Improvements in Telephonic Switching Apparatus.
Year: 1891

page_text: Date of Application, 27th Oct., 1891
Complete Specification Left, 26th Aug., 1892-Accepted, 1st Oct., 1892
PROVISIONAL SPECIFICATION.
Improvements in Telephonic Switching Apparatus.
I, ALFRED ROSLING BENNETT, of 22, St. Albans Road, Harlesden, London,
N.W., Electrical Engineer, do hereby declare the nature of this invention to be as
follows:-
The object of my invention is in a telephone exchange operated by metallic
circuits and multiple switch-boards to provide a method by means of which the
operator at any one section of the switch-board can, by applying a test, discover
whether any particular metallic circuit is connected at any other section.
The test enables each operator to tell whether a connecting cord is in at any
section other than her own.
Each connecting cord for metallic circuit switching necessarily consists of two
insulated conductors. From each conductor of such a cord, preferably from its
centre, I lead a branch wire or conductor to earth through a resistance which may
conveniently consist of a coil of insulated wire wound on a bobbin but it may also
be made up of liquid, carbon or any suitable conductor. The resistance may or
may not be combined with cores, sheaths or condensers for the purpose of
increasing the electro-magnetic inertia and static_capacity of the branch to earth.
To attain the best results it is necessary that the branch to earth from each of the
conductors in a cord should be as nearly as possible alike in resistance, self-
induction, magnetic inertia and capacity. To prevent variations due to changes of
temperature the material employed should be alike in each branch and to save
space, when wire is used both resistances may be wound on the same bobbin. All
the connecting cords used with the switch-board should be so fitted.
Each operator is provided with a testing apparatus consisting of a flexible
conducting cord tipped with metal, which cord is permanently connected to earth
through a source of electricity such as a voltaic battery or magneto generator and
through an instrument capable of indicating visibly or audibly when a current
passes through it.
To test, the operator applies the metal tip of her testing cord to the metal
socket of the metallic circuit which is before her on her section of the switch-
board.
If there is no connecting cord in a socket of the same metallic circuit on any
other section the indicating instrument will not be affected since the current from
the source of electricity contained in the circuit of the cord cannot find an outlet
through the metallic circuit which is everywhere insulated from the earth. The
absence of a signal will show that the metallic circuit wanted is not engaged at
any other section and the operator may accordingly connect it on hers.
But if a connecting cord is in a socket on any other section the indicating
instrument will reveal a current the moment the tip of the testing cord is applied
to the socket, since the battery current will now find an outlet to earth through
one of the branches to earth from the connecting cord that may so be in. The
operator will then know that the metallic circuit wanted is engaged and must not
be connected on her section.
Instead of a source of electricity being in the circuit of the testing cord it may
be placed in the branches between the connecting cords and earth. In this case
the operator's testing apparatus consists only of a flexible cord, tipped, connected
through the indicating instrument to earth. On applying the tip to the socket she
obtains a current only when a connecting cord is in.
The indicating instrument used for testing may consist of a galvanometer, an

page_text: electro-magnet, a voltameter, a telephone or any device adapted to respond
promptly to the passage of a current.
As the branches between the connecting cords and earth are of equal resistance,
self-induction, inertia and capacity, the efficiency of the metallic circuits will not
be impaired by the derivation to earth.
The invention may be varied in detail. For instance, instead of connecting
each conductor of a cord to earth through a resistance as hereinbefore described
each conductor may be connected to one plate of a condenser, or to one circuit of
an induction coil or of a phonopore, the source of electricity and earth being
joined to the other plate of the condenser or circuit of the coil or phonopore.
When either of these plans is adopted the source of electricity employed should
preferably be one capable of furnishing intermittent currents or reversals and the
operator's indicating instrument should be adapted to reveal the passage through it
of such currents.
As in the plan first described the source of electricity may be placed in circuit
with the testing cord. In that case the earth is joined direct to the second plate
of the condenser or circuit of induction coil or phonopore.
The condensers, coils or phonopores connected to the conductors of a cord
should be as nearly as possible equal in capacity, resistance and dimensions.
For the convenience of description it has herein before been stated that the
branches from the cords and the operator's indicating instrument are respectively
connected to the earth. But this is not really necessary. In every case a direct
metallic connection, from which the earth is excluded, may be made between the
branches and the indicating instrument.
The invention may be applied to any of the types of multiple switch-boards
ordinarily employed.
Dated this Tighteenth dN L
1001
COMPLETE SPECIFICATION.
Improvements in Telephonic Switching Apparatus.
I, ALFRED ROSLING BENNETT, of 22 St. Albans Road, Harlesden in the
County of Middlesex, Electrical Engineer, do hereby declare the nature of this
invention and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
My invention relates to telephonic switching apparatus and is designed for use
in conjunction with multiple switch-boards of any type when the telephonic
currents are carried by means of a complete metallic circuit.
Multiple switchboards in telephone exchanges are set up in sections, each
section being constructed to serve a certain number of subscribers and being
controlled by one operator. An operator on being asked to put any subscriber in
a section into communication with another subscriber on another section must
before making the connection ascertain whether the latter subscriber is engaged.
When complete metallic circuits are employed the connecting together of two
subscribers' line wires is usually accomplished by means of a flexible connecting
cord which is made up of two insulated wires jointed to divided plugs at each end
of the said cord. The plugs are adapted to fit and make connection with the
contact-blocks or sockets on the switchboard to which the subscribers' wires
are led.
In the Specification of a former Patent dated 23rd July 1891 No. 12484 I have
described an arrangement wherein I lead from each conductor in a connecting
cord a branch wire through resistances which are preferably the same for every
branch wire. These resistances may be of any suitable kind and may or may not
be con:bined with iron cores or sheaths or with condensers. The said branch wires
are all joined together after passing through the said resistances and their common

