Volume I Part 24 (1/2)
[Ill.u.s.tration: Fig. 190. K.B. Lock-Out Station]
A complete K.B. lock-out telephone is shown in Fig. 190. This is the type of instrument that is usually furnished when new equipment is ordered. If, however, it is desired to use the K.B. system in connection with telephones of the ordinary bridging type that are already in service, the lock-out and selective mechanism, which is shown on the upper inner face of the door in Fig. 190, is furnished separately in a box that may be mounted close to the regular telephone and connected thereto by suitable wires, as shown in Fig. 191. It is seen that this instrument employs a local battery for talking and also a magneto generator for calling the central office.
The central-office equipment consists of a dial connected with an impulse wheel, together with suitable keys by which the various circuits may be manipulated. This dial and its a.s.sociated mechanism may be mounted in the regular switchboard cabinet, or it may be furnished in a separate box and mounted alongside of the cabinet in either of the positions shown at _1_ or _2_ of Fig. 192.
In order to send the proper number of impulses to the line to call a given party, the operator places her finger in the hole in the dial that bears the number corresponding to the station wanted and rotates the dial until the finger is brought into engagement with the fixed stop shown at the bottom of the dial in Fig. 192. The dial is then allowed to return by the action of a spring to its normal position, and in doing so it operates a switch within the box to make and break the battery circuit the proper number of times.
_Operation._ A complete description of the operation may now be had in connection with Fig. 193, which is similar to Fig. 189, but contains the details of the calling arrangement at the central office and also of the talking circuits at the various subscribers' stations.
[Ill.u.s.tration: Fig. 191. K.B. Lock-Out Station]
Referring to the central-office apparatus the usual ringing key is shown, the inside contacts of which lead to the listening key and to the operator's telephone set as in ordinary switchboard practice.
Between the outside contact of this ringing key and the ringing generator there is interposed a pair of contact springs _8-8_ and another pair _9-9_. The contact springs _8_ are adapted to be moved backward and forward by the impulse wheel which is directly controlled by the dial under the manipulation of the operator. When these springs _8_ are in their normal position, the ringing circuit is continued through the release-key springs _9_ to the ringing generator. These springs _8_ occupy their normal position only when the dial is in its normal position, this being due to the notch _10_ in the contact wheel.
At all other times, _i.e._, while the impulse wheel is out of its normal position, the springs _8-8_ are either depressed so as to engage the lower battery contacts, or else held in an intermediate position so as to engage neither the battery contacts nor the generator contacts.
[Ill.u.s.tration: Fig. 192. Calling Apparatus K.B. System]
When it is desired to call a given station, the operator pulls the subscriber's number on the dial and holds the ringing key closed, allowing the dial to return to normal. This connects the impulse battery to the subscriber's line as many times as is required to move the subscriber's sectors to the proper position, and in such direction as to cause the stepping movement of the various relays. As the impulse wheel comes to its normal position, the springs _8_, a.s.sociated with it, again engage their upper contacts, by virtue of the notch _10_ in the impulse wheel, and this establishes the connection between the ringing generator and the subscriber's line, the ringing key being still held closed. The pulling of the transmitter dial and holding the ringing key closed, therefore, not only sends the stepping impulses to line, but also follows it by the ringing current. The sending of five impulses to line moves all of the sectors to the sixth notch, and this corresponds to the position necessary to make the fourth station operative. Such a condition is shown in Fig. 193, it being a.s.sumed that the subscriber at Station C originated the call and pressed his own b.u.t.ton so as to prevent his sector from being moved out of its normal position. As a result of this, the talking circuit at Station C is left closed, and the talking and the ringing circuit of Station D, the called station, are closed, while both the talking and the ringing circuits of all the other stations are left open. Station D may, therefore, be rung and may communicate with Station C, while all of the other stations on the line are locked out, because of the fact that both their talking and ringing circuits are left open.
[Ill.u.s.tration: Fig. 193. Circuit K.B. System]
When conversation is ended, the operator is notified by the usual clearing-out signal, and she then depresses the release b.u.t.ton, which brings the springs _9_ out of engagement with the generator contact but into engagement with the battery contact in such relation as to send a battery current on the line in the reverse direction from that sent out by the impulse wheel. This sends current through all of the relays in such direction as to withdraw both the moving and the holding pawls from the segments and thus allow all of the segments to return to their normal positions. Of course, in thus establis.h.i.+ng the release current, it is necessary for the operator to depress the ringing key as well as the release key.
A one-half microfarad condenser is placed in the receiver circuit at each station so that the line will not be tied up should some subscriber inadvertently leave his receiver off its hook. This permits the pa.s.sage of voice currents, but not of the direct currents used in stepping the relays or in releasing them.
