Improvements in Metallic Telephone Circuits.
Year: 1889

page_text: Date of Application, 4th June, 1889
Complete Specification Left, 21st Feb., 1890-Accepted, 12th Apr., 1890
PROVISIONAL SPECIFICATION.
Improvements in Metallic Telephone Circuits.
I, HENRY LYMAN BURBANK of 227, Montgomery Street, Jersey City, in the
County of Hudson, and State of New Jersey, United States of America, do hereby
declare the nature of this invention to be as follows :-
My Invention relates to the art of telephonic communication and more especially to
systems utilizing metallic circuits. For the more perfect transmission of speech and
to the end that the voice currents may not be interfered with by extraneous induced
and earth currents, it is now a well-known expedient to provide a return wire for the
telephone circuit instead of using an earth return; thus forming a metallic circuit the
direct and return wires whereof are practically equidistant from any external electric
wire. By this expedient, not only are disturbing electric currents due to induction
neutralized, but the ordinary earth connection of the telephone line being dispensed
with earth currents traceable to that source are also avoided. But, since metallic
circuits require two wires and since, to obtain the best results, one station only should
practically be connected with each circuit, this class of construction is very expensive.
The object of my invention is to obtain the benefits accruing from the use of
metallic circuits but to reduce the costliness of such circuits by making each double
wire circuit subserve as the conducting medium for two or more sub-stations, and
to do this while providing that each station shall when using the line for telephonic
conversation have a clear line unencumbered by interposed electro-magnets; that each
station shall be enabled to ascertain when the circuit is already being used by another;
and that while any one station is using the line, none of the others shall be able to
disturb the conversation by sending a call signal.
To this end, my invention consists in certain arrangements of the circuits with
respect to the several appliances at the several stations whereby when all the stations
are at rest the several cali bell helices and call bell generators are serially connected
in circuit so that each station may receive a call from, or send a call to, the central
station or exchange calls with each other and whereby the calling circuit of both is
opened when either station is using the line for conversation. This is effected, in the
simplest embodiment of my invention, by causing one of the two main wires to pass
directly to one of the working, and one of the resting, contacts of the automatic
telephone switch of the sub-station most distant from the central station; by causing
the other main wire to terminate in the other resting contact of said switch at said
most distant station after passing successively through the movable member and
resting contacts of the switch and the bell and generator helices at the intermediate
station and then through the generator and bell helices at the said most distant station ;
by constructing a branch circuit from one of the working contacts of the switch at the
intermediate station through the telephones at said station to a junction with the first
named of said lines, and a similar branch from the telephone contact of the terminal
station to a junction with the second line, both junctions being located between the
central station and the movable member of the automatic switches to which the said
lines mainly lead.
When the system is extended to embrace a larger number of stations, modifications
become requisite which will hereinafter be adverted to. But, in both cases and in all
embodiments of my invention, it is required that the calling appliances of all stations
shall be (when the circuit is at rest) serially included therein, so that each generator
may be in serial circuit with all of the call bells and with the central station call
receiving appliance; the said circuit thus extends virtually in normally-closed loops
to each sub-station and the said loops include the said call apparatus; furthermore, in
order that each sub-station may have a clear talking circuit and that the act of

page_text: establishing said talking circuit may disestablish the ringing circuit, my invention also
contemplates in all cases that each station shall also be provided with normally-open
talking loops, one side of each of these being formed of that side of the closed loops
which includes the switch lever, the other side being formed of the telephone branch
circuit extended from the working contact of said switch through the telephones to a
junction with that one of the main lines with which the branch terminating in the said
switch lever is not connected, as herein indicated.
