Whether or not the lamp will be lighted at this time depends on whether the relay _1_ is energized or not, and this, it will be seen, depends on whether the subscriber"s receiver is off or on its hook. If off its hook, current will flow through the metallic circuit of the line for energizing the subscriber"s transmitter, and as whatever current goes to the subscriber"s line must flow through the relay _1_, that relay will be energized and prevent the lighting of the supervisory lamp _5_. If, on the other hand, the subscriber"s receiver is on its hook, no current will flow through the line, the supervisory relay will not be energized, and the lamp _5_ will be lighted.
In a nutsh.e.l.l, the sleeve supervisory relay normally prevents the lighting of the corresponding supervisory lamp, but as soon as the operator inserts a plug into the jack of the line, the relay _2_ establishes such a condition as to make possible the lighting of the supervisory lamp, and the lighting of this lamp is then controlled entirely by the relay _1_, which is, in turn, controlled by the position of the subscriber"s switch hook.
_Battery Feed._ A 2-microfarad condenser is included in each strand of the cord, and battery is fed through the relay windings to the calling and called subscribers on opposite sides of these condensers, in accordance with the combined impedance coil and condenser method described in Chapter XIII. Here the relay windings do double duty, serving as magnets for operating the relays and as r.e.t.a.r.dation coils in the system of battery supply.
_Complete Cord and Line Circuits._ The complete cord and line circuits of the Kellogg two-wire system are shown in Fig. 352. In the more recent installations of the Kellogg Company the cord and line circuits have been slightly changed from those shown in Figs. 350 and 351, and these changes have been incorporated in Fig. 352. The principles of operation described in connection with the simplified figures remain, however, exactly the same. One of the changes is, that the tip side of the lines is permanently connected to the tips of the jacks instead of being normally cut off by the cut-off relay, as was done in the system as originally developed. Another change is, that the line relay is a.s.sociated with the tip side of the line, rather than with the sleeve side, as was formerly done. The cord circuit shown in Fig. 352 shows exactly the same arrangement of supervisory relays and exactly the same method of battery feed as in the simplified cord circuit of Fig. 351, but in addition to this the detailed connections of the operator"s talking set and of her order-wire keys are indicated, and also the ringing equipment is indicated as being adapted for four-party harmonic work.
[Ill.u.s.tration: Fig. 352. Kellogg Two-Wire Board]
In connection with this ringing key it may be stated that the springs _7_, _8_, _9_, and _10_ are individually operated by the pressure of one of the ringing key b.u.t.tons, while the spring _17_, connected with the sleeve side of the calling plug, is always operated simultaneously with the operation of any one of the other springs. As a result the proper ringing circuit is established, it being understood that the upper contacts of the springs _7_, _8_, _9_, and _10_ lead to the terminals of their respective ringing generators, the other terminals of which are grounded. The circuit is, therefore, from the generator, through the ringing key, out through the tip side of the line, back over the sleeve side of the line, and to ground through the spring _17_, resistance _11_, and the battery, which is one of the cord-circuit batteries. The object of this coil _11_ and the battery connection through it to the ringing-key spring is to prevent the falling back of the cut-off relay when the ringing key is operated. This will be clear when it is remembered that the cut-off relay is energized by battery current fed over the sleeve strand of the cord, and obviously, since it is necessary when the ringing key is operated to cut off the supply wire back of the key, this would de-energize the cut-off relay when the ringing key was depressed, and the falling back of the cut-off relay contacts would make it impossible to ring because the sleeve side of the line would be cut off. The battery supply through the resistance _11_ is, therefore, subst.i.tuted on the sleeve strand of the cord for the battery supply through the normal connection.
_Busy Test._ The busy test depends on all of the test rings being at zero potential on an idle line and at a higher potential on a busy line.
Obviously, when the line is not switched, the test rings are at zero potential on account of a ground through the cut-off relay. When, however, a plug is inserted in either the answering or multiple jacks, the test rings will all be raised in potential due to being connected with the live side of the battery through the sleeve strand of the cord.