The circuit of Fig. 193 is somewhat simplified from that in actual practice, and it should be remembered that the hook switch, which is not shown in this figure, controls in the usual way the continuity of the receiver and the transmitter circuits as well as of the generator circuits, the generator being attached to the line as in an ordinary telephone.
Broken-Line System. The broken-line method of accomplis.h.i.+ng selective signaling and locking-out on telephone party lines is due to Homer Roberts and his a.s.sociates.
[Ill.u.s.tration: Fig. 194. Roberts Latching Relay]
To understand just how the principles ill.u.s.trated in Figs. 186 and 187 are put into effect, it will be necessary to understand the latching relay shown diagrammatically in its two possible positions in Fig. 194, and in perspective in Fig. 195. Referring to Fig. 194, the left-hand cut of which shows the line relay in its normal position, it is seen that the framework of the device resembles that of an ordinary polarized ringer. Under the influence of current in one direction flowing through the left-hand coil, the armature of this device depresses the hard rubber stud _4_, and the springs _1_, _2_, and _3_ are forced downwardly until the spring _2_ has pa.s.sed under the latch carried on the spring _5_. When the operating current through the coil _6_ ceases, the pressure of the armature on the spring _1_ is relieved, allowing this spring to resume its normal position and spring _3_ to engage with spring _2_. The spring _2_ cannot rise, since it is held by the latch _5_, and the condition shown in the right-hand cut of Fig.
194 exists. It will be seen that the spring _2_ has in this operation carried out just the same function as the switch lever performed as described in connection with Figs. 186 and 187. An a.n.a.lysis of this action will show that the normal contact between the springs _1_ and _2_, which contact controls the circuit through the relay coil and the bell, is not broken until the coil _6_ is de-energized, which means that the magnet is effective until it has accomplished its work. It is impossible, therefore, for this relay to cut itself out of circuit before it has caused the spring _2_ to engage under the latch _5_. If current of the proper direction were sent through the coil _7_ of the relay, the opposite end of the armature would be pulled down and the hard rubber stud at the left-hand end of the armature would bear against the bent portion of the spring _5_ in such manner as to cause the latch of this spring to release the spring _2_ and thus allow the relay to a.s.sume its normal, or unlatched, position.
A good idea of the mechanical construction of this relay may be obtained from Fig. 195. The entire selecting function of the Roberts system is performed by this simple piece of apparatus at each station.
[Ill.u.s.tration: Fig. 195. Roberts Latching Relay]
The diagram of Fig. 196 shows, in simplified form, a four-station line, the circuits being given more in detail than in the diagrams of Chapter XVI.
It will be noticed that the ringer and the relay coil _6_ at the first station are bridged across the sides of the line leading to the central office. In like manner the bell and the relay magnets are bridged across the two limbs of the line leading into each succeeding station, but this bridge at each of the stations beyond Station A is ineffective because the line extension _R__{x} is open at the next station nearest the central office.
[Ill.u.s.tration: Fig. 196. Simplified Circuits of Roberts System]
In order to ring Station A it is only necessary to send out ringing current from the central office. This current is in such direction as not to cause the operation of the relay, although it pa.s.ses through the coil _6_. If, on the other hand, it is desired to ring Station B, a preliminary impulse would be sent over the metallic circuit from the central office, which impulse would be of such direction as to operate the relay at Station A, but not to operate the bell at that station.
The operation of the relay at Station A causes the spring _2_ of this relay to engage the spring _3_, thus extending the line on to the second station. After the spring _2_ at Station A has been forced into contact with the spring _3_, it is caught by the latch of the spring _5_ and held mechanically. When the impulse from the central office ceases, the spring _1_ resumes its normal position, thus breaking the bridge circuit through the bell at that station. It is apparent now that the action of coil _6_ at Station A has made the relay powerless to perform any further action, and at the same time the line has been extended on to the second station. A second similar impulse from the central office will cause the relay at Station B to extend the line on to Station C, and at the same time break the circuit through the operating coil and the bell at Station B. In this way any station may be picked out by sending the proper number of impulses to operate the line relays of all the stations between the station desired and the central office, and having picked out a station it is only necessary to send out ringing current, which current is in such direction as to ring the bell but not to operate the relay magnet at that station.
In Fig. 197, a four-station line, such as is shown in Fig. 196, is ill.u.s.trated, but the condition shown in this is that existing when two preliminary impulses have been sent over the line, which caused the line relays at Station A and Station B to be operated. The bell at Station C is, therefore, the only one susceptible to ringing current from the central office.
[Ill.u.s.tration: Fig. 197. Simplified Circuits of Roberts System]