page_text: terminal is put to earth and thereby connected with the operators' testing
apparatus, which is likewise earthed.
The testing apparatus described in the said Specification consists of a suitable
device for giving audible or visual signals when a current flows through the same
and of a flexible conductor connected thereto by one extremity and provided at its
other extremity with a metallic tip or point. A battery of suitable size is placed
in circuit with the branch wires or at any other convenient point in the testing
circuit. The switchboards have the contacts arranged in parallel as described in
the Specification of a former Patent No. 19486 dated November 29th 1890.
According to the said Specification an operator at any section wishing to
ascertain if any particular subscriber is already connected on another section of
the board makes connection by the test tip or point between the testing apparatus
and the contact block or socket on the said operator's section which is attached to
that subscribers line wire; the absence of any signal on the testing apparatus will
indicate that the subscriber is free, since if the subscriber be connected to some
other line on another section a current will flow through the testing apparatus
directly the test point is applied to the said subscriber's block.
According to my present invention I employ a complete metallic circuit insulated
from the earth for connecting the said testing apparatus with the subscribers'
lines and I use a multiple switchboard with the contacts arranged in series, with
complete metallic circuits for the subscribers' lines though in some instances I can
use a multiple switch-board with the contacts in parallel. Moreover in some cases
I substitute condensers for the resistances or coils in the branch wires.
In order that my invention may be clearly understood I will now proceed to
describe the same with reference to the accompanying drawings in which :-
Figure 1 is a diagram showing my invention applied to one connecting cord
connected by a complete metallic circuit with the testing apparatus.
Figure 2 is a diagram showing the general arrangement when the multiple
switchboard used is one in which the sockets or contact-blocks are connected in
series.
Figure 3 is a similar diagram, the contact-blocks or spring-jacks being connected
in parallel and which shows the modification of my invention wherein I use
condensers instead of resistances. When it is desired to employ annunciator drops
or electro-magnetic indicators with the switchboards for the purpose of attracting
the operators' attention such may be looped in the metallic circuits at any
convenient point although not shown in the drawings.
Similar letters of reference denote corresponding parts thronghout the drawings.
C'C' are the two conductors of the flexible connecting cord which are furnished
with insulated contact pieces A. B B are the branch wires joined to the
conductors C¹ C at about the middle of their length and R' R' are resistance
coils in series with the branch wires B' B. For practical purposes these
resistances may
each be of 3000 Ohms. The branch wires are joined at B to a
wire E which is connected direct to the operator's test apparatus as shown. The
operator's test apparatus comprises a flexible conductor T¹ furnished with a
metallic tip or point T and a suitable device for giving an audible or visual
signal when a current passes therethrough. This device may, as shown in
Figure 1, take the form of a relay I which operates to close a local circuit
containing a battery L and a telephone O or an electric bell or other signalling
device. The wire from the signal device is joined direct to the wire E as shown,
a battery V which may consist of ten ordinary Leclanché cells being inserted at
any suitable place in the circuit.
In Figure 2 five metallic circuits connected to three sections of a multiple
switchboard are shewn. P¹ to P' are the subscribers' telephones, M' to M' their
lines which are shown connected in series to the switch- or contact-blocks S'S' S
on three respective sections. The three wires B* leading from the junctions of
the branch wires B¹ B' are shown connected to a common lead E which is joined to
the testing devices. The testing battery V is shown inserted in the common

page_text: lead E but it may be obviously inserted at any point in the circuit or a separate
battery may be placed in each of the wires B₁ or conductors T. In Figure 3
which shows a similar arrangement the contact-blocks or spring-jacks S¹ S S are
put in parallel with the line wires.
In the Figures 2 and 3 four of the subscribers are shown connected two and two
by the connecting cords C¹ C³, and the fifth subscriber is shown disengaged. It is
obvious that if one of the metallic points T' T' T' be applied to one of the
contact-blocks or spring-jacks of the engaged lines a current will flow through the
test apparatus and a signal will thereby be given to the operator, whereas if
the line be disengaged no current will flow, the connecting cords being insulated
from the lines.
The same figure illustrates a modified form of my invention in which the source
of electricity V in the testing circuit is preferably of such a form as will give
intermittent, vibratory, or alternating currents as for instance a revolving
magneto or dynamo machine, a pole-changer, a make-and-break interrupter or
induction coil. For the signal apparatus in the test circuit a telephone, phonic
relay or some other delicate electro-magnet may very advantageously be used.
The resistances R' R2 which are shown in Figures 1 and 2 are replaced by two
ordinary two-plate condensers X' X² which are preferably of equal capacity or by
a three-plate condenser X. For practical purposes the capacity of each of these
condensers may be about half a microfarad.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
First. In a telephone system with complete metallic circuits for the subscribers'
lines and multiple switch-boards the combination with the same of a testing
apparatus substantially as hereinabove described having an entirely metallic
connection, insulated from the earth, between the operators' testing tip or point
and the connecting cords for the purpose specified.
Second. In a telephone exchange the combination with a flexible connecting
cord containing insulated twin conductors of a bridge including two condensers
and having a point between the said condensers joined through a source of
electricity and a signalling device to the operators' testing cord substantially as
described for the purpose specified.
Third. In combination with a telephone testing system such as that herein
described the employment of multiple switches with the contacts arranged in series
for the purpose specified.
Deted this 26th dox of Ano 1800
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.