Conditions on the line external to the central office cannot make an idle line test busy because, owing to the presence of the cut-off relay, the sleeve contacts of all the jacks are disconnected from the line when it is idle. The test circuit from the tip of the calling plug to ground at the operator"s set pa.s.ses through the tip strand of the cord, thence through a pair of normally closed extra contacts on the supervisory relay _4_, thence in series through all the ringing key springs _10_, _9_, _8_, and _7_, thence through an extra pair of springs _12_ and _13_ on the listening key--closed only when the listening key is operated--and thence to ground through a r.e.t.a.r.dation coil _14_. No battery or other source of potential exists in this circuit between ground and the tip of the calling plug and, therefore, the tip is normally at ground potential. The sleeve ring of the jack being at ground potential if the line is idle, no current will flow and no click will be produced in testing such a line. If, however, the line is busy, the test ring will be at a higher potential and, therefore, current will flow from the tip of the calling plug to ground over the path just traced, and this will cause a rise in potential at the terminal of the condenser _15_ and a momentary flow of current through the tertiary winding _16_ of the operator"s induction coil; hence the click.
[Ill.u.s.tration: SWITCH ROOM OF CITIZENS" TELEPHONE COMPANY, GRAND RAPIDS, MICH. One of the Earliest Large Automatic Offices.]
Obviously the testing circuit from the tip of the calling plug to ground at the operator"s set is only useful during the time when the calling plug is not in a jack, and as the tip strand of the calling plug has to do double duty in testing and in serving as a part of the talking circuit, the arrangement is made that the testing circuit will be automatically broken and the talking circuit through the tip strand automatically completed when the plug is inserted into a jack in establishing a connection. This is accomplished by means of the extra contact on the relay _4_, which relay, it will be remembered, is held energized when its corresponding plug is inserted in a jack. During the time when the plug is not inserted, this relay is not energized and the test circuit is completed through the back contact of its right-hand armature. When connection is made at the jack, this relay becomes energized and the tip strand of the cord circuit is made complete by the right-hand lever being pulled against the front contact of this relay.
The keys shown to the right of the operator"s set are order-wire keys.
_Summary of Operation._ We may give a brief summary of the operation of this system as shown in Fig. 352. The left-hand station calls and the line relay pulls up, lighting the lamp. The operator inserts an answering plug in the answering jack, thus energizing the cut-off relay which operates to cut off the line relay and to complete the connection between the jacks and the external line. The act of plugging in by the operator also raises the potential of all the test rings so as to guard the line against intrusion by other callers. The supervisory lamp _5_ remains unlighted because, although the relay _2_ is operated, the relay _1_ is also operated, due to the calling subscriber"s receiver being off its hook. The operator throws her listening key, communicates with the subscriber, and, learning that the right-hand station is wanted, proceeds to test that line. If the line is idle, she will get no click, because the tip of her calling plug and the tested ring will be at the same ground potential. She then plugs in and presses the proper ringing-key b.u.t.ton to send out the proper frequency to ring the particular subscriber on the line--if there be more than one--the current from the battery through the coil _11_ and spring _17_ serving during this operation to hold up the cut-off relay.
As soon as the operator plugs in with the calling plug, the supervisory lamp _6_ lights, a.s.suming that the called subscriber had not already removed his receiver from its hook, due to the fact that the relay _4_ is energized and the relay _3_ is not. As soon as the called subscriber responds, the relay _3_ becomes energized and the supervisory lamp goes out. If the line called for had been busy by virtue of being plugged at another section, the tip of the operator"s plug in testing would have found the test ring raised to a potential above the ground, and, as a consequence, current would have flowed from the tip of this plug through the back contact of the right-hand lever of relay _4_, thence through the ringing key springs and the auxiliary listening-key springs to ground through the r.e.t.a.r.dation coil _14_. This would have produced a click by causing a momentary flow of current through the tertiary winding _16_ of the operator"s set.