My. Invention is practically disclosed by the consideration of a single circuit which
has two sub-stations which I will term M and N. At the central station the two
wires, which I will term 1 and 2, of both the circuits are connected with spring
contacts of several spring jacks. Normally, these springs rest on insulated contact
points, the points of the two spring jacks of each line being united by wires in both
cases passing through the respective call annunciators which are thereby normally
included in their respective circuits. The circuits are interconnected by means of the
jacks and a flexible cord which carries two insulated conductors and ends with double
connecting plugs, the two members of these plugs being inserted in the jacks. The
springs of all of the jacks are thus raised out of contact with their insulated pins and
the annunciator branches are thereby disconnected, while the ends of the two wires
of the two circuits are at the same time united.
Instead of normally uniting the two wires of each circuit together through their
annunciator helices, one may be connected directly to earth and the other to earth
through the annunciator helix. This would not interfere with, or disturb, the condition
of the two wires of the circuit with respect to the sub-stations.
Each sub-station is, as usual, provided with a call sending generator a call receiving
bell receiving and transmitting telephone instruments and a circuit-changing switch
which in one position, that of rest, when controlled by the receiver directs the main
circuit through the calling appliances and which, when the receiver is removed
therefrom, automatically operates and transfers the main circuit to the telephone
instruments. One of the main wires, say 1, extends outwardly to the terminal
station N and there is united with a contact of the switch bar and also to a working
contact. Supposing the circuit to be at rest and both receiving telephones in place,
the circuit is continued through the resting contact at station N by a wire, call bell,
call generator, and wire, back to the intermediate station M. Here it leads successively
through call generator, call bell, wire, resting contact, and switch to the second
resting contact to the main wire 2, which can now be traced back to the central station.
When this is the condition of the sub-stations, either station operator may operate his
generator and drop the annunciator at the central station and may also ring the other
sub-station. In like manner the central station may signal either sub-station. The
sub-station bells being in series, oach will hear the others ring and will thus be apprised
that the line is about to be used. A definite signal must be assigned as the call for
each sub-station or some simple individual call may be employed.
From any point on wire 1 between station N and the central station a branch wire
is led to station M and is there connected with the upper or telephone switch contact.
The telephone instruments are connected in circuit with this branch of which a wire
forms a continuation by way of the working contacts. In like manner, a similar
branch circuit leaves any point on the line wire 2, between sub-station M and the
central station and passing through the telephones at station N terminates in the
telephone contact of the switch contact there. This construction gives a clear
metallic circuit for each sub-station telephone whereby the telephonic transmission
of both remains unobstructed by the call magnets of the other and which cannot
be broken, disconnected, or disturbed, by the operation of the calling apparatus of
the other. For suppose the operator at station M to be talking. His telephone is
removed from the switch holder and the switch makes contact with the upper contacts.
The circuit is from the central station through sub-station M and back by way of
main wire 1, junction-point, branch, transmitter, wire, receiver, wire, contact-piece,
switch bar and contact, to main wire 2. Sub-station N cannot disturb the operation
by ringing, because, although wire 1 continues to that station and there connects

page_text: with the calling apparatus, the circuit of the said apparatus, continued by wire, is
open at the resting contact at station M. Thus though N may actuate his generator,
he cannot send a signal and furthermore he is immediately informed that M is using
the line, by the refusal of his own bell to ring. He therefore waits until he hears
the disconnecting ring of M. And in like manner should N be using the line,
M cannot interfere by ringing because his ringing circuit is made discontinuous by
the disconnection at station N of the switch from its resting contacts and N has a
clear metallic circuit by way of wire 1 to station N switch telephone contacts wire,
receiver, wire, and transmitter, all at station N and the wire to the junction point
and thence back to the central station over wire 2; the main line continuation of said
wire 2 through the calling apparatus of both stations being, as already indicated, open
at station N.
The cost of such additional construction as may be required need not be great, as
any two stations which are to be operated together on the same circuit will naturally
be comparatively close together; and it will be seen that two wires only are required
between the central and the nearest sub-station, the third and additional wire being
required between the intermediate and terminal sub-station only; of course the con-
nections of the two lines with respect to the two stations may, if desired, be reversed.
