ps of the office trunk t is ready to be inserted in the jack K of the interstation
trunk line T. One of the plugs P of the link e³ is ready to be inserted in the
R

page_text: other jack K' of said trunk while its mate is in the jack 2 of the second office
trunk, and finally the plug end of the said office trunk 2 is inserted in the jack j³ of
substation circuit L. When all of the plugs are so inserted, the compound
conversation circuit is completed and when both substation circuits are metallic, is
a double conductor circuit throughout, while when one of the substation circuits is
earth completed, the said compound circuit is still practically of the double
conductor type, since its trunk line is composed of two wires and since one at least
of its ends is looped.
For the purposes of initially signalling either central station from the other, over
the main trunk T; of indicating at either end thereof the occupied condition of
said trunk line at the other; and for the prompt and automatic announcement at
either terminal station of connections and disconnections made at the other, I
associate with the said trunk a system of reciprocal visual signs whereby the simple
act of making a connection with the trunk line by inserting a plug in its spring-
jack, or of making a disconnection therewith by withdrawing a plug from its spring
jack at either end, is enabled to operate a visual indicator of the class to which I
have hercinbefore referred, such an indicator being responsive both to said con-
nections and disconnections and remaining set as long as the use of the line
continues. When both ends of such a system are located in the same station, it is
sufficiently economical to provide special conductors for the said system; but
when the trunk line extends between two stations, it is desirable on account of
considerations both of economy and simplicity, to dispense with all conductors
which are not absolutely essential; and in view of such considerations I have
devised means for operating the necessary automatic visual signals reciprocally
using no other main conductors than the direct and return conductors 9 and 10 of
the conversation circuit trunk line itself, these constituting respectively and
severally the main conductors also of the two independent signalling circuits
required. To accomplish this condensers M, M2, M³, and M are interposed
between each end of both conductors 9 and 10 and their respective spring jack
contacts. I have found condensers with a capacity of three micro-farads or
thereabout to answer perfectly. These form no bar to the practical continuity of
the circuit with respect to conversation currents, while they render it impassable for
steady currents such as are used in operating the visual signals.
The trunk line spring jack K at O, has two local contact springs 7 and 8 for
signalling circuits, and the spring jack K2 at 02 also has two local signalling
circuit spring contacts h and u. These last may if desired be electrically united.
A branch conductor 23 is led to an earth connection at station O from any point
on the main trunk conductor 10 outside of the condenser, and in its circuit is
included the visual signal indicator and an electro-magnetic or inductive
resistance R². At the same station a second branch 24 is extended from the
point on the main trunk conductor 9 to the local spring jack contact r of the
jack K. At station O' the main conductor 9 which is thus left normally open in
the jack K at O, is provided with an earth branch 25 joining it at the point f³,
and included in the circuit of said branch is the visual signal indicator and the
electro-magnetic resistance R³; while the main conductor 10 which has a closed
circuit branch to earth at station O, is at O' provided with a branched conductor 31
connecting directly to the normally insulated contact springs h and u of the jack K³.
At station O, the office trunk plug P has a conducting contact surface w adapted
when inserted in jack K to register and make contact with the spring contact r.
This surface is united to an extra conductor 20 leading to an earth connection
through the visual signal indicator v, the electro-magnetic resistance R, and an
electrical generator E which conveniently may be a primary or secondary
battery. And in like manner at station 02 the plug P4 has conducting surface
contacts w³ and w' adapted when the said plug is inserted in the jack K' to register
and make contact with the spring contacts and u. These surfaces are
both united to a branch conductor 29 leading through the inductive resistance R*
and battery E to earth. Thus the two conductors 9 and 10 of the conversation