_Wiring of Line Circuit._ The more complete wiring diagram of a single subscriber"s line, Fig. 353, shows the placing in the circuits of the terminals and jumper wires of the main distributing frame and of the intermediate distributing frame, and also shows how the pilot lamps and night-alarm circuits are a.s.sociated with a group of lines. The main distributing frame occupies the same relative position in this line circuit as in the Western Electric, being located in the main line circuit outside of all the switchboard apparatus. The intermediate distributing frame occupies a different relative position from that in the Western Electric line. It will be recalled by reference to Fig. 348 that the line lamp and the answering jack were permanently a.s.sociated with the line and cut-off relays, such mutations of arrangement as were possible at the intermediate distributing frame serving only to vary the connection between the multiple of a line and one of the various groups of apparatus consisting of an answering jack and line lamp and a.s.sociated relays. In the Kellogg arrangement, Fig. 353, the line and cut-off relays, instead of being permanently a.s.sociated with the answering jack and line lamp, are permanently a.s.sociated with the multiple jacks, no changes, of which the intermediate or main frames are capable, being able to alter the relation between a group of multiple jacks and its a.s.sociated line and cut-off relays. In this Kellogg arrangement the intermediate distributing frame may only alter the connection of an answering jack and line lamp with the multiple and its permanently a.s.sociated relays. The pilot and night alarm arrangements of Fig. 353 should be obvious from the description already given of other similar systems.
[Ill.u.s.tration: Fig. 353. Kellogg Two-Wire Line Circuit]
=Dean Multiple Board.= In Fig. 354 are shown the circuits of the multiple switchboard of the Dean Electric Company. The subscriber"s station equipment shown at Station _A_ and Station _B_ will be recognized as the Wheatstone-bridge circuit of the Dean Company.
_Line Circuit._ The line circuit is easily understood in view of what has been said concerning the Western Electric line circuit, the line relay _1_ being single wound and between the live side of the battery and the ring side of the line. The cut-off relay _2_ is operated whenever a plug is inserted in a jack and serves to sever the connection of the line with the normal line signaling apparatus.
_Cord Circuit._ The cord circuit is of the four-relay type, but employs three conductors instead of two, as in the two-wire system. The relay _3_, being in series between the battery and the sleeve contact on the plug, is energized whenever a plug is inserted in the jack, its winding being placed in series with the cut-off relay of the line with which the plug is connected. This completes the circuit through the a.s.sociated supervisory lamp unless the relay _4_ is energized, the local lamp circuit being controlled by the back contact of relay _4_ and the front contact of relay _3_. It is through the two windings of the relay _4_ that current is fed to the subscriber"s station, and, therefore, the armature of this relay is responsive to the movements of the subscriber"s hook. As the relay _3_ holds the supervisory lamp circuit closed as long as a plug is inserted in a jack of the line, it follows that during a connection the relay _4_ will have entire control of the supervisory lamp.
_Listening Key._ The listening key, as usual, serves to connect the operator"s set across the talking strands of the cord circuit, and the action of this in connection with the operator"s set needs no further explanation.
_Ringing Keys._ The ringing-key arrangement ill.u.s.trated is adapted for use with harmonic ringing, the single springs _5_, _6_, _7_, and _8_ each being controlled by a separate b.u.t.ton and serving to select the particular frequency that is to be sent to line. The two springs _9_ and _10_ always act to open the cord circuit back of the ringing keys, whenever any one of the selective b.u.t.tons is depressed, in order to prevent interference by ringing current with the other operations of the circuit.
Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356 is an end view of the entire set. In Fig. 355 the listening key is shown at the extreme right and the four selective b.u.t.tons at the left. When a b.u.t.ton is released it rises far enough to cause the disengagement of the contacts, but remains partially depressed to serve as an indication that it was last used. The group of springs at the extreme left of Fig. 355 are the ones represented at _9_ and _10_ in Fig. 354 and by the anvils with which those springs co-operate.
[Ill.u.s.tration: Fig. 354. Dean Multiple Board Circuits]
_Test._ The test in this Dean system is simple, and, like the Western Electric and Kellogg systems, it depends on the raising of the potential of the test thimbles of all the line jacks of a line when a connection is made with that line by a plug at any position. When an operator makes a test by applying the tip of the calling plug to the test thimble of a busy line, current pa.s.ses from the test thimble through the tip strand of the cord to ground through the left-hand winding of the calling supervisory relay _4_. The drop of potential through this winding causes the tip strand of the cord to be raised to a higher potential than it was before, and as a result the upper plate of the condenser _11_ is thus altered in potential and this change in potential across the condenser results in a click in the operator"s ear.
[Ill.u.s.tration: Fig. 355. Dean Party Line Ringing Key]
[Ill.u.s.tration: Fig. 356. Dean Party Line Ringing Key]
=Stromberg-Carlson Multiple Board.= _Line Circuit._ In Fig. 357 is shown the multiple common-battery switchboard circuits employed by the Stromberg-Carlson Telephone Manufacturing Company. The subscriber"s line circuits shown in this drawing are of the three-wire type and, with the exception of the subscriber"s station, are the same as already described for the Western Electric Company"s system.