I will now describe an extension of the principle to three sub-stations which I will
designate M, N, and H. Bearing in mind that all of the calling appliances are in
series when the line is at rest it is not difficult to trace the circuits. The station
appliances are unchanged consisting, as before, at each station of a call generator and
bell, transmitting and receiving telephones and an automatic switch.
The normal or calling circuit extends from the central station over wire 1, succes-
sively through sub-stations M, N, and H, and back by wire 2; the route being as
follows; wire 1 to station M there by contact, switch, contact, wire, and calling
appliances, out; then by wire to station N there through contact, switch, contact,
wire, and calling appliances out; then by wire to station H and there through bell,
generator, wire, contact switch, and contact, and then back by wire 2 to the central
station. To complete the telephone circuit of station M, a branch terminating at
the telephone contacts of the switch leads by a wire, receiver, wire, transmitter,
and wire to the junction point on the main line 2. The telephone circuit of station N
extends (normally open, of course) from telephone contacts of the switch by wire,
receiver, wire, transmitter and wire, to the junction point on return line 2.. The
telephonic facilities of station H are provided by means of a branch circuit extending
from the telephone contacts of the switch there, through the telephones and by wire
to the junction point of the main line 1. In addition to the above, a shunt wire is
required for the intermediate station, this extending from a subsidiary telephone
contact to a junction point on a branch of the main line 1. Were it not for this
shunt, the telephonic currents to and from station N would be required to traverse
the calling appliances at station M. The results attained in this latter organization
are, in every respect, analogous to those obtained in the simpler organization of but
two stations.
Calls sent to, or from, the central station by, or for, any sub-station are heard by
all alike, all having their calling appliances in series. When any sub-station is using
the line for conversation, the others are debarred from sending call signals, their
calling circuit being thereby made discontinuous; and each sub-station has a clear
metallic circuit extending to the central station; no electro-magnets, except those
actually appertaining to the telephones in use, being in the said circuit.
It only remains to be stated that when a plurality of stations are connected upon
the same circuit in accordance with my invention, each can call and converse with
any or all of the others; the calling appliances being in series and the several
telephones being in multiple arc with each other.
Dated this 4th day of June 1889.
H. L. BURBANK.
By A. M. & Wm. Clark,
53, Chancery Lane, London, Patent Agents.

page_text: COMPLETE SPECIFICATION.
Improvements in Metallic Telephone Circuits.
I, HENRY LYMAN BURBANK, of 227, Montgomery Street, Jersey City, in the County
of Hudson, 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 :-
My Invention relates to the art of telephonic communication and more especially
to systems utilizing metallic circuits.
For the more perfect transmission of speech and to the end that the voice currents
may not be interfered with by extraneous induced and earth currents, it is now a
well-known expedient to provide a return wire for the telephone circuit instead of
using an earth return; thus forming a metallic circuit the direct and return wires
whereof are practically equidistant from any external electric wire.
By this expedient, not only are disturbing electric currents due to induction
neutralized, but the ordinary earth connection of the telephone line being dispensed
with, earth currents traceable to that source are also avoided. But, since metallic
circuits require two wires and since, to obtain the best results, one station only should
practically be connected with each circuit, this class of construction is very expensive.
The object of my invention is to obtain the benefits accruing from the use of metallic
circuits but to reduce the costliness of such circuits by making each double wire
circuit subserve as the conducting medium for two or more sub-stations, and to do
this while providing that each station shall when using the line for telephonic con-
versation have a clear line unencumbered by interposed electro-magnets; that each
station shall be enabled to ascertain when the circuit is already being used by another;
and that while any one station is using the line, none of the others shall be able to
disturb the conversation by sending a call signal.
