Volume I Part 29 (1/2)
The series of figures showing the cycle of operations of the magneto switchboard about to be discussed are typical of this type of switchboard almost regardless of make. The apparatus is in each case represented symbolically, the representations indicating type rather than any particular kind of apparatus within the general cla.s.s to which it belongs.
_Normal Condition of Line._ In Fig. 240 is shown the circuit of an ordinary magneto line. The subscriber's sub-station apparatus, shown at the left, consists of the ordinary bridging telephone but might with equal propriety be indicated as a series telephone. The subscriber's station is shown connected with the central office by the two limbs of a metallic-circuit line. One limb of the line terminates in the spring _1_ of the jack, and the other limb in the sleeve or thimble _2_ of the jack. The spring _1_ normally rests on the third contact or anvil _3_ in the jack, its construction being such that when a plug is inserted this spring will be raised by the plug so as to break contact with the anvil _3_. It is understood, of course, that the plug a.s.sociated with this jack has two contacts, referred to respectively as the tip and the sleeve; the tip makes contact with the tip spring _1_ and the sleeve with the sleeve or thimble _2_.
[Ill.u.s.tration: Fig. 240. Normal Condition of Line]
The drop or line signal is permanently connected between the jack sleeve and the anvil _3_. As a result, the drop is normally bridged across the circuit of the line so as to be in a receptive condition to signaling current sent out by the subscriber. It is evident, however, that when the plug is inserted into the jack this connection between the line and the drop will be broken.
In this normal condition of the line, therefore, the drop stands ready at the central office to receive the signal from the subscriber and the generator at the sub-station stands ready to be bridged across the circuit of the line as soon as the subscriber turns its handle.
Similarly the ringer--the call-receiving device at the sub-station--is permanently bridged across the line so as to be responsive to any signal that may be sent out from the central office in order to call the subscriber. The subscriber's talking apparatus is, in this normal condition of the line, cut out of the circuit by the switch hook.
_Subscriber Calling._ Fig. 241 shows the condition of the line when the subscriber at the sub-station is making a call. In turning his generator the two springs which control the connection of the generator with the line are brought into engagement with each other so that the generator currents may pa.s.s out over the line. The condition at the central office is the same as that of Fig. 240 except that the drop is shown with its shutter fallen so as to indicate a call.
[Ill.u.s.tration: Fig. 241. Subscriber Calling]
[Ill.u.s.tration: A SPECIALLY FORMED CABLE FOR KEY SHELF OF MONARCH SWITCHBOARD]
_Operator Answering._ The next step is for the operator to answer the call and this is shown in Fig. 242. The subscriber has released the handle of his generator and the generator has, therefore, been automatically cut out of the circuit. He also has removed his receiver from its hook, thus bringing his talking apparatus into the line circuit. The operator on the other hand has inserted one of the plugs _P__{a} into the jack. This action has resulted in the breaking of the circuit through the drop by the raising of the spring _1_ from the anvil _3_, and also in the continuance of the line circuit through the conductors of the cord circuits. Thus, the upper limb of the line is continued by means of the engagement of the tip spring _1_ with the tip _4_ of the plug to the conducting strand _6_ of the cord circuit; likewise the lower limb of the line is continued by the engagement of the thimble _2_ of the jack with the sleeve contact _5_ of the plug _P__{a} to the strand _7_ of the cord circuit. The operator has also closed her listening key _L.K._ In doing so she has brought the springs _8_ and _9_ into engagement with the anvils _10_ and _11_ and has thus bridged her head telephone receiver with the secondary of her induction coil across the two strands _6_ and _7_ of the cord.
a.s.sociated with the secondary winding of her receiver is a primary circuit containing a transmitter, battery, and the primary of the induction coil. It will be seen that the conditions are now such as to permit the subscriber at the calling station to converse with the operator and this conversation consists in the familiar ”Number Please” on the part of the operator and the response of the subscriber giving the number of the line that is desired. Neither the plug _P__{c}, nor the ringing key _R.K._, shown in Fig. 242, is used in this operation. The clearing-out drop _C.O._ is bridged permanently across the strands _6-7_ of the cord, but is without function at this time; the fact that it is wound to a high resistance and impedance prevents its having a harmful effect on the transmission.