_Cord Circuit._ The cord circuit employed is of the two-conductor type, the plugs being so constructed as to connect the ring and thimble contacts of the jack when inserted. This cord circuit is somewhat similar to that employed by the Kellogg Switchboard and Supply Company, shown in Fig. 352, except that only one battery is employed, and that certain functions of this circuit are performed mechanically by the inter-action of the armatures of the relays.
_Supervisory Signals._ When the answering plug is inserted in a jack, in response to a call, the current pa.s.sing to the subscriber"s station and also through the cut-off relay must flow through the relay _1_, thus energizing it. As the calling subscriber"s receiver is at this time removed from the hook switch, the path for current will be completed through the tip of the jack, thence through the tip of the plug, through relay _2_ to ground, causing relay _2_ to be operated and to break the circuit of the answering supervisory lamp. The two relays _1_ and _2_ are so a.s.sociated mechanically that the armature of _1_ controls the armature of _2_ in such a manner as to normally hold the circuit of the answering supervisory lamp open. But, however, when the plug is inserted in a jack, relay _1_ is operated and allows the operation of relay _2_ to be controlled by the hook switch at the subscriber"s station. The supervisory relay _3_ a.s.sociated with the calling cord is operated when the calling plug is placed in a jack, and this relay normally holds the armature of relay _4_ in an operated position in a similar manner as the armature of relay _1_ controlled that of relay _2_. Supervisory relay _4_ is under the control of the hook switch at the called subscriber"s station.
_Test._ In this circuit, as in several previously described, when a plug is inserted in a jack of a line, the thimble contacts of the jacks a.s.sociated with that line are raised to a higher potential than that which they normally have. The operator in testing a busy line, of course having previously moved the listening key to the listening position, closes a path from the test thimble of the jack, through the tip of the calling plug, through the contacts of the relay _4_, the inside springs of the listening key, thence through a winding of the induction coil a.s.sociated with her set to ground. The circuit thus established allows current to flow from the test thimble of the jack through the winding of her induction coil to ground, causing a click in her telephone receiver.
The arrangement of the ringing circuit does not differ materially from that already described for other systems and, therefore, needs no further explanation.
[Ill.u.s.tration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]
=Multiple Switchboard Apparatus.= Coming now to a discussion of the details of apparatus employed in multiple switchboards, it may be stated that much of the apparatus used in the simpler types is capable of doing duty in multiple switchboards, although, of course, modification in detail is often necessary to make the apparatus fit the particular demands of the system in which it is to be used.
_Jacks._ Probably the most important piece of apparatus in the multiple switchboard is the jack, its importance being increased by the fact that such very large numbers of them are sometimes necessary. Switchboards having hundreds of thousands of jacks are not uncommon. The multiple jacks are nearly always mounted in strips of twenty and the answering jacks usually in strips of ten, the length of the jack strip being the same in each case in the same board and, therefore, giving twice as wide a s.p.a.cing in the answering as in the multiple jacks. The distance between centers in the multiple jacks varies from a quarter of an inch--which is perhaps the extreme minimum--to half an inch, beyond which larger limit there seems to be no need of going in any case. It is customary that the jack strip shall be made of the same total thickness as the distance between the centers of two of its jacks, and from this it follows that the strips when piled one upon the other give the same vertical distance between jack centers as the horizontal distance.
In Fig. 358 is shown a strip of multiple and a strip of answering jacks of Western Electric make, this being the type employed in the No. 1 standard switchboards for large exchanges. In Fig. 359 are shown the multiple and answering jacks employed in the No. 10 Western Electric switchboard. The multiple jacks in the No. 1 switchboard are mounted on 3/8-inch centers, the jacks having three branch terminal contacts. The multiple jacks of the No. 10 switchboard indicated in Fig. 359 are mounted on 1/2-inch centers, each jack having five contacts as indicated by the requirement of the circuits in Fig. 349.