page_text: circuit trunk line, each have a closed earth branch including a signal receiving
instrument at one end, and a normally open earth branch at the other, the open
branch of each being located at the station where is located the closed branch of
the other. Each also has an inductive resistance in each earth branch; and a
battery in one of them; and each also has a suitable circuit closer in the open
branch, operating to close such branch by the insertion of the conversation circuit
connecting plug, to maintain the battery current flowing therein, and through the
indicator at the other station, as long as said plug remains inserted; and to open
such branch and thereby the circuit of said battery by the withdrawal of the said
plug. The battery of course may be placed at any point in the branch at either
end.
The inductive resistance is made in a manner well understood, by winding
insulated copper wire over a properly shaped iron core or cores, the iron preferably
having a closed magnetic circuit, and may be wound to a real resistance of about
fifty ohms, which alone it practically offers to a steady current such as that
employed in the signalling circuit, but adapted in virtue of the arrangement and
construction of its winding and iron core to offer an apparent resistance of thirty
thousand ohms or thereabout to electrical changes occurring with a velocity of six
hundred per second. These appliances are well known in the art, and are variously
termed electro-magnetic or inductive resistances, and retardation, choking or self
induction coils. Their presence prevents the diversion of the conversation currents
through the earth branches while permitting the free passage of the steady
signalling current; and on the other hand the condensers prevent the passage of
the signalling currents, but present no obstacle to the transmission of speech. It
will therefore be seen that a connection made with the double conductor trunk line
at station O to form a through telephone circuit, automatically closes the circuit
of the battery E through one of the conductors 9 of such circuit and the visual
signal at station O2, which thereupon is set, that the withdrawal of the said plug
opens the said circuit, and thereupon effects the disappearance of such signal, and
that in consequence of these operations, no connection or disconnection can be
made at O with said trunk without notifying station 03 of such action. And in
the same way a connection made at O' with the said trunk line closes the circuit
of the battery E' through the visual signal and gives thereby one signal
indicating that the connection called for is made, while a disconnection produces a
reversal signal and indicates that such trunk line is free at O³.
I have not shown or described any call key or telephone loop key system in
association with the link connections e and at the stations O and O2, as no such
apparatus is required for a clear understanding of my invention, but it will of
course be understood that any of the well known systems of apparatus in which a
cam key is supplied with each link, controlling the terminals of a telephone and a
generator loop may be employed.
Although I have described the reciprocal signalling feature of my invention as
being associated with my system of divided switch boards, I desire it to be
understood that it is by no means restricted to such association, but is adapted for
use in connection with double conductor trunk lines in any system of switching
apparatus.
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 substantially as hereinbefore described, of two divided
switchboards located at different central telephone stations, and each comprising a
terminal switchboard at which calls are answered, and a connecting switchboard at
which connections called for are made; a series of substation lines centering at each
of the said central stations, each passing through a connection device on the
connecting board to a second connection device on the terminal board; an office

page_text: trunk line at each central station extending from a plug socket at the terminal
board to a connecting plug at the connecting board; and a series of link con-
nections at each terminal board, each comprising a flexible conductor and a
connecting plug at both ends, with an inter-station trunk line uniting the connecting
board at one station with the terminal board at the other station, and represented
at each by a plug socket, the said trunk line comprising a main line section, two
inductively connected terminal sections therefor connected with the said plug
sockets respectively, and a condensor interposed between each terminal section and
the main line to constitute the said inductive connection thereof.
2. The combination of two divided switchboards located at different central
telephone stations, each comprising a terminal switchboard at which calls are
answered, and a connecting switchboard at which connections called for are made;
a series of substation circuits converging to each of the said central stations, each
passing through a plug socket on the connection board to a second plug socket on
the terminal board; an office trunk circuit at each central station joining a plug
socket at the terminal board to a plug connector at the connecting board; and link
connections at each terminal board, each formed of a flexible conductor fitted at
both ends with a connecting plug whereby the plug sockets of the substation and
trunk circuits may be united; with an interstation trunk line uniting a plug socket
on the connecting board at one central station, to a plug socket on the terminal
board at the other station, the said trunk line comprising a main line section, two
inductively connected terminal sections therefor connected with the said plug
sockets respectively, and a condensor interposed at the junction between each
terminal section and the main line section; and a system of automatic signals
associated with said inter-office trunk line and consisting of a branch circuit at each
end, permanently connected to the main line section thereof; one of the said
branch circuits being closed through a signal indicator and a retardation coil, and
the other including a battery, a retardation coil and a circuit closer being normally
open but adapted to be closed by any connection made with its associated plug
socket of said trunk, substantially as hereinbefore described.
3. The combination of two divided switchboards located at different central
telephone stations, each comprising a terminal switchboard at which calls are
answered, and a connecting switchboard where connections called for are made;
a series of substation circuits centering at each of the said central stations, each
passing through a connection device on the connecting board to a similar device
on the terminal board; an office trunk line at each central station fitted at the
terminal board with a plug socket, and at the connecting board with a plug
connector; and link connections at each terminal board, each formed of a
flexible conductor fitted at both ends with a connecting plug adapted for insertion
in the line and office trunk sockets; with a double conductor or metallic circuit
inter station trunk line uniting a plug socket on the connecting board at one
central station to a plug socket on the terminal board at the other central
station, the said trunk line comprising a double conductor main line section,
two inductively connected terminal sections therefor looping to the said plug
sockets respectively, and condensors interposed at the junctions between the two
ends of each terminal loop section and the corresponding ends of the two con-
ductors of the main line section; and a reciprocal system of automatic signals
for said trunk line comprising a branch circuit at each end of both conductors
thereof, permanently connected with its main line sections, one branch of each
of the said trunk conductors at opposite terminal stations being closed through
a signal indicator and a retardation coil, and the other including a battery, a
retardation coil, and a circuit closer being normally open in the plug socket of
said trunk line, and adapted to be closed by the insertion of a plug therein,
whereby connections and disconnections of said trunk line at either end thereof
may be automatically signalled to the other end substantially as specified herein.
4. A double conductor trunk line extending between two telephone stations, the
f

