Operation in Detail

With these general statements in mind we may take up in some detail the various operations of a telephone system wherein the lines center in a magneto switchboard. This may best be done by considering the circuits involved, without special regard to the details of the apparatus. The series of figures showing the cycle of operations of the magneto switchboard about to be discu ed are typical of this type of switchboard almost regardle 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 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 ociated 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 . IMG:384091999325875926_fig240_t.gif.png:Illustration_ Fig. 240. Normal Condition of Line Fig. 240. Normal Condition of Line View full size illustration. The drop or line signal is permanently connected between the jack sleeve and the anvil 3 . As a result, the drop is normally bridged acro 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 acro 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 acro 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 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. IMG:384091999325875926_fig241_t.gif.png:Illustration_ Fig. 241. Subscriber Calling Fig. 241. Subscriber Calling View full size illustration. 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 acro the two strands 6 and 7 of the cord. A ociated 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 acro 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 transmi ion. IMG:384091999325875926_fig242_t.gif.png:Illustration_ Fig. 242. Operator Answering Fig. 242. Operator Answering View full size illustration. It may be stated at this point that the two plugs of an a ociated 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 establishing connection with the called subscriber. IMG:384091999325875926_fig243_t.gif.png:Illustration_ Fig. 243. Operator Calling Fig. 243. Operator Calling View full size illustration. 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. IMG:384091999325875926_fig244_t.gif.png:Illustration_ Fig. 244. Subscribers Connected for Conversation Fig. 244. Subscribers Connected for Conversation View full size illustration. Subscribers Conversing. The only other thing nece ary 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 ociated with the circuit at the central office is the clearing-out drop C. O. , which remains bridged acro the cord circuit. This, like the two ringers at the respective connected stations, which also remain bridged acro 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 ing up through the secondary winding of the induction coil at the calling station, it pa es 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. IMG:384091999325875926_fig245_t.gif.png:Illustration_ Fig. 245. Clearing-Out Signal Fig. 245. Clearing-Out Signal View full size illustration. 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 pre her listening key L.K. , bridge her telephone set acro the circuit of the two connected lines, and listen to the conversation or converse with either of the subscribers in case of nece ity. 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 acro the line by the opening of the automatic switch contacts a ociated with those generators. E entials 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 nece ary in connection with the simple switchboard, are added. The e ential 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.