THE MAGNETO MULTIPLE SWITCHBOARD
=Field of Utility.= The principles of the multiple switchboard set forth
in the last chapter were all developed long before the common-battery
system came into existence, and consequently all of the first multiple
switchboards were of the magneto type. Although once very widely used,
the magneto multiple switchboard has almost passed out of existence,
since it has become almost universal practice to equip exchanges large
enough to employ multiple boards with common-battery systems.
Nevertheless there is a field for magneto multiple switchboards, and in
this field it has recently been coming into increasing favor. In those
towns equipped with magneto systems employing simple switchboards or
transfer switchboards, and which require new switchboards by virtue of
having outgrown or worn out their old ones, the magneto multiple
switchboard is frequently found to best fit the requirements of economy
and good practice. The reason for this is that by its use the magneto
telephones already in service may be continued, no change being required
outside of the central office. Furthermore, with the magneto multiple
switchboard no provision need be made for a power plant, which, in towns
of small size, is often an important consideration. Again, many
companies operate over a considerable area, involving a collection of
towns and hamlets. It may be that all of these towns except one are
clearly of a size to demand magneto equipment and that magneto equipment
is the standard throughout the entire territory of the company. If,
however, one of the towns, by virtue of growth, demands a multiple
switchboard, this condition affords an additional argument for the
employment of the magneto multiple switchboard, since the same standards
of equipment and construction may be maintained throughout the entire
territory of the operating company, a manifest advantage. On the other
hand, it may be said that the magneto multiple switchboard has no proper
place in modern exchanges of considerable size--say, having upward of
one thousand subscribers--at least under conditions found in the United
States.
Notwithstanding the obsolescence of the magneto multiple switchboard for
large exchanges, a brief discussion of some of the early magneto
multiple switchboards, and particularly of one of the large ones, is
worth while, in that a consideration of the defects of those early
efforts will give one a better understanding and appreciation of the
modern multiple switchboard, and particularly of the modern multiple
common-battery switchboard, the most highly organized of all the manual
switching systems. Brief reference will, therefore, be made to the
so-called series multiple switchboard, and then to the branch terminal
multiple switchboard, which latter was the highest type of switchboard
development at the time of the advent of common-battery working.
[Illustration: Fig. 337. Series Magneto Multiple Switchboard]
=Series-Multiple Board.= In Fig. 337 are shown the circuits of a series
magneto multiple switchboard as developed by the engineers of the
Western Electric Company during the eighties. As is usual, two
subscribers' lines and a single cord circuit are shown. One side of each
line passes directly from the subscriber's station to one side of the
drop, and also branches off to the sleeve contact of each of the jacks.
The other side of the line passes first to the tip spring of the first
jack, thence to the anvil of that jack and to the tip spring of the next
jack, and so on in series through all of the jacks belonging in that
line to the other terminal of the drop coil. Normally, therefore, the
drop is connected across the line ready to be responsive to the signal
sent from the subscriber's generator. The cord circuit is of the
two-conductor type, the plugs being provided with tip and sleeve
contacts, the tips being connected by one of the flexible conductors
through the proper ringing and listening key springs, and the sleeve
being likewise connected through the other flexible conductor and the
other springs of the ringing and listening keys. It is obvious that when
any plug is inserted into a jack, the circuit of the line will be
continued to the cord circuit and at the same time the line drop will be
cut out of the circuit, because of the lifting of the tip spring of the
jack from its anvil. Permanently connected between the sleeve side of
the cord circuit and ground is a retardation coil _1_ and a battery.