[Ill.u.s.tration: Fig. 242. Operator Answering]
It may be stated at this point that the two plugs of an a.s.sociated pair are commonly referred to as the answering and calling plugs. The answering plug is the one which the operator always uses in answering a call as just described in connection with Fig. 242. The calling plug is the one which she next uses in connecting with the line of the called subscriber. It lies idle during the answering of a call and is only brought into play after the order of the calling subscriber has been given, in which case it is used in establis.h.i.+ng connection with the called subscriber.
[Ill.u.s.tration: Fig. 243. Operator Calling]
_Operator Calling._ We may now consider how the operator calls the called subscriber. The condition existing for this operation is shown in Fig. 243. The operator after receiving the order from the calling subscriber inserts the calling plug _P__{c} into the jack of the line of the called station. This act at once connects the limbs of the line with the strands _6_ and _7_ of the cord circuit, and also cuts out the line drop of the called station, as already explained. The operator is shown in this figure as having opened her listening key _L.K._ and closed her ringing key _R.K._ As a result, ringing current from the central-office generator will flow out over the two ringing key springs _12_ and _13_ to the tip and sleeve contacts of the calling plug _P__{c}, then to the tip spring _1_ and the sleeve or thimble _2_ of the jack, and then to the two sides of the metallic-circuit line to the sub-station and through the bell there. This causes the ringing of the called subscriber's bell, after which the operator releases the ringing key and thereby allows the two springs _12_ and _13_ of that key to again engage their normal contacts _14_ and _15_, thus making the two strands _6_ and _7_ of the cord circuit continuous from the contacts of the answering plug _P__{a} to the contacts of the calling plug _P__{c}. This establishes the condition at the central office for conversation between the two subscribers.
[Ill.u.s.tration: Fig. 244. Subscribers Connected for Conversation.]
_Subscribers Conversing._ The only other thing necessary to establish a complete set of talking conditions between the two subscribers is for the called subscriber to remove his receiver from its hook, which he does as soon as he responds to the call. The conditions for conversation between the two subscribers are shown in Fig. 244. It is seen that the two limbs of the calling line are connected respectively to the two limbs of the called line by the two strands of the cord circuit, both the operator's receiver and the central-office generator being cut out by the listening and ringing keys, respectively. Likewise the two line drops are cut out of circuit and the only thing left a.s.sociated with the circuit at the central office is the clearing-out drop _C. O._, which remains bridged across the cord circuit. This, like the two ringers at the respective connected stations, which also remain bridged across the circuit when bridging instruments are used, is of such high resistance and impedance that it offers practically no path to the rapidly fluctuating voice currents to leak from one side of the line circuit to the other. Fluctuating currents generated by the transmitter at the calling station, for instance, are converted by means of the induction coil into alternating currents flowing in the secondary of the induction coil at that station. Considering a momentary current as pa.s.sing up through the secondary winding of the induction coil at the calling station, it pa.s.ses through the receiver of that station through the upper limb of the line to the spring _1_ of the line jack belonging to that line at the central office; thence through the tip _4_ of the answering plug to the conductor _6_ of the cord; thence through the pair of contacts _14_ and _12_ forming one side of the ringing key to the tip _4_ of the calling plug; thence to the tip spring _1_ of the jack of the called subscriber's line; thence over the upper limb of his line through his receiver and through the secondary of the induction to one of the upper switch-hook contacts; thence through the hook lever to the lower side of the line, back to the central office and through the sleeve contact _2_ of the jack and the sleeve contact _5_ of the plug; thence through the other ringing key contacts _13_ and _15_; thence through the strand _7_ of the cord to the sleeve contact _5_ and the sleeve contact _2_ of the answering plug and jack, respectively; thence through the lower limb of the calling subscriber's line to the hook lever at his station; thence through one of the upper contacts of this hook to the secondary of the induction coil, from which point the current started.
[Ill.u.s.tration: Fig. 245. Clearing-Out Signal]
Obviously, when the called subscriber is talking to the calling subscriber the same path is followed. It will be seen that at any time the operator may press her listening key _L.K._, bridge her telephone set across the circuit of the two connected lines, and listen to the conversation or converse with either of the subscribers in case of necessity.