In Fig. 360 are shown the answering and multiple jacks of the Kellogg Switchboard and Supply Company"s two-wire system. The extreme simplicity of these is particularly well shown in the cut of the answering jack, and these figures also show clearly the customary method of numbering jacks. In very large multiple boards it has been the practice of the Kellogg Company to s.p.a.ce the multiple jacks on 3/10-inch centers, and in their smaller multiple work, they employ the 1/2-inch s.p.a.cing. With the 3/10-inch s.p.a.cing that company has been able to build boards having a capacity of 18,000 lines, that many jacks being placed within the reach of each operator.
In all modern multiple switchboards the test thimble or sleeve contacts are drawn up from sheet bra.s.s or German silver into tubular form and inserted in properly s.p.a.ced borings in strips of hard rubber forming the faces of the jacks. These strips sometimes are reinforced by bra.s.s strips on their under sides. The springs forming the other terminals of the jack are mounted in milled slots in another strip of hard rubber mounted in the rear of and parallel to the front strip and rigidly attached thereto by a suitable metal framework. In this way desired rigidity and high insulation between the various parts is secured.
[Ill.u.s.tration: Fig. 358. Answering and Multiple Jacks for No. 1 Board]
_Lamp Jacks._ The lamp jacks employed in multiple work need no further description in view of what has been said in connection with lamp jacks for simple common-battery boards. The lamp jack s.p.a.cing is always the same as the answering jack s.p.a.cing, so that the lamps will come in the same vertical alignment as their corresponding answering jacks when the lamp strips and answering jack strips are mounted in alternate layers.
[Ill.u.s.tration: Fig. 359. Answering and Multiple Jacks for No. 10 Board]
[Ill.u.s.tration: Fig. 360. Answering and Multiple Jacks for Kellogg Two-Wire Board]
_Relays._ Next in order of importance in the matter of individual parts for multiple switchboards is the relay. The necessity for reliability of action in these is apparent, and this means that they must not only be well constructed, but that they must be protected from dust and moisture and must have contact points of such a nature as not to corrode even in the presence of considerable sparking and of the most adverse atmospheric conditions. Economy of s.p.a.ce is also a factor and has led to the almost universal adoption of the single-magnet type of relay for line and cut-off as well as supervisory purposes.
[Ill.u.s.tration: Fig. 361. Type of Line Relay]
[Ill.u.s.tration: Fig. 362. Type of Cut-Off Relay]
The Western Electric Company employs different types of relays for line, cut-off, and supervisory purposes. This is contrary to the practice of most of the other companies who make the same general type of relay serve for all of these purposes. A good idea of the type of Western Electric line relay, as employed in its No. 1 board, may be had from Fig. 361. As is seen this is of the tilting armature type, the armature rocking back and forth on a knife-edge contact at its base, the part on which it rests being of iron and of such form as to practically complete, with the armature and core, the magnetic circuit. The cut-off relay, Fig. 362, is of an entirely different type. The armature in this is loosely suspended by means of a flexible spring underneath two L-shaped polar extensions, one extending up from the rear end of the core and the other from the front end. When energized this armature is pulled away from the core by these L-shaped pieces and imparts its motion through a hard-rubber pin to the upper pair of springs so as to effect the necessary changes in the circuit.
[Ill.u.s.tration: Fig. 363. Western Electric Combined Line and Cut-off Relay]
[Ill.u.s.tration: Fig. 364. Western Electric Supervisory Relay]
[Ill.u.s.tration: Fig. 365. Line Relay No. 10 Board]
Much economy in s.p.a.ce and in wiring is secured in the type of switchboards employing cut-off as well as line relays by mounting the two relays together and in making of them, in fact, a unitary piece of apparatus. Since the line relay is always a.s.sociated with the cut-off relay of the same line and with no other, it is obvious that this unitary arrangement effects a great saving in wiring and also secures a great advantage in the matter of convenience of inspection. Such a combined cut-off and line relay, employed in the Western Electric No. 1 relay board, is shown in Fig. 363. These are mounted in banks of ten pairs, a common dust cap of sheet iron covering the entire group.
The Western Electric supervisory relay, Fig. 364, is of the tilting armature type and is copper clad. The dust cap in this case fits on with a bayonet joint as clearly indicated. In Fig. 365 is shown the line relay employed in the Western Electric No. 10 board.
[Ill.u.s.tration: Fig. 366. Kellogg Line and Cut-off Relays]
[Ill.u.s.tration: Fig. 367. Strip of Kellogg Line and Cut-Off Relays]