Another retardation coil _2_ is connected between the ground and a
point on the operator's telephone circuit between the operator's head
receiver and the secondary of her induction coil. These two retardation
coils have to do with the busy test, the action of which is as follows:
normally, or when a line is not switched at the central office, the test
thimbles will all be at substantially ground potential, _i. e._, they
are supposed to be. The point on the operator's receiver circuit which
is grounded through the retardation coil _2_ will also be of ground
potential because of that connection to ground. In order to test, the
operator always has to throw her listening key _L.K._ into the listening
position. She also has to touch the tip of the calling plug _P_c to a
sleeve or jack of the line that is being tested. If, therefore, a test
is made of an idle or non-busy line, the touching of the tip of the
calling plug with the test thimble of that line will result in no flow
of current through the operator's receiver, because there will be no
difference of potential anywhere in the test circuit, which test circuit
may be traced from the test thimble of the line under test to the tip of
the calling plug, thence through the tip strand of the cord to the
listening key, thence to the outer anvil of the listening key on that
side, through the operator's receiver to ground through the impedance
coil _2_. If, however, the line had already been switched at some other
section by the insertion of either a calling or answering plug, all of
the test thimbles of that line would have been raised to a potential
above that of the ground, by virtue of the battery connected with the
sleeve side of the cord circuit through the retardation coil _1_. If the
operator had made a test of such a line, the tip of her testing plug
would have found the thimble raised to the potential of the battery and,
therefore, a flow of current would occur which would give her the busy
click. The complete test circuit thus formed in testing a busy line
would be from the ungrounded pole of the battery through the impedance
coil _1_ associated with the cord that was already in connection with
the line, thence to the sleeve strand of that cord to the sleeve of the
jack at which the line was already switched, thence through that portion
of the line circuit to which all of the sleeve contacts were connected,
and therefore to the sleeve or test thimble of the jack at which the
test is made, thence through the tip of the calling plug employed in
making the test through the tip side of that cord circuit to the outer
listening key contact of the operator making the test, and thence to
ground through the operator's receiver and the impedance coil _2_. The
resultant click would be an indication to the operator that the line was
already in use and that, therefore, she must not make the connection.
The condenser _3_ is associated with the operator's talking set and with
the extra spring in the listening key _L.K._ in such a manner that when
the listening key is thrown, the tip strand of the cord circuit is
divided and the condenser included between them. This is for the purpose
of preventing any potentials, which might exist on the line with which
the answering plug _P_a was connected, from affecting the busy-test
conditions.
_Operation._ The operation of the system aside from the busy-test
feature is just like that described in connection with the simple
magneto switchboard. Assuming that the subscriber at Station _A_ makes
the call, he turns his hand generator, which throws the drop on his line
at the central office. The operator, seeing the signal, inserts the
answering plug of one of her idle pairs of cords into the answering jack
and throws her listening key _L.K._ This enables the operator to talk
with the calling subscriber, and having found that he desires a
connection with the line extending to Station _B_, she touches the tip
of her calling plug to the multiple jack of that line that is within her
reach, it being remembered that each one of the multiple jacks shown is
on a different section. She leaves the listening key in the listening
position when she does this. If the line is busy, the click will notify
her that she must not make the connection, in which case she informs the
calling subscriber that the line is busy and requests him to call again.
If, however, she received no click, she would insert the calling plug
into the jack, thus completing the connection between the two lines. She
would then press the ringing key associated with the calling plug and
that momentarily disconnects the calling plug from the answering plug
and at the same time establishes connection between the ringing
generator and the called line. The release of the ringing key again
connects the calling and answering plugs and, therefore, connects the
two subscribers' lines ready for conversation. All that is then
necessary is that the called subscriber shall respond and remove his
receiver from its hook, the calling subscriber already having done this.
When the conversation is finished, both of the subscribers (if they
remember it) will operate their ringing generators, which will throw the
clearing-out drop as a signal to the operator for disconnection. If it
should become necessary for the operator to ring back on the line of the
calling subscriber, she may do so by pressing the ringing key associated
with the calling plug.
Frequently this multiple switchboard arrangement was used with grounded
lines, in which case the single line wire extending from the
subscriber's station to the switchboard was connected with the tip
spring of the first jack, the circuit being continued in series through
the jack to the drop and thence to ground through a high non-inductive
resistance.
_Defects._ This series multiple magneto system was used with a great
many variations, and it had a good many defects. One of these defects
was due to the necessary extending of one limb of the line through a
large number of series contacts in the jacks. This is not to be desired
in any case, but it was particularly objectionable in the early days
before jacks had been developed to their present high state of
perfection. A particle of dust or other insulating matter, lodging
between the tip spring and its anvil in any one of the jacks, would
leave the line open, thus disabling the line to incoming signals, and
also for conversation in case the break happened to occur between the
subscriber and the jack that was used in connecting with the line.
Another defect due to the same cause was that the line through the
switchboard was always unbalanced by the insertion of a plug, one limb
of the line always extending clear through the switchboard to the drop
and the other, when the plug was inserted, extending only part way
through the switchboard and being cut off at the jack where the
connection was made. The objection will be apparent when it is
remembered that the wires in the line circuit connecting the multiple
jacks are necessarily very closely bunched together and, therefore,
there is very likely to be cross-talk between two adjacent lines unless
the two limbs of each line are exactly balanced throughout their entire
length.
Again the busy-test conditions of this circuit were not ideal. The fact
that the test rings of the line were connected permanently with the
outside line circuit subjected these test rings to whatever potentials
might exist on the outside lines, due to any causes whatever, such as a
cross with some other wire; thus the test rings of an idle line might by
some exterior cause be raised to such a potential that the line would
test busy. It may be laid down as a fundamental principle in good
multiple switchboard practice that the busy-test condition should be
made independent of any conditions on the line circuit outside of the
central office, and such is not the case in this circuit just described.
