An Improved System of Telephonic Exchange Signalling.
Year: 1891

page_text: Date of Application, 26th Nov., 1891
Complete Specification Left, 8th Sept., 1892-Accepted, 8th Oct., 1892
PROVISIONAL SPECIFICATION.
An Improved System of Telephonic Exchange Signalling.
We, ALFRED ROSLING BENNETT of 22, St. Albans Road, Harlesden, London,
N.W., Electrical Engineer, and ROBERT MCLEAN of 38, Freme Street, Chorlton-
on-Medlock, Manchester, Electrical Engineer, do hereby declare the nature of
this invention to be as follows:-
The object of our invention is to enable the subscribers to a telephone exchange
worked with metallic circuits to speak to the operator at will without any pre-
liminary signalling by means of bells or indicators and without the employment of
a special signalling or service wire. In the telephone exchange signalling systems
known as the Law and the Mann the operators are always listening at telephones
in connection with special service wires on to which the subscribers switch them-
selves temporarily for the purpose of giving instructions. Our invention is worked
in a similar manner to the Law and Maun systems, but while yielding equal
facilities dispenses with the special service wire.
According to our invention the instrument used at the end of each subscriber's
loop comprises a device of the nature of a switch, key or lever, which is
hereinafter termed a lever.
The lever is preferably kept in such a position by a spring that the subscriber's
speaking apparatus is in circuit with the metallic loop, but on the lever being
depressed by the finger or otherwise the speaking apparatus is placed between the
metallic loop and earth and so continues until the lever is allowed to rise to its
normal position. With this exception the subscriber's instrument may be of any
of the usual patterns.
At the exchange each metallic loop is provided with a spring-jack or equivalent
contact device having three contacts which for the purposes of description are
hereinafter referred to as contacts Nos. 1, 2 and 3. One wire of the metallic loop
is connected to contact No. 1 which in its normal position is in electrical connection
with contact No. 3 and through it with the operator's telephone and earth. The
second wire of the metallic loop is connected to contact No. 2 which normally is
not in connection with any other contact, although it may be advantageous under
some circumstances to add a fourth contact to the spring-jack through which
contact No. 2 may obtain connection to earth when out of use. Contact No. 3 is
permanently connected to one terminal of the operator's telephone of which the
other terminal is to earth.
When the subscriber depresses his lever he finds himself in communication with
the operator's telephone since an earth circuit then exists through his speaking
apparatus, one wire of the metallic loop, contact No. 1, contact No. 3 and
operator's telephone. On allowing his lever to rise the earth is cut off from his
speaking apparatus and he cannot communicate with the operator.
The switch plugs or pegs are so designed that when a plug is inserted in a
spring-jack contact No. 1 is separated from contact No. 3 which then although
still connected with the operator's telephone becomes insulated from everything
else. Each plug has two contacts which are in permanent connection by means of
insulated conductors contained in a flexible cord with the two corresponding
contacts of a second plug. When a plug is inserted one of its contacts touches
contact No. 1 of the spring-jack while its other contact touches contact No. 2.
Any two metallic circuits can consequently be joined together by inserting plugs
in their spring-jacks. When this has been done the two subscribers are in com-
munication with each other. It is now necessary to provide means by which the

page_text: subscribers can notify the operator to disconnect when their conversation is finished.
This is effected in the following manner.
From each conductor of each connecting cord, preferably from its centre, we lead
a branch wire to earth through a resistance and through the operator's telephone.
The resistances should preferably be equal or nearly so in resistance, static
capacity, self-induction and magnetic inertia, and may or may not be fitted with
cores or sheaths. The resistances should be so constructed that the speaking and
ringing between subscribers is not sensibly weakened by the derivation across the
metallic loop which they form. So long as the subscribers converse together the
operator can hear nothing of what passes, but as soon as one depresses his lever an
earth circuit is constituted through his speaking apparatus, both wires of his
metallic loop, contacts Nos. 1 and 2 of the spring-jack, both contacts of the
switch-plug, both conductors in the flexible cord, both branch wires and resistances
and the operator's telephone. One or both subscribers can consequently notify
the termination of a conversation and give fresh instructions to the operator by
depressing their levers.
