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 establishing 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 shield 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]
These drops may be mounted individually on the face of the switchboard, but it is more usual to mount them in strips of five or ten. A strip of five drops, as manufactured by the Kellogg Switchboard and Supply Company, is shown in Fig. 248. The front strip on which these drops are mounted is usually of bra.s.s or steel, copper plated, and is sufficiently heavy to provide a rigid support for the entire group of drops that are mounted on it. This construction greatly facilitates the a.s.sembling of the switchboard and also serves to economize s.p.a.ce--obviously, the thing to economize on the face of a switchboard is s.p.a.ce as defined by vertical and horizontal dimensions.
These tubular drops, having but one coil, are readily mounted on 1-inch centers, both vertically and horizontally. Sometimes even smaller dimensions than this are secured. The greatest advantage of this form of construction, however, is in the absolute freedom from cross-talk between two adjacent drops. So completely is the magnetic field of force kept within the material of the sh.e.l.l, that there is practically no stray field and two such drops may be included in two different talking circuits and the drops mounted immediately adjacent to each other without producing any cross-talk whatever.
_Night Alarm._ Switchboard drops in falling make but little noise, and during the day time, while the operator is supposed to be needed continually at the board, the visual signal which they display is sufficient to attract her attention. In small exchanges, however, it is frequently not practicable to keep an operator at the switchboard at night or during other comparatively idle periods, and yet calls that do arrive during such periods must be attended to. For this reason some other than a visual signal is necessary, and this need is met by the so-called night-alarm attachment. This is merely an arrangement by which the shutter in falling closes a pair of contacts and thus completes the circuit of an ordinary vibrating bell or buzzer which will sound until the shutter is restored to its normal position.
Such contacts are shown in Fig. 249 at _1_ and _2_. Night-alarm contacts have a.s.sumed a variety of forms, some of which will be referred to in the discussion of other types of drops and jacks.
[Ill.u.s.tration: Fig. 249. Drop with Night-Alarm Contacts]
_Jack Mounting._ Jacks, like drops, though frequently individually mounted are more often mounted in strips. An individually mounted jack is shown in Fig. 250, and a strip of ten jacks in Fig. 251. In such a strip of jacks, the strips supporting the metallic parts of the various jacks are usually of hard rubber reinforced by bra.s.s so as to give sufficient strength. Various forms of supports for these strips are used by different manufacturers, the means for fastening them in the switchboard frame usually consisting of bra.s.s lugs on the end of the jack strip adapted to be engaged by screws entering the stationary portion of the iron framework; or sometimes pins are fixed in the framework, and the jack is held in place by nuts engaging screw-threaded ends on such pins.
[Ill.u.s.tration: Fig. 250. Individual Jack]
[Ill.u.s.tration: Fig. 251. Strip of Jacks]
_Methods of a.s.sociating Jacks and Drops._ There are two general methods of arranging the drops and jacks in a switchboard. One of these is to place all of the jacks in a group together at the lower portion of the panel in front of the operator and all of the drops together in another group above the group of jacks. The other way is to locate each jack in immediate proximity to the drop belonging to the same line so that the operator"s attention will always be called immediately to the jack into which she must insert her plug in response to the display of a drop. This latter practice has several advantages over the former. Where the drops are all mounted in one group and the jacks in another, an operator seeing a drop fall must make mental note of it and pick out the corresponding jack in the group of jacks. On the other hand, where the jacks and drops are mounted immediately adjacent to each other, the falling of a drop attracts the attention of the operator to the corresponding jack without further mental effort on her part.
The immediate a.s.sociation of the drops and jacks has another advantage--it makes possible such a mechanical relation between the drop and its a.s.sociated jack that the act of inserting the plug into the jack in making the connection will automatically and mechanically restore the drop to its raised position. Such drops are termed _self-restoring drops_, and, since a drop and jack are often made structurally a unitary piece of apparatus, they are frequently called _combined_ drops and jacks.
_Manual vs. Automatic Restoration._. There has been much difference of opinion on the question of manual versus automatic restoration of drops. Some have contended that there is no advantage in having the drops restored automatically, claiming that the operator has plenty of time to restore the drops by hand while receiving the order from the calling subscriber or performing some of her other work. Those who think this way have claimed that the only place where an automatically restored drop is really desirable is where, on account of the lack of s.p.a.ce on the front of the switchboard, the drops are placed on such a portion of the board as to be not readily reached by the operator.
This resulted in the electrically restored drop, mention of which will be made later.
Others have contended that even though the drop is mounted within easy reach of the operator, it is advantageous that the operator should be relieved of the burden of restoring it, claiming that even though there are times in the regular performance of the operator"s duties when she may without interfering with other work restore the drops manually, such requirement results in a double use of her attention and in a useless strain on her which might better be devoted to the actual making of connections.