[Illustration: CABLE RUN FROM INTERMEDIATE FRAME TO MULTIPLE Cortlandt
Office, New York Telephone Co.]
=Branch-Terminal Multiple Board.= The next important step in the
development of the magneto multiple switchboard was that which produced
the so-called branch-terminal board. This came into wide use in the
various Bell operating companies before the advent of the common-battery
systems. Its circuits and the principles of operation may be understood
in connection with Fig. 338. In the branch-terminal system there are no
series contacts in the jacks and no unbalancing of the line due to a
cutting off of a portion of the line circuit when a connection was made
with it. Furthermore, the test circuits were entirely local to the
central office and were not likely to be affected by outside conditions
on the line. This switchboard also added the feature of the automatic
restoration of the drops, thus relieving the operator of the burden of
doing that manually, and also permitting the drops to be mounted on a
portion of the switchboard that was not available for the mounting of
jacks, and thus permitting a greater capacity in jack equipment.
[Illustration: Fig. 338. Branch-Terminal Magneto Multiple Switchboard]
Each jack has five contacts, and the answering and multiple jacks are
alike, both in respect to their construction and their connection with
the line. The drops are the electrically self-restoring type shown in
Fig. 263. The line circuits extended permanently from the subscriber's
station to the line winding of the drop and the two limbs of the line
branched off to the tip and sleeve contacts _1_ and _2_ respectively of
each jack. Another pair of wires extended through the multiple parallel
to the line wires and these branched off respectively to the contact
springs _3_ and _4_ of each of the jacks. This pair of wires formed
portions of the drop-restoring circuit, including the restoring coil _6_
and the battery _7_, as indicated. The test thimble _5_ of each of the
jacks is connected permanently with the spring _3_ of the corresponding
jack and, therefore, with the wire which connects through the restoring
coil _6_ of the corresponding drop to ground through the battery _7_.
The plugs were each provided with three contacts. Two of these were the
usual tip and sleeve contacts connected with the two strands of the cord
circuit. The third contact _8_ was not connected with any portion of the
cord circuit, being merely an insulated contact on the plug adapted,
when the plug was fully inserted, to connect together the springs _3_
and _4_. The cord circuit itself is readily understood from the drawing,
having two features, however, which merit attention. One is the
establishing of a grounded battery connection to the center portion of
the winding of the receiver for the purposes of the busy test, and the
other is the provision of a restoring coil and restoring circuit for the
clearing-out drop, this circuit being closed by an additional contact on
the listening key so as to restore the clearing-out drop whenever the
listening key was operated.
_Operation._ An understanding of the operation of this system is easy.
The turning of the subscriber's generator, when the line was in its
normal condition, caused the display of the line signal. The insertion
of the answering plug, in response to this call, did three things: (1)
It extended the line circuit to the tip and sleeve strand of the cord
circuit. (2) It energized the restoring coil _6_ of the drop by
establishing the circuit from the contact spring _3_ through the plug
contact _8_ to the other contact spring _4_, thus completing the circuit
between the two normally open auxiliary wires. (3) The connecting of the
springs _3_ and _4_ established a connection from ground to the test
thimbles of all the jacks on a line, the spring _4_ being always
grounded and the spring _3_ being always connected to the test thimble
_5_.
It is to be noted that on idle lines the test rings are always at the
same potential as the ungrounded pole of the battery _7_, being
connected thereto through the winding _6_ of the restoring coil. On all
busy lines, however, the test rings are dead grounded through the
contact _8_ of the plug that is connected with the line.
The tip of the testing plug at the time of making a test will also be at
the same potential as that of the ungrounded pole of the battery _7_,
since this pole of the battery _7_ is always connected to the center
portion of the operator's receiver winding, and when the listening key
is thrown the tip of the calling plug is connected therewith and is at
the same potential. When, therefore, the operator touches the tip of
the calling plug to the test thimble of an idle line, she will get no
click, since the tip of the plug and the test thimble will be at the
same potential. If, however, the line has already been switched at
another section of the board, there will be a difference of potential,
because the test thimble will be grounded, and the circuit, through
which the current which causes the click flows, may be traced from the
ungrounded pole of the battery _7_ to the center portion of the
operator's receiver, thence through one-half of the winding to the tip
of the calling plug, thence to the test thimble of the jack under test,
thence to the spring _3_ of the jack on another section at which the
connection exists, through the contact _8_ on the plug of that jack to
the spring _4_, and thence to ground and back to the other terminal of
the battery _7_.
_Magnet Windings._ Coils of the line and clearing-out drops by which
these drops are thrown, are wound to such high resistance and impedance
as to make it proper to leave them permanently bridged across the
talking circuit. The necessity for cutting them out is, therefore, done
away with, with a consequent avoidance, in the case of the line drops,
of the provision of series contacts in the jacks.
_Arrangement of Apparatus._ In boards of this type the line and
clearing-out drops were mounted in the extreme upper portion of the
switchboard face so as to be within the range of vision of the operator,
but yet out of her reach. Therefore, the whole face of the board that
was within the limit of the operator's reach was available for the
answering and multiple jacks. A front view of a little over one of the
sections of the switchboard, involving three complete operator's
positions, is shown in Fig. 339, which is a portion of the switchboard
installed by the Western Electric Company in one of the large exchanges
in Paris, France. (This has recently been replaced by a common-battery
multiple board.) In this the line drops may be seen at the extreme top
of the face of the switchboard, and immediately beneath these the
clearing-out drops. Beneath these are the multiple jacks arranged in
banks of one hundred, each hundred consisting of five strips of twenty.
At the extreme lower portion of the jack space are shown the answering
jacks and beneath these on the horizontal shelf, the plugs and keys.
These jacks were mounted on 1/2-inch centers, both vertically and
horizontally and each section had in multiple 90 banks of 100 each,
making 9,000 in all. Subsequent practice has shown that this involves
too large a reach for the operators and that, therefore, 9,000 is too
large a number of jacks to place on one section if the jacks are not
spaced closer than on 1/2-inch centers. With the jack involving as many
parts as that required by this branch terminal system, it was hardly
feasible to make them smaller than this without sacrificing their
durability, and one of the important features of the common-battery
multiple system which has supplanted this branch-terminal magneto system
is that the jacks are of such a simple nature as to lend themselves to
mounting on 3/8-inch centers, and in some cases on 3/10-inch centers.
[Illustration: Fig. 339. Face of Magneto Multiple Switchboard]
=Modern Magneto Multiple Board.= Coming now to a consideration of modern
magneto multiple switchboards, and bearing in mind that such boards are
to be found in modern practice only in comparatively small installations
and then only under rather peculiar conditions, as already set forth, we
will consider the switchboard of the Monarch Telephone Manufacturing
Company as typical of good practice in this respect.
[Illustration: Fig. 340. Monarch Magneto Multiple Switchboard Circuits]
_Line Circuit._ The line and cord circuits of the Monarch system are
shown in Fig. 340. It will be seen that each jack has in all five
contacts, numbered from _1_ to _5_ respectively, of which _1_ and _4_
are the springs which register with the tip and ring contacts of the
plug and through which the talking circuit is continued, while _2_ and
_3_ are series contacts for cutting off the line drop when a plug is
inserted, and _5_ is the test contact or thimble adapted to register
with the sleeve contact on the plug when the plug is fully inserted. The
line circuit through the drop may be traced normally from one side of
the line through the drop coil, thence through all of the pairs of
springs _2_ and _3_ in the jacks of that line, and thence to spring _1_
of the last jack, this spring always being strapped to the spring _2_ in
the last jack, and thence to the other side of the line. All the ring
springs _1_ are permanently tapped on to one side of the line, and all
of the tip springs _4_ are permanently tapped to the other side of the
line. This system may, therefore, properly be called a branch-terminal
system. It is seen that as soon as a plug is inserted into any of the
jacks, the circuit through the drop will be broken by the opening of the
springs _2_ and _3_ in that jack. The drop shown immediately above the
answering jack is so associated mechanically with that jack as to be
mechanically self-restored when the answering plug is inserted into the
answering jack in response to a call. The arrangement in this respect is
the same as that shown in Fig. 259, illustrating the Monarch combined
drop and jack.
_Cord Circuit._ The cord circuit needs little explanation. The tip and
ring strands are the ones which carry the talking current and across
these is bridged the double-wound clearing-out drop, a condenser being
included in series in the tip strand between the two drop windings in
the manner already explained in connection with Fig. 284. The third or
sleeve strand of the cord is continuous from plug to plug, and between
it and the ground there is permanently connected a retardation coil.
_Test._ The test is dependent on the presence or absence of a path to
ground from the test thimbles through some retardation coil associated
with a cord circuit. Obviously, in the case of an idle line there will
be no path to ground from the test thimbles, since normally they are
merely connected to each other and are insulated from everything else.
When, however, a plug is inserted into a multiple or answering jack, the
test thimbles of that line are connected to ground through the
retardation coil associated with the third strand of the plug used in
making the connection. When the operator applies the tip of the calling
plug to a test contact of a multiple jack there will be no path to
ground afforded if the line is idle, while if it is busy the potential
of the tip of the test plug will cause a current to flow to ground
through the impedance coil associated with the plug used in making the
connection. This will be made clearer by tracing the test circuit. With
the listening key thrown this may be traced from the live side of the
battery through the retardation coil _6_, which is common to an
operator's position, thence through the tip side of the listening key to
the tip conductor of the calling cord, and thence to the tip of the
calling plug and the thimble of the jack under test. If the line is idle
there will be no path to ground from this point and no click will
result, but if the line is busy, current will flow from the tip of the
test plug to the thimble of the jack tested, thence by the test wire in
the multiple to the thimble of the jack at which a connection already
exists, and thence to ground through the third strand of the cord used
in making that connection and the impedance coil associated therewith.
The current which flows in this test circuit changes momentarily the
potential of the tip side of the operator's telephone circuit, thus
unbalancing her talking circuit and causing a click.
[Illustration: Fig. 341. Magneto Multiple Switchboard]
If this test system were used in a very large board where the multiple
would extend through a great many sections, there would be some
liability of a false test due to the static capacity of the test
contacts and the test wire running through the multiple. For small
boards, however, where the multiple is short, this system has proven
reliable. A multiple magneto switchboard employing the form of circuits
just described is shown in Fig. 341. This switchboard consists of three
sections of two positions each. The combined answering jacks and drops
may be seen at the lower part of the face of the switchboard and
occupying somewhat over one-half of the jack and drop space. The
multiple jacks are above the answering jacks and drops and it may be
noted that the same arrangement and number of these jacks is repeated in
each section. This switchboard may be extended by adding more sections
and increasing the multiple in those already installed to serve 1,600
lines.
_Assembly._ In connection with the assembly of these magneto multiple
switchboards, as installed by the Monarch Company, Fig. 342 shows the
details of the cord rack at the back of the board. It shows how the ends
of the switchboard cords opposite to the ends that are fastened to the
plugs are connected permanently to terminals on the cord rack, at which
point the flexible conductors are brought out to terminal clips or
binding posts, to which the wires leading from the other portions of the
cord circuit are led. In order to relieve the conductors in the cords
from strain, the outer braiding of the cord at the rack end is usually
extended to form what is called a _strain cord_, and this attached to an
eyelet under the cord rack, so that the weight of the cord and the cord
weights will be borne by the braiding rather than by the conductors.
This leaves the insulated conductors extending from the ends of the
cords free to hang loose without strain and be connected to the
terminals as shown. This method of connecting cords, with variations in
form and detail, is practically universal in all types of switchboards.
[Illustration: Fig. 342. Cord-Rack Connectors]
A detail of the assembly of the drops and jacks in such a switchboard
is shown in Fig. 343. The single pair of clearing-out drops is mounted
in the lower part of the vertical face of the switchboard just above the
space occupied by the plug shelf. Vertical stile strips extend above the
clearing-out drop space for supporting the drops and jacks. A single row
of 10 answering jacks and the corresponding line drops are shown in
place. Above these there would be placed, in the completely assembled
board, the other answering jacks and line signals that were to occupy
this panel, and above these the strips of multiple jacks. The rearwardly
projecting pins from the stile strips are for the support of the
multiple jack strips, these pins supporting the strips horizontally by
suitable multiple clips at the ends of the jack strips; the jack strips
being fastened from the rear by means of nuts engaging the screw
threads on these pins. This method of supporting drops and jacks is one
that is equally adaptable for use in other forms of boards, such as the
simple magneto switchboard.
[Illustration: Fig. 343. Drop and Jack Mounting]
[Illustration: Fig. 344. Keyboard Wiring]
In Fig. 344 is shown a detail photograph of the key shelf wiring in one
of these Monarch magneto switchboards. In this the under side of the
keys is shown, the key shelf being raised on its hinge for that purpose.
The cable, containing all of the insulated wires leading to these keys,
enters the space under the key shelf at the extreme left and from the
rear. It then passes to the right of this space where a "knee" is
formed, after which the cable is securely strapped to the under side of
the key shelf. By this construction sufficient flexibility is provided
for in the cable to permit the raising and lowering of the key shelf,
the long reach of the cable between the "knee" and the point of entry at
the left serving as a torsion member, so that the raising of the shelf
will give the cable a slight twist rather than bend it at a sharp angle.