page_text: two conductors thereof constituting respectively the direct and return conductors.
of a conversation circuit, and each severally forming a portion of an independent
signalling circuit.
5. A double conductor telephone or conversation circuit, and two independent.
signalling circuits extending between two telephone stations, the two conductors of
the said telephone circuit constituting respectively the main conductors of the said
two signalling circuits, substantially as described.
6. In combination with a double conductor inter office trunk telephone circuit
adapted to unite the telephone circuits centering in one office, with those centering
in the other; a reciprocal signalling system comprising two independent signalling
circuits for indicating at each end of said trunk a connection or disconnection made
at the other, the two conductors of said telephone circuit forming respectively the
main conductors of the two circuits of said signalling system substantially as.
described.
7. A double conductor trunk line extending between two distant telephone-
stations, constituting a main line section common to independent telephone and
signalling circuits, the terminals of the remaining portions of said telephone circuit
being inductively connected therewith through condensers and the terminals of the
remaining portion of the said signalling circuits being conductively connected with
the two main conductors respectively through electro magnetic resistances, or-
retardation coils, substantially as described.
8. An inter office trunk line; a telephone branch or loop therefor at both.
terminal offices; and a condenser interposed at each terminus between the said
trunk line and loop; combined with a normally closed signal branch including a
signal receiving appliance, and an electro magnetic or inductive resistance, connected
with said trunk at one of the said terminal offices; a normally open signal
transmitting branch therefor, a source of electricity, and means as indicated for
automatically closing the said transmitting branch and for including the said source
therein at the other of said terminal offices substantially as described.
9. The combination with a double conductor trunk telephone line extending
between two stations to form a part of a metallic telephone circuit, and a condenser
in the circuit of each conductor at both stations; of an earth terminal branch
connected at each end of each of the said conductors the two earth branches of
each conductor including respectively signal receiving and signal transmitting
devices, the receiving devices of one and the transmitting devices of the other
conductor being at the same station; a battery in one of the earth branches of
both conductors, and an electro-magnetic resistance or self induction coil in both
earth branches of each conductor whereby the two conductors of said trunk line
are enabled severally to form portions of independent earth completed signalling
circuits, substantially as described herein.
10. The combination substantially as herein before described, of a metallic circuit
or double conductor trunk telephone line extending between two telephone stations
and having its two main conductors at each of the said stations connected through
condensers respectively, with telephone loops to constitute a conversation circuit ;.
with an associated reciprocal signalling system comprising the said two main
conductors; a closed branch permanently connected with each at opposite terminal
stations, and including a visual signal indicator; and an electro magnetic resistance
or retardation coil; and a normally open branch for the other end of each, at the
said opposite stations, including a battery or like generator, a similar electro
magnetic resistance, and a circuit closer adapted to be operated, and to close and
open the signalling circuit by connections or disconnections made with the
telephone loop at the same station.
11. The combination of a metallic circuit or double conductor trunk line uniting
two central telephone stations and composed of a double conductor main line
section, two inductively connected terminal loop sections therefor, one at each
station, a spring jack in each of the said loop sections, and condensers interposed
between the several ends of said loops and the corresponding ends of the main.

page_text: sections to constitute the said inductive connection; with two independent earth
completed signalling circuits associated with said double conductor trunk to
indicate at either end thereof a connection or disconnection made therewith at the
other end; one of the said two signalling circuits being normally closed, and the
other normally open at each of the two terminal stations, and each being composed
of a separate one of the trunk main line sections, a closed and an open branch
permanently connected with said section, an electro magnetic resistance in each of
the said branches a visual signalling indicator responding to the closing and
opening of the circuit in the closed branch, a circuit closer in the open branch
mounted in the trunk spring jack, and an electrical generator, whereby the
insertion or withdrawal of a plug in or from the spring jack at either end of the
trunk line is enabled to operate the signal at the other without necessitating special
signalling conductors, substantially as described.
Dated this 7th day of February 1894.
CARPMAEL & Co.,
Agents for the Applicant.