To this end, my invention consists in certain arrangements of the circuits with
respect to the several appliances at the several stations whereby when all the stations
are at rest the several call bell helices and call bell generators are serially connected
in circuit so that each station may receive a call from, or send a call to, the central
station or exchange calls with each other and whereby the calling circuit of both is
opened when either station is using the line for conversation. This is effected, in the
simplest embodiment of my invention, by causing one of the two main wires to pass
directly to one of the working, and one of the resting contacts of the automatic
telephone switch of the sub-station most distant from the central station; by causing
the other main wire to terminate in the other resting contact of said switch at said
most distant station after passing successively through the movable member and
resting contacts of the switch and the bell and generator helices at the intermediate
station and then through the generator and bell helices at the said most distant
station; by constructing a branch circuit from one of the working contacts of the
switch at the intermediate station through the telephones at said station to a junction
with the first named of said lines, and a similar branch from the telephone contact of
the terminal station to a junction with the second line, both junctions being located
between the central station and the movable member of the automatic switches to
which the said lines mainly lead.
When the system is extended to embrace a larger number of stations, modifications
become requisite which will hereinafter be adverted to. But, in both cases shewn and
in all embodiments of my invention, it is required that the calling appliances of all
stations shall be (when the circuit is at rest) serially included therein, so that each
generator may be in serial circuit with all of the call bells and with the central station
call receiving appliance; the said circuit thus extends virtually in normally-closed
loops to each sub-station and the said loops include the said call apparatus; further-
more, in order that each sub-station may have a clear talking circuit and that the act
of establishing said talking circuit may disestablish the ringing circuit, my invention

page_text: also contemplates in all cases that each station shall also be provided with normally-
open talking loops, one side of each of these being formed of that side of the closed
loops which includes the switch lever, the other side being formed of the telephone
branch circuit extended from the working contact of said switch through the telephones
to a junction with that one of the main lines with which the branch terminating in the
said switch lever is not connected, as herein indicated.
I have illustrated this specification by two diagrams; figure 1 shewing my invention
in its simplest form, while figure 2 shews the same principle but extended to three
sub-stations.
In figure 1, A is supposed to be a central telophone station to which metallic or
double wire telephone circuits B and C together with others converge.
My Invention is practically disclosed by the consideration of a single circuit and but
one circuit accordingly is delineated in full. The said circuit B has two sub-
stations M and N. At the central station the two wires 1 and 2 of circuit Bas
well as those 3 and 4 of circuit C are shewn as being connected with the spring
contacts s of the several spring jacks a, a', a², and a³. Normally these springs rest
on the insulated contact points t, the points of the two spring jacks of each line being
united by the wires 21 and 22 in both cases passing through the respective call
annunciators "", which are thereby normally included in their respective circuits. The
circuits B and C are represented in the drawing as being interconnected by means of
the jacks and a flexible cord c which carries two insulated conductors and ends with
double connecting plugs P P1, the two members p and p₁ of the end P being inserted
in the jacks a and a', while the two members p² and p³ of the end P¹ in like
manner are inserted in the jacks a² and a³ of line C. The springs s of all of the jacks
are thus raised out of contact with their insulated pins, and the annunciator branches
are thereby disconnected while the ends of the two wires 1 and 2 of circuit B are at
the same time united to the ends of the two wires 3 and 4 of the circuit C. Instead
of normally uniting the two wires of each circuit together through their annunciator
helices one may be connected directly to earth and the other to earth through the
annunciator helix. This would not interfere with or disturb the condition of the two
wires of the circuit with respect to the sub-stations. Each sub-station is, as usual,
provided with a call sending generator G a call. receiving bell b receiving and
transmitting telephone instruments R and T and a circuit-changing switch which in
one position, that of rest, when controlled by the receiver directs the main circuit
through the calling appliances and which, when the receiver is removed therefrom,
automatically operates and transfers the main circuit to the telephone instruments.
One of the main wires, say 1, extends outwardly to the terminal station N and is there
united with contact 31 of switch bar h and also to working contact 32. Supposing
the circuit to be at rest and both receiving telephones in place, the circuit is continued
through the resting contact 16 at station N by way of the wire 17, call bell b, call
generator G, and wire 18, back to the intermediate station M. Here it leads
successively through call generator G, call bell b, wire 17, resting contact 16, and
switch h, to the second resting contact 31 to main wire 2, which can now be traced
back to the central station. When this is the condition of the sub-stations, either
station operator may operate his generator and drop the annunciator at the central
station and may also ring the other sub-station. In like manner the central station
may signal either sub-station. The sub-station bells being in series, each will hear the
others ring and will thus be apprised that the line is about to be used. A definite
signal must be assigned as the call for each sub-station or some simple individual call
may be employed.
From any point 9 on wire 1 between station N and the central station A a branch
wire 11 is led to station M and is there connected with the upper or telephone switch
contact 14. The telephone instruments T and R are connected in circuit with this
branch of which wire 15 is a continuation by way of working contacts 14 and 32. In
like manner, a similar branch circuit 6 leaves any point 7 on the line wire 2, between
sub-station M and the central station and passing through the telephones at station N

page_text: terminates in the telephone contact 14 of the switch contact there. This construction
gives a clear metallic circuit for each sub-station telephone whereby the telephonic
transmission of both remains unobstructed by the call magnets of the other and which
cannot be broken, disconnected, or disturbed, by the operation of the calling apparatus
of the other. For suppose the operator at station M to be talking. His telephone is
removed from the switch holder and the switch makes contact with upper
contacts 14 and 32. The circuit is from the central station through sub-station M
and back by way of main wire 1, junction-point 9. branch 11, transmitter T, wire 5,
receiver R, wire 15, contact-piece 14, switch bar h and contact 32, to main wire 2.
Sub-station N cannot disturb the operation by ringing, because, although wire 1.
continues to that station and there connects with the calling apparatus, the circuit of
the said apparatus, continued by wire 18, is open at the resting contact 16 at station M.
Thus though N may actuate his generator, he cannot send a signal and furthermore
he is immediately informed that M is using the line, by the refusal of his own bell to
ring. He therefore waits until he hears the disconnecting ring of M. And in like
manner should N be using the line, M cannot interfere by ringing because his ringing
circuit is made discontinuous by the disconnection at station N of the switch / from
its resting contacts 16 and 31, and N has a clear metallic circuit by way of wire 1 to
station N switch / telephone contacts 14 and 32, wire 15, receiver R, wire 5, and
transmitter T, all at station N and wire 6 to the junction point 7 and thence back to
the central station over wire 2; the main line continuation of said wire 2 through
the calling apparatus of both stations being, as already indicated, open at 16 and 31
station N.
The cost of such additional construction as may be required need not be great, as
any two stations which are to be operated together on the same circuit will naturally
be comparatively close together; and it will be seen that two wires only are required
between the central and the nearest sub-station, the third and additional wire being
required between the intermediate and terminal sub-station only; of course the
connections of the two lines with respect to the two stations may, if desired, be
reversed.
Figure 2 represents an extension of the principle to three sub-stations M, N, and H.
Bearing in mind that all of the calling appliances are in series when the line is at
rest it is not difficult to trace the circuits. The station appliances are unchanged
consisting, as before, at each station of a call generator G and bell b, transmitting and
receiving telephones T and R and an automatic switch h. The normal or calling
circuit extends from the central station over wire 1, successively through sub-
stations M, N, and H, and back by wire 2; the route being as follows; wire 1 to
station M there by contact 31, switch h, contact 16, wire 17, and calling appliances G,
out; then by wire 18 to station N there through contact 31, switch /, contact 16,
wire 17, and calling appliances G out; then by wire 12 to station H and there through
bell b, generator G, wire 17, contact 16 switch h, and contact 31, and then back by
wire 2 to the central station. To complete the telephone circuit of station M, a branch
terminating, as in figure 1, at the telephone contacts 14 and 32 of the switch / leads
by way of wire 15, receiver R, wire 5, transmitter T, and wire 6 to the junction
point 7 on main line 2. The telephone circuit of station N extends (normally open,
of course) from telephone contacts 14 and 32 of the switch h by way of wire 15,
receiver R, wire 5, transmitter T and wire 13, to the junction point 20 on return
line 2. The telephonic facilities of station H are as in figure 7 provided by means of
a branch circuit extending from the telephone contacts of the switch there, through
the telephones and by wire 11 to the junction point 9 of main line 1. In addition to
the above, a shunt wire 21 is required for the intermediate station, this extending
from a subsidiary telephone contact 32 to a junction point 22 on the branch 11 of
main line 1. Were it not for this shunt, the telephonic currents to and from station N
would be required to traverse the wire 18 and the calling appliances at station M.