When the operator withdraws the plugs from the spring-jacks the connection
with her telephone through the conductors of the flexible cord is interrupted and
that through contact No. 3 of the spring-jack reëstablished. A subscriber can
therefore always put himself in communication with the operator by depressing his
lever whether he is connected through to another subscriber or not.
During busy hours the No. 3 contacts of a number of metallic circuits are
connected to the operator's telephone at which she is continually listening. At
night or during slack hours the telephone is replaced by an electro-magnetic or
other suitable indicator and battery which are operated by the depression of a
subscriber's lever.
When the employment of the earth is undesirable a third wire between the
operator's telephone and the subscriber's instrument may be substituted.
The resistances may be replaced by condensers. For instance two ordinary two-
plate condensers of sufficient capacity may be joined across the conductors of the
flexible cord and a connection to earth through the operator's telephone taken off
from between them. Or a three-plate condenser may be used the middle plate
being connected to the telephone. The resistances may also be replaced by an
induction coil, one circuit of the coil being bridged across the conductors and the
operator's telephone included in the other circuit with or without earth. Similarly
a phonopore may be utilised.
By a modification of our invention a spring-jack or contact device having only
two contacts may be employed. In this case one wire of the metallic loop is
connected to contact No. 1 and the other wire to contact No. 2 which has also a
permanent branch to earth through the operator's telephone either with or without
the intervention of resistances, condensers, induction coils or phonopores. Since
the metallic loop is everywhere else insulated from the earth the existence of the earth
derivation at the exchange will not matter when two subscribers are connected
through and the depression of their levers will at any time connect them to the
operator. This method obviates the necessity of any branch connections being
taken off the conductors of the flexible cords.
Either modification of our invention may be applied to multiple switch-boards
of any known type. When the multiple system is one in which the spring-jacks
of the various sections are coupled in series, one method of applying our invention
is to take one wire of a metallic circuit to contact No. 1 of the first jack;
contact No. 3 of the first jack is joined to contact No. 1 of the second jack
and contact No. 3 of the second jack to contact No. 1 of the third jack and so on,
contact No. 3 of the last jack being connected to the operator's telephone. The
other wire of the metallic circuit joined permanently to contact No. 2 of each
jack. Or spring-jacks with only two contacts may be used and a permanent
operator's connection with or without the intervention of resistances, condensers,
induction coils or phonopores taken off at any convenient point a method which

page_text: may likewise be adopted when the multiple system is one in which the spring-jacks
or contact devices are arranged in parallel.
By another modification of our invention operators at two different switch-rooms
situated either in the same or in different towns, which switch-rooms are connected
by metallic circuit trunk or junction lines, are enabled to communicate at will with
each other without disturbing conversations between subscribers which may be
proceeding on the said metallic circuits. Substantially this is done by giving the
operators power to establish an earth circuit between each other's telephones when
communication between them is desired, the said earth circuit being arranged so
as to avoid interference with the efficiency of the metallic circuits for through
conversations.
Dated this Twenty-fourth day of November 1891
A. R. BENNETT.
ROBERT MCLEAN.
COMPLETE SPECIFICATION.
An Improved System of Telephonic Exchange Signalling:
We, ALFRED ROSLING BENNETT of 22 St. Albans Road, Harlesden in the
County of Middlesex and ROBERT MCLEAN, of 38 Freme Street, Chorlton-on-
Medlock, Manchester, in the County of Lancaster, both Electrical Engineers 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:-
Our invention relates to telephonic exchange signalling, and is applicable to those
telephone systems which are worked with metallic circuits.
The object of our invention is to enable the subscribers to a telephone exchange
worked with metallic circuits to speak to the operator at will without any pre-
liminary signalling by means of bells or indicators and without the employment of
a special signalling or service wire. In the telephone exchange signalling systems
known as the "Law" and the "Mann" the operators are always listening at
telephones in connection with special service wires, on to which the subscribers
switch themselves temporarily for the purpose of giving instructions. In practice
our improved apparatus is worked in a similar manner to the Law and Mann
systems. While our invention yields equal facilities with the Law and Mann
system it is much simpler and less expensive, inasmuch as it dispenses with the
special service wire.
According to our invention, we provide means at the subscribers' stations
whereby a subscriber by pressing a key or lever puts his speaking apparatus in
circuit between his metallic loop and the earth, the said key or lever being normally
held back by a spring so that the speaking apparatus is in circuit with the
metallic loop. The subscribers' instruments may be of any of the usual
patterns.
