Chapter XXI: The Simple Magneto Switchboard (1)

Definitions. As already stated those switchboards which are adapted to work in conjunction with magneto telephones are called magneto switchboards. The signals on such switchboards are electromagnetic devices capable of responding to the currents of the magneto generators at the subscribers' stations. Since, as a rule, magneto telephones are equipped with local batteries, it follows that the magneto switchboard does not need to be arranged for supplying the subscribers' stations with talking current. This fact is accountable for magneto switchboards often being referred to as local-battery switchboards, in contradistinction to common-battery switchboards which are equipped so as to supply the connected subscribers' stations with talking current.

The term _simple_ as applied in the headings of this and the next chapter, is employed to designate switchboards adapted for so small a number of lines that they may be served by a single or a very small group of operators; each line is provided with but a single connection terminal and all of them, without special provision, are placed directly within the reach of the operator, or operators if there are more than one. This distinction will be more apparent under the discussion of transfer and multiple switchboards.

Mode of Operation. The cycle of operation of any simple manual switchboard may be briefly outlined as follows: The subscriber desiring a connection transmits a signal to the central office, the operator seeing the signal makes connection with the calling line and places herself in telephonic communication with the calling subscriber to receive his orders; the operator then completes the connection with the line of the called subscriber and sends ringing current out on that line so as to ring the bell of that subscriber; the two subscribers then converse over the connected lines and when the conversation is finished either one or both of them may send a signal to the central office for disconnection, this signal being called a clearing-out signal; upon receipt of the clearing-out signal, the operator disconnects the two lines and restores all of the central-office apparatus involved in the connection to its normal position.

Component Parts. Before considering further the operation of manual switchboards it will be well to refer briefly to the component pieces of apparatus which go to make up a switchboard.

_Line Signal._ The line signal in magneto switchboards is practically always in the form of an electromagnetic annunciator or drop. It consists in an electromagnet adapted to be included in the line circuit, its armature controlling a latch, which serves to hold the drop or shutter or target in its raised position when the magnet is not energized, and to release the drop or shutter or target so as to permit the display of the signal when the magnet is energized. The symbolic representation of such an electromagnetic drop is shown in Fig. 233.

_Jacks and Plugs._ Each line is also provided with a connection terminal in the form of a switch socket. This assumes many forms, but always consists in a cylindrical opening behind which are arranged one or more spring contacts. The opening forms a receptacle for plugs which have one or more metallic terminals for the conductors in the flexible cord in which the plug terminates. The arrangement is such that when a plug is inserted into a jack the contacts on the plug will register with certain of the contacts in the jack and thus continue the line conductors, which terminate in the jack contacts, to the cord conductors, which terminate in the plug contacts. Usually also when a plug is inserted certain of the spring contacts in the jack are made to engage with or disengage other contacts in the jack so as to make or break auxiliary circuits.

A simple form of spring jack is shown in section in Fig. 234. In Fig. 235 is shown a sectional view of a plug adapted to co-operate with the jack of Fig. 234. In Fig. 236 the plug is shown inserted into the jack. The cylindrical portion of the jack is commonly called the _sleeve_ or _thimble_ and it usually forms one of the main terminals of the jack; the spring, forming the other principal terminal, is called the _tip spring_, since it engages the tip of the plug. The tip spring usually rests on another contact which may be termed the _anvil_. When the plug is inserted into the jack as shown in Fig. 236, the tip spring is raised from contact with this anvil and thus breaks the circuit leading through it. It will be understood that spring jacks are not limited to three contacts such as shown in these figures nor are plugs limited to two contacts. Sometimes the plugs have three, and even more, contacts, and frequently the jacks corresponding to such plugs have not only a contact spring adapted to register with each of the contacts of the plug, but several other auxiliary contacts also, which will be made or broken according to whether the plug is inserted or withdrawn from the jack. Symbolic representations of plugs and jacks are shown in Fig. 237. These are employed in diagrammatic representations of circuits and are supposed to represent the essential elements of the plugs and jacks in such a way as to be suggestive of their operation. It will be understood that such symbols may be greatly modified to express the various peculiarities of the plugs and jacks which they represent.