The results attained in this latter organization are, in every respect, analogous to those
obtained in the simpler organization of but two stations.

page_text: Calls sent to, or from, the central station by, or for, any sub-station are heard by
all alike, all having their calling appliances in series. When any sub-station is using
the line for conversation, the others are debarred from sending call signals, their
calling circuit being thereby made discontinuous; and each sub-station has a clear
metallic circuit extending to the central station; no electro-magnets, except those
actually appertaining to the telephones in use, being in the said circuit.
It only remains to be stated that when a plurality of stations are connected upon
the same circuit in accordance with my invention, each can call and converse with
any or all of the others; the calling appliances being in series and the several
telephones. being in multiple arc with each other..
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, 1 declare that what I
claim is:
1. The combination with a metallic or double line telephone circuit having two or
more sub-stations connected thereon; of calling appliances at each sub-station, the
said calling appliances being normally included in series in the said circuit ; a branch
circuit from each of the said sub-stations to one of the said main lines as indicated
herein, the said branch being normally open at the said sub-station; telephones
included in the said branch circuit; and a switch at each sub-station having its
movable element in electrical connection with the main line not connected with the
telephone branch of its own station, the said switch being adapted to maintain the
continuity of the metallic circuit through the call apparatus of both sub-stations, when
the line is at rest; and through the telephones of its own station breaking the calling
circuit of other stations when the said telephones are being used; substantially as
described.
2. The combination, substantially as hereinbefore set forth, in a metallic circuit
system of telephonic communication, of a central station, two sub-stations, each being
provided with telephones, calling appliances, and an automatic telephone switch
actuated by the removal and replacement of the receiving telephone; two main line
wires; one extending from the central station to the switch lever of the terminal
station forming one side of a metallic electric circuit and the other extending from
the central station to the switch lever of the intermediate station and forming a
portion of the other side of the said metallic circuit; a continuation line extending
from the resting or normal switch contact of the intermediate station through the
calling appliances of both stations, to the resting contact of the terminal station
forming the remainder of the second side of the metallic circuit and normally uniting
the two switch levers (the receiving telephone being in place) whereby the calling
appliances of both stations are normally included in circuit; and an independent
normally-open branch line for each station, extending from the working or telephone
switch contact through the telephones of said station to a junction with the main line
leading to the switch lever of the other station, the said branch being in each case
adapted to serve as a substitute for that portion of the metallic circuit which includes
the several calling appliances of other stations, the removal of the receiving telephone
from its holder at any station being thereby adapted to close the telephone branch at
said station and to open the circuit of the calling apparatus at other stations.
3. The combination with a main metallic telephone circuit extending between a
main station and a plurality of sub-stations and normally closed loops, one for each
sub-station, the loops being in series in the said circuit and each including the calling
apparatus of its respective sub-stations; of a series of normally open loops, one for
each sub-station; the said loops being in multiple arc with each other, and each
including the telephones of its respective sub-station; and an automatic switch at
each sub-station connected with one of the main circuit wires and adapted to form a
part of one side of the loops of either class according to its position, the calling
apparatus and telephones in their several loops being uniformly upon the other side,
whereby the use of the circuit at either station for telephony is adapted to prevent

page_text: the other stations from using their calling apparatus, and whereby the several stations
are enabled to simultaneously use their telephones for communication with each other,
substantially as hereinbefore described.
Dated this 21st day of February 1800
H. L. BURBANK.
By A. M. & Wm. Clark,
53, Chancery Lane, London, Patent Agents.