We moreover arrange the connections and switch contacts at the exchange so
that the operator's telephone attached to any one section of the switchboard, is
always connected between the earth and the metallic loop of each subscriber who is
served by that section of the switchboard.
Any subscriber therefore can, by the depression of the key or lever provided as
hereinabove mentioned, be put in telephonic connection with the operator of
his section at the exchange.
We may so arrange the connections of the operator's telephone that any sub-
scriber may call "up" the operator through his telephone and metallic loop
without making any earth connection, while the call "off" is effected by the
establishment of such earth connection as hereinabove described. The subscriber
is not able to communicate to the operator through the metallic loop without the

page_text: establishment of an earth connection when once connection has been made at the
exchange to a second subscriber.
In order that our invention may be clearly understood we will now proceed to
explain the same with reference to the accompanying drawings in which,
Figure 1 shews a convenient form of apparatus for use at the subscribers'
stations.
Figures 2 and 3 are diagrams shewing the method of connecting the operator's
telephone to a metallic loop when triple-contact spring jacks are employed in the
switch-board.
Figure 4 shews a contact plug for use when triple-contact spring jacks are
employed on the switch-board after the manner shewn in Figures 2 and 3.
Figure 5 is a diagram shewing the connections of two metallic loops on a switch-
board when ordinary double-contact spring jacks are employed.
Figure 6 is a diagram shewing a modification of our system.
Figures 7, 8, and 9 are diagrams shewing a portion of a switch-board to which
our invention is applied after three different methods.
Like letters of reference denote similar parts in all the drawings.
In Figure 1, M, M is the subscriber's metallic loop, E is the earth wire, P the
telephone which when not in use is hung upon the automatic contact arm S which
thereby cuts the telephone out of circuit. K is a special key or lever which is
normally held up by a spring K', in which position of the arm, as shewn in full
lines in the figure the telephone P is included in circuit with the metallic
loop M, M². When the lever K is depressed as shewn in dotted lines, one wire
from the telephone is put direct to earth as shewn by the dotted lines, the other
wire from the telephone being joined to both wires M1, M2 of the metallic loop, as
shewn in chain lines in the figure. I may modify the construction shewn in
Figure 1 by arranging that the depression of the key or lever K shall connect the
subscriber's telephone to only one of the wires of his metallic loop instead of to
both of the said wires as hereinbefore described.
Referring to Figures 2 and 3, a form of triple-contact spring jack as shewn
at S is used on the switch-board at the exchange having three contacts, 1, 2, and 3.
The operator's telephone O is joined between contact 2 of the spring jack and the
earth. Hence if a subscriber puts his telephone to earth, by depressing the key or
lever K, Figure 1, he can call up the operator through the metallic loop and
contact 2 of the spring jack.
In Figure 3 two subscribers' telephones P1, P2 are shewn connected together at
the exchange by means of the ordinary flexible connecting cord C¹ C². The two
conductors C1, C2 of the connecting cord are joined by a bridge wire, having
resistance coils R1, R2 included in circuit, the middle point B. of the said bridge
wire being joined to one end of the operator's telephone as shewn. A suitable form
of plug for use in this system is shewn in Figure 4. A', A are the two conducting
portions of the plug which are joined to the two wires of the flexible connecting
cord. The part A of the plug is adapted to make connection with the contact 1
of the spring jack, the part A being adapted to connect with contact 3 of the
spring jack, the contact 2 of the spring jack being insulated when the plug is in
place, as shewn in Figure 3.
The operator's telephone is then joined between the earth and the metallic
loop of either of the subscribers, by the bridge wire and the conductors C¹, C² of
the flexible connecting cord.
At night or during slack hours the operator's telephone may be replaced by an
electro-magnetic or other suitable indicator and battery, which are operated by the
depression of a subscriber's lever, as above described.
When the employment of the earth is undesirable a third wire between the
operator's telephone and the subscriber's instrument may be substituted.