_Keys_. Other important elements of manual switchboards are ringing and listening keys. These are the devices by means of which the operator may switch the central-office generator or her telephone set into or out of the circuit of the connected lines. The details of a simple ringing and listening key are shown in Fig. 238. This consists of two groups of springs, one of four and one of six, the springs in each group being insulated from each other at their points of mounting. Two of these springs _1_ and _2_ in one group--the ringing group--are longer than the others, and act as movable levers engaging the inner pair of springs _3_ and _4_ when in their normal positions, and the outer pair _5_ and _6_ when forced into their alternate positions. Movement is imparted to these springs by the action of a cam which is mounted on a lever, manipulated by the operator. When this lever is moved in one direction the cam presses the two springs _1_ and _2_ apart, thus causing them to disengage the springs _3_ and _4_ and to engage the springs _5_ and _6_.

The springs of the other group constitute the switching element of the listening key and are very similar in their action to those of the ringing key, differing in the fact that they have no inner pair of springs such as _3_ and _4_. The two long springs _7_ and _8_, therefore, normally do not rest against anything, but when the key lever is pressed, so as to force the cam between them, they are made to engage the two outer springs _9_ and _10_.

The design and construction of ringing and listening keys assume many different forms. In general, however, they are adapted to do exactly the same sort of switching operations as that of which the device of Fig. 238 is capable. Easily understood symbols of ringing and listening keys are shown in Fig. 239; the cam member which operates on the two long springs is usually omitted for ease of illustration. It will be understood in considering these symbols, therefore, that the two long curved springs usually rest against a pair of inner contacts in case of the ringing key or against nothing at all in case of the listening key, and that when the key is operated the two springs are assumed to be spread apart so as to engage the outer pair of contacts with which they are respectively normally disconnected.

_Line and Cord Equipments._ The parts of the switchboard that are individual to the subscriber's line are termed the _line equipment;_ this, in the case of a magneto switchboard, consists of the line drop and the jack together with the associated wiring necessary to connect them properly in the line circuit. The parts of the switchboard that are associated with a connecting link--consisting of a pair of plugs and associated cords with their ringing and listening keys and clearing-out drop--are referred to as a _cord equipment_. The circuit of a complete pair of cords and plugs with their associated apparatus is called a _cord circuit_. In order that there may be a number of simultaneous connections between different pairs of lines terminating in a switchboard, a number of cord circuits are provided, this number depending on the amount of traffic at the busiest time of the day.

_Operator's Equipment._ A part of the equipment that is not individual to the lines or to the cord circuits, but which may, as occasion requires, be associated with any of them is called the _operator's equipment_. This consists of the operator's transmitter and receiver, induction coil, and battery connections together with the wiring and other associated parts necessary to co-ordinate them with the rest of the apparatus. Still another part of the equipment that is not individual to the lines nor to the cord circuits is the calling-current generator. This may be common to the entire office or a separate one may be provided for each operator's position.

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 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 class 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 associated 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_.

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 pass 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.

_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. Associated 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.

It may be stated at this point that the two plugs of an associated 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.

_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.

_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 associated 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 passing up through the secondary winding of the induction coil at the calling station, it passes 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.

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 associated 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.

Liability to Cross-Talk:--This type of drop is suitable for use only on small switchboards where space 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 space, 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 space 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 shell 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 shell 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 shell. The armature carries a latch rod extending the entire length of the shell 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.

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 brass 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 assembling of the switchboard and also serves to economize space--obviously, the thing to economize on the face of a switchboard is space 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 shell, 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 assumed a variety of forms, some of which will be referred to in the discussion of other types of drops and jacks.

_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 brass 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 brass 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.

_Methods of Associating 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 association of the drops and jacks has another advantage--it makes possible such a mechanical relation between the drop and its associated 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 space 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 space 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 space 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 space on the face of the switchboard.

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 brass 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 passes 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.

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 principally 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.

A feature that is made much of in recently designed drops and jacks for magneto service is that which provides for the ready removal of the drop coil, from the rest of the structure, for repair. The drop and jack of the Western Electric Company, just described, embodies this feature, a single screw being so arranged that its removal will permit the withdrawal of the coil without disturbing any of the other parts or connections. The coil windings terminate in two projections on the front head of the spool, and these register with spring clips on the inside of the shell so that the proper connections for the coil are automatically made by the mere insertion of the coil into the shell.

Dean Type:--The combined drop and jack of the Dean Electric Company is illustrated in Figs. 256 and 257. The two perspective views show the general features of the drop and jack and the method by which the magnet coil may be withdrawn from the shell. As will be seen the magnet is wound on a hollow core which slides over the iron core, the latter remaining permanently fixed in the shell, even though the coil be withdrawn.

Fig. 258 shows the structural details of the jack employed in this combination and it will be seen that the restoring spring for the drop is not the tip spring itself, but another spring located above and insulated from it and mechanically connected therewith.

Monarch Type:--Still another combined drop and jack is that of the Monarch Telephone Manufacturing Company of Chicago, shown in sectional view in Fig. 259. This differs from the usual type in that the armature is mounted on the front end of the electromagnet, its latch arm retaining the shutter in its normal position when raised, and releasing it when depressed by the attraction of the armature. As is shown, there is within the core of the magnet an adjustable spiral spring which presses forward against the armature and which spring is compressed by the attraction of the armature of the magnet. The night-alarm contact is clearly shown immediately below the strip which supports the drop, this consisting of a spring adapted to be engaged by a lug on the shutter and pressed upwardly against a stationary contact when the shutter falls. The method of restoration of the shutter in this case is by means of an auxiliary spring bent up so as to engage the shutter and restore it when the spring is raised by the insertion of a plug into the jack.

_Code Signaling._ On bridging party lines, where the subscribers sometimes call other subscribers on the same line and sometimes call the switchboard so as to obtain a connection with another line, it is not always easy for the operator at the switchboard to distinguish whether the call is for her or for some other party on the line. On such lines, of course, code ringing is used and in most cases the operator's only way of distinguishing between calls for her and those for some sub-station parties on the line is by listening to the rattling noise which the drop armature makes. In the case of the Monarch drop the adjustable spring tension on the armature is intended to provide for such an adjustment as will permit the armature to give a satisfactory buzz in response to the alternating ringing currents, whether the line be long or short.

The Monarch Company provides in another way for code signaling at the switchboard. In some cases there is a special attachment, shown in Fig. 260, by means of which the code signals are repeated on the night-alarm bell. This is in the nature of a special attachment placed on the drop, which consists of a light, flat spring attached to the armature and forming one side of a local circuit. The other side of the circuit terminates in a fixture which is mounted on the drop frame and is provided with a screw, having a platinum point forming the other contact point; this allows of considerable adjustment. At the point where the screw comes in contact with the spring there is a platinum rivet. When an operator is not always in attendance, this code-signaling attachment has some advantages over the drop as a signal interpreter, in that it permits the code signals to be heard from a distance. Of course, the addition of spring contacts to the drop armature tends to complicate the structure and perhaps to cut down the sensitiveness of the drop, which are offsetting disadvantages.

For really long lines, this code signaling by means of the drop is best provided for by employing a combined drop and ringer, although in this case whatever advantages are secured by the mechanical restoration of the shutter upon plugging in are lost. Such a device as manufactured by the Dean Electric Company is shown in Fig. 261. In this the ordinary polarized ringer is used, but in addition the tapper rod carries a latch which, when vibrated by the ringing of the bell, releases a shutter and causes it to fall, thus giving a visual as well as an audible signal.

_Electrical_. Coming now to the electrical restoration of drop shutters, reference is made to Fig. 262, which shows in side section the electrical restoring drop employed by the Bell companies and manufactured by the Western Electric Company. In this the coil _1_ is a line coil, and it operates on the armature _2_ to raise the latch lever _3_ in just the same manner as in the ordinary tubular drop. The latch lever _3_ acts, however, to release another armature _4_ instead of a shutter. This armature _4_ is pivoted at its lower end at the opposite end of the device from the armature _2_ and, by falling outwardly when released, it serves to raise the light shutter _5_. The restoring coil of this device is shown at _6_, and when energized it attracts the armature _4_ so as to pull it back under the catch of the latch lever _3_ and also so as to allow the shutter _5_ to fall into its normal position. The method of closing the restoring circuit is by placing coil _6_ in circuit with a local battery and with a pair of contacts in the jack, which latter contacts are normally open but are bridged across by the plug when it enters the jack, thus energizing the restoring coil and restoring the shutter.

A perspective view of this Western Electric electrical restoring drop is shown in Fig. 263, a more complete mention being made of this feature under the discussion of magneto multiple switchboards, wherein it found its chief use. It is mentioned here to round out the methods that have been employed for accomplishing the automatic restoration of shutters by the insertion of the plug.

Switchboard Plugs. A switchboard plug such as is commonly used in simple magneto switchboards is shown in Fig. 264 and also in Fig. 235. The tip contact is usually of brass and is connected to a slender steel rod which runs through the center of the plug and terminates near the rear end of the plug in a connector for the tip conductor of the cord. This central core of steel is carefully insulated from the outer shell of the plug by means of hard rubber bushings, the parts being forced tightly together. The outer shell, of course, forms the other conductor of the plug, called the sleeve contact. A handle of tough fiber tubing is fitted over the rear end of the plug and this also serves to close the opening formed by cutting away a portion of the plug shell, thus exposing the connector for the tip conductor.

_Cord Attachment._ The rear end of the plug shell is usually bored out just about the size of the outer covering of the switchboard cord, and it is provided with a coarse internal screw thread, as shown. The cord is attached by screwing it tightly into this screw-threaded chamber, the screw threads in the brass being sufficiently coarse and of sufficiently small internal diameter to afford a very secure mechanical connection between the outer braiding of the cord and the plug. The connection between the tip conductor of the cord and the tip of the plug is made by a small machine screw connection as shown, while the connection between the sleeve conductor of the plug and the sleeve conductor of the cord is made by bending back the latter over the outer braiding of the cord before it is screwed into the shank of the plug. This results in the close electrical contact between the sleeve conductor of the cord and the inner metal surface of the shank of the plug.

Switchboard Cords. A great deal of ingenuity has been exerted toward the end of producing a reliable and durable switchboard cord. While great improvement has resulted, the fact remains that the cords of manual switchboards are today probably the most troublesome element, and they need constant attention and repairs. While no two manufacturers build their cords exactly alike, descriptions of a few commonly used and successful cords may be here given.

_Concentric Conductors._ In one the core is made from a double strand of strong lock stitch twine, over which is placed a linen braid. Then the tip conductor, which is of stranded copper tinsel, is braided on. This is then covered with two layers of tussah silk, laid in reverse wrappings, then there is a heavy cotton braid, and over the latter a linen braid. The sleeve conductor, which is also of copper tinsel, is then braided over the structure so formed, after which two reverse wrappings of tussah silk are served on, and this is covered by a cotton braid and this in turn by a heavy linen or polished cotton braid. The plug end of the cord is reinforced for a length of from 12 to 18 inches by another braiding of linen or polished cotton, and the whole cord is treated with melted beeswax to make it moisture-proof and durable.

_Steel Spiral Conductors._ In another cord that has found much favor the two conductors are formed mainly by two concentric spiral wrappings of steel wire, the conductivity being reinforced by adjacent braidings of tinsel. The structure of such a cord is well shown in Fig. 265. Beginning at the right, the different elements shown are, in the order named, a strand of lock stitch twine, a linen braiding, into the strands of which are intermingled tinsel strands, the inner spiral steel wrapping, a braiding of tussah silk, a linen braiding, a loose tinsel braiding, the outer conductor of round spiral steel, a cotton braid, and an outside linen or polished cotton braid. The inner tinsel braiding and the inner spiral together form the tip conductor while the outer braiding and spiral together form the sleeve conductor. The cord is reinforced at the plug end for a length of about 14 inches by another braiding of linen. The tinsel used is, in each case, for the purpose of cutting down the resistance of the main steel conductor. These wrappings of steel wire forming the tip and sleeve conductors respectively, have the advantage of affording great flexibility, and also of making it certain that whatever strain the cord is subjected to will fall on the insulated braiding rather than on the spiral steel which has in itself no power to resist tensile strains.

_Parallel Tinsel Conductors._ Another standard two-conductor switchboard cord is manufactured as follows: One conductor is of very heavy copper tinsel insulated with one wrapping of sea island cotton, which prevents broken ends of the tinsel or knots from piercing through and short-circuiting with the other conductor. Over this is placed one braid of tussah silk and an outer braid of cotton. This combines high insulation with considerable strength. The other conductor is of copper tinsel, not insulated, and this is laid parallel to the thrice insulated conductor already described. Around these two conductors is placed an armor of spring brass wire in spiral form, and over this a close, stout braid of glazed cotton. This like the others is reinforced by an extra braid at the plug end.

Ringing and Listening Keys. The general principles of the ringing key have already been referred to. Ringing keys are of two general types, one having horizontal springs and the other vertical.

_Horizontal Spring Type._ Various Bell operating companies have generally adhered to the horizontal spring type except in individual and four-party-line keys. The construction of a Western Electric Company horizontal spring key is shown in Fig. 266. In this particular key, as illustrated, there are two cam levers operating upon three sets of springs. The cam lever at the left operates the ordinary ringing and listening set of springs according to whether it is pushed one way or the other. In ringing on single-party lines the cam lever at the left is the one to be used; while on two-party lines the lever at the left serves to ring the first party and the ringing key at the right the second party.

In order that the operator may have an indication as to which station on a two-party line she has called, a small target _1_ carried on a lever _2_ is provided. This target may display a black or a white field, according to which of its positions it occupies. The lever _2_ is connected by the links _3_ and _4_ with the two key levers and the target is thus moved into one position or the other, according to which lever was last thrown into ringing position.

It will be noticed that the springs are mounted horizontally and on edge. This on-edge feature has the advantage of permitting ready inspection of the contacts and of avoiding the liability of dust gathering between the contacts. As will be seen, at the lower end of each switch lever there is a roller of insulating material which serves as a wedge, when forced between the two long springs of any set, to force them apart and into engagement with their respective outer springs.

_Vertical Spring Type._ The other type of ringing and listening key employing vertical springs is almost universally used by the various independent manufacturing companies. A good example of this is shown in Fig. 267, which shows partly in elevation and partly in section a double key of the Monarch Company. The operation of this is obvious from its mode of construction. The right-hand set of springs of the right-hand key in this cut are the springs of the listening key, while the left-hand set of the right-hand key are those of the calling-plug ringing key. The left-hand set of the left-hand key may be those of a ring-back key on the answering plug, while the right-hand set of the left-hand key may be for any special purpose. It is obvious that these groups of springs may be grouped in different combinations or omitted in part, as required. This same general form of key is also manufactured by the Kellogg Company and the Dean Company, that of the Kellogg Company being illustrated in perspective, Fig. 268. The keys of this general type have the same advantages as those of the horizontal on-edge arrangement with respect to the gathering of dust, and while perhaps the contacts are not so readily get-at-able for inspection, yet they have the advantage of being somewhat more simple, and of taking up less horizontal space on the key shelf.

_Party-Line Ringing Keys._ For party-line ringing the key matter becomes somewhat more complicated. Usually the arrangement is such that in connection with each calling plug there are a number of keys, each arranged with respect to the circuits of the plug so as to send out the proper combination and direction of current, if the polarity system is used; or the proper frequency of current if the harmonic system is used; or the proper number of impulses if the step-by-step or broken-line system is used. The number of different kinds of arrangements and combinations is legion, and we will here illustrate only an example of a four-party line ringing key adapted for harmonic ringing. A Kellogg party-line listening and ringing key is shown in Fig. 269. In this, besides the regular listening key, are shown four push-button keys, each adapted, when depressed, to break the connection back of the key, and at the same time connect the proper calling generator with the calling plug.