_Clearing Out_. At the close of the conversation, either one or both of the subscribers may send a clearing-out signal by turning their generators after hanging up their receivers. This condition is shown in Fig. 245. The apparatus at the central office remains in exactly the same position during conversation as that of Fig. 244, except that the clearing-out drop shutter is shown as having fallen. The two subscribers are shown as having hung up their receivers, thus cutting out their talking apparatus, and as operating their generators for the purpose of sending the clearing-out signals. In response to this act the operator pulls down both the calling and the answering plug, thus restoring them to their normal seats, and bringing both lines to the normal condition as shown in Fig. 240. The line drops are again brought into operative relation with their respective lines so as to be receptive to subsequent calls and the calling generators at the sub-stations are removed from the bridge circuits across the line by the opening of the automatic switch contacts a.s.sociated with those generators.
_Essentials of Operation_. The foregoing sequence of operations while described particularly with respect to magneto switchboards is, with certain modifications, typical of the operation of nearly all manual switchboards. In the more advanced types of manual switchboards, certain of the functions described are sometimes done automatically, and certain other functions, not necessary in connection with the simple switchboard, are added. The essential mode of operation, however, remains the same in practically all manual switchboards, and for this reason the student should thoroughly familiarize himself with the operation and circuits of the simple switchboard as a foundation for the more complex and consequently more-difficult-to-understand switchboards that will be described later on.
Commercial Types of Drops and Jacks. _Early Drops_. Coming now to the commercial types of switchboard apparatus, the first subject that presents itself is that of magneto line signals or drops. The very early forms of switchboard drops had, in most cases, two-coil magnets, the cores of which were connected at their forward ends by an iron yoke and the armature of which was pivoted opposite the rear end of the two cores. To the armature was attached a latch rod which projected forwardly to the front of the device and was there adapted to engage the upper edge of the hinged shutter, so as to hold it in its raised or undisplayed position when the armature was unattracted.
Such a drop, of Western Electric manufacture, is shown in Fig. 246.
[Ill.u.s.tration: Fig. 246 Old-Style Drop]
Liability to Cross-Talk:--This type of drop is suitable for use only on small switchboards where s.p.a.ce is not an important consideration, and even then only when the drop is entirely cut out of the circuit during conversation. The reason for this latter requirement will be obvious when it is considered that there is no magnetic s.h.i.+eld around the winding of the magnet and no means for preventing the stray field set up by the talking currents in one of the magnets from affecting by induction the windings of adjacent magnets contained in other talking circuits. Unless the drops are entirely cut out of the talking circuit, therefore, they are very likely to produce cross-talk between adjacent circuits. Furthermore, such form of drop is obviously not economical of s.p.a.ce, two coils placed side by side consuming practically twice as much room as in the case of later drops wherein single magnet coils have been made to answer the purpose.
_Tubular Drops._ In the case of line drops, which usually can readily be cut out of the circuit during conversation, this cross-talk feature is not serious, but sometimes the line drops, and always the clearing-out drops must be left in connection with the talking circuit.
On account of economy in s.p.a.ce and also on account of this cross-talk feature, there has come into existence the so-called tubular or iron-clad drop, one of which is shown in section in Fig. 247. This was developed a good many years ago by Mr. E.P. Warner of the Western Electric Company, and has since, with modifications, become standard with practically all the manufacturing companies. In this there is but a single bobbin, and this is enclosed in a sh.e.l.l of soft Norway iron, which is closed at its front end and joined to the end of the core as indicated, so as to form a complete return magnetic path for the lines of force generated in the coil. The rear end of the sh.e.l.l and core are both cut off in the same plane and the armature is made in such form as to practically close this end of the sh.e.l.l. The armature carries a latch rod extending the entire length of the sh.e.l.l to the front portion of the structure, where it engages the upper edge of the pivoted shutter; this, when released by the latch upon the attraction of the armature, falls so as to display a target behind it.
[Ill.u.s.tration: Fig. 247. Tubular Drop]
[Ill.u.s.tration: Fig. 248. Strip of Tubular Drops]