The resistances may be replaced by condensers. For instance two ordinary
two-plate condensers of sufficient capacity may be joined across the conductors of
the flexible cord and a connection to earth through the operator's telephone taken

page_text: off from between them Or a three-plate condenser may be used the middle plate
being connected to the telephone. The resistances may also be replaced by an
induction coil, one circuit of the coil being bridged across the conductors, and the
operator's telephone included in the other circuit, with or without earth. Similarly
a phonopore may be utilised.
By a modification of our invention a spring-jack or contact device having only
two contacts may be employed as shewn in Figure 5. In this case one wire of the
metallic loop is connected to contact No. 1 and the other wire to contact No. 2
which has also a permanent branch to earth through the operator's telephone, either
with or without the intervention of resistances, condensers, induction coils or
phonopores. Since the metallic loop is everywhere else insulated from the carth
the existence of the earth derivation at the exchange will not matter when two
subscribers are connected through, and the depression of their levers will at any
time connect them to the operator. This method obviates the necessity of any
branch connections being taken off the conductors of the flexible cords, and may be
further modified by arranging the connections as in Figure 6, which shows a system
by which a subscriber's call "up to the operator is made through the metallic
loop without the aid of an earth connection, while the call "off" is effected by
establishing an earth connection by means of such a lever as has already been
described. Thus in Figure 6 the course of a telephone current from a
subscriber P prior to the establishment of a connection would be, Pl, one wire of
the metallic loop M₁, contacts 1 and 2 of the triple-contact spring jack S₁,
operator's telephone O, contact 3 of spring jack S', second wire of loop M², back
to P. The course of a current used to call "off" from subscriber P', during the
existence of a connection say with subscriber P2, would be, E, P¹, one wire of
metallic loop M', contact 1 of spring jack S¹, one conductor of the connecting
cord C to contact 3 of spring jack S2, operator's telephone O to earth E.
"
Either modification of our invention may be applied to multiple switch-boards
of any known type. When the multiple system is one in which the spring jacks
of the various sections are coupled in series, one method of applying our invention
is shewn in Figure 7. One wire of a metallic circuit is taken to contact No. 1.
of the first jack St. Contact No. 2 of the first jack is joined to contact No. 1 of
the second jack S³, and contact No. 2 of the second jack to contact No. 1 of the
third jack S, and so on, contact No. 2 of the last jack being connected to the
operator's telephone. The other wire of the metallic circuit is joined permanently
to contact No. 3 of each jack.
The course of the telephonic currents used in calling "up" the operator, or
calling “off” to shew that a conversation is finished and disconnection desired, is
the same as herein before described with reference to Figure 2 of the drawings.
Figures 8 and 9 shew diagrammatically portions of switch-boards provided with
only two contacts for the switch plugs. In Figure 8, the plug contact-blocks are
arranged in series on the metallic loops, in Figure 9 the contact blocks are
arranged in parallel. The operator's telephone is joined in permanent connection
with one of the lines of the several metallic loops, and we may if desirable include
in the operator's telephone circuit resistances, condensers, induction or phonoporic
coils. The subscribers may call "up" or call "off" in the manner hereinbefore
described with reference to Figure 5 of the drawings.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed, we declare that what we
claim is :-
1. In a metallic circuit telephone exchange system, the means for calling up
prior to connection, of the operator by a subscriber, said means comprising a
special circuit including the subscriber's telephone, one or both wires of the metallic
loop, the operator's telephone, and the earth or a third wire, which special circuit
is established by the subscriber when required.
2. In a metallic circuit telephone exchange system the means for calling up

page_text: during the existence of a connection, of the operator by a subscriber, said means
comprising a special circuit including the subscriber's telephone, one or both wires
of the metallic loop, the operator's telephone placed in a branch or derivation from
the conductors of the connecting cord, and the earth or a third wire, which special
circuit is established by the subscriber when required.
3. In a telephone system worked with complete metallic circuits, the devices
whereby a subscriber can put his telephone in connection with the earth and with
both wires of his metallic loop, substantially as described with reference to
Figure 1 of the accompanying drawings, or the modification thereof wherein the
subscriber's telephone is connected to only one wire of the metallic loop, for the
purpose specified.
4. In a telephone system worked with complete metallic circuits, the combination
with the subscribers' metallic loops, of the spring jacks, connecting cords, and
operators' telephones, substantially as described with reference to any of the forms
shown in the accompanying drawings, for the purposes specified.
1 1
2
1000
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicants