Volume Ii Part 5 (1/2)
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 establis.h.i.+ng 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.