Until recently the various Bell operating companies have adhered, in their small exchange work, to the manual restoring method, while most of the so-called independent operating companies have adhered to the automatic self-restoring drops.
Methods of Automatic Restoration. Two general methods present themselves for bringing about the automatic restoration of the drop.
First, the mechanical method, which is accomplished by having some moving part of the jack or of the plug as it enters the jack force the drop mechanically into its restored position. This usually means the mounting of the drop and the corresponding jack in juxtaposition, and this, in turn, has usually resulted in the unitary structure containing both the drop and the jack. Second, the electrical method wherein the plug in entering the jack controls a restoring circuit, which includes a battery or other source of energy and a restoring coil on the drop, the result being that the insertion of the plug into the jack closes this auxiliary circuit and thus energizes the restoring magnet, the armature of which pulls the shutter back into its restored position. This practice has been followed by Bell operating companies whenever conditions require the drop to be mounted out of easy reach of the operator; not otherwise.
_Mechanical--Direct Contact with Plug._ One widely used method of mechanical restoration of drops, once employed by the Western Telephone Construction Company with considerable success, was to hang the shutter in such position that it would fall immediately in front of the jack so that the operator in order to reach the jack with the plug would have to push the plug directly against the shutter and thus restore it to its normal or raised position. In this construction the coil of the drop magnet was mounted directly behind the jack, the latch rod controlled by the armature reaching forward, parallel with the jack, to the shutter, which, as stated, was hung in front of the jack. This resulted in a most compact arrangement so far as the s.p.a.ce utilization on the front of the board was concerned and such combined drops and jacks were mounted on about 1-inch centers, so that a bank of one hundred combined drops and jacks occupied a s.p.a.ce only a little over 10 inches square.
A modification of this scheme, as used by the American Electric Telephone Company, was to mount the drop immediately over the jack so that its shutter, when down, occupied a position almost in front of, but above, the jack opening. The plug was provided with a collar, which, as it entered the jack, engaged a cam on the base of the shutter and forced the latter mechanically into its raised position.
Neither of these methods of restoring--_i.e._, by direct contact between the shutter or part of it and the plug or part of it--is now as widely used as formerly. It has been found that there is no real need in magneto switchboards for the very great compactness which the hanging of the shutter directly in front of the drop resulted in, and the tendency in later years has been to make the combined drops and jacks more substantial in construction at the expense of some s.p.a.ce on the face of the switchboard.
[Ill.u.s.tration: Fig. 252. Kellogg Drop and Jack]
Kellogg Type:--A very widely used scheme of mechanical restoration is that employed in the Miller drop and jack manufactured by the Kellogg Switchboard and Supply Company, the principles of which may be understood in connection with Fig. 252. In this figure views of one of these combined drops and jacks in three different positions are shown.
The jack is composed of the framework _B_ and the hollow screw _A_, the latter forming the sleeve or thimble of the jack and being externally screw-threaded so as to engage and bind in place the front end of the framework _B_. The jack is mounted on the lower part of the bra.s.s mounting strip _C_ but insulated therefrom. The tip spring of the jack is bent down as usual to engage the tip of the plug, as better shown in the lower cut of Fig. 252, and then continues in an extension _D_, which pa.s.ses through a hole in the mounting plate _C_.
This tip spring in its normal position rests against another spring as shown, which latter spring forms one terminal of the drop winding.
The drop or annunciator is of tubular form, and the shutter is so arranged on the front of the mounting strip _C_ as to fall directly above the extension _D_ of the tip spring. As a result, when the plug is inserted into the jack, the upward motion of the tip spring forces the drop into its restored position, as indicated in the lower cut of the figure. These drops and jacks are usually mounted in banks of five, as shown in Fig. 253.
[Ill.u.s.tration: Fig. 253. Strip of Kellogg Drops and Jacks]
Western Electric Type:--The combined drop and jack of the Western Electric Company recently put on the market to meet the demands of the independent trade, differs from others princ.i.p.ally in that it employs a spherical drop or target instead of the ordinary flat shutter. This piece of apparatus is shown in its three possible positions in Fig.
254. The shutter or target normally displays a black surface through a hole in the mounting plate. The sphere forming the target is out of balance, and when the latch is withdrawn from it by the action of the electromagnet it falls into the position shown in the middle cut of Fig. 254, thus displaying a red instead of a black surface to the view of the operator. When the operator plugs in, the plug engages the lower part of an =S=-shaped lever which acts on the pivoted sphere to restore it to its normal position. A perspective view of one of these combined line signals and jacks is shown in Fig. 255.