_Ribbon Fuses_. A point of interest with relation to tubular fuses is

that in some of the best types of such fuses, the resistance material

is not in the form of a round wire but in the form of a flat ribbon.

This arrangement disposes the necessary amount of fusible metal in a

form to give the greatest amount of surface, while a round wire offers

the least surface for a given weight of metal--a circle encloses its

area with less periphery than any other figure. The reason for giving

the fuse the largest possible surface area is to decrease the

likelihood of the fuse being ruptured by lightning. The fact that such

fuses do withstand lightning discharges much more thoroughly than

round fuses of the same rating is an interesting proof of the

oscillating nature of lightning discharges, for the density of the

current of those discharges is greater on and near the surface of the

conductor than within the metal and, therefore, flattening the fuse

increases its carrying capacity for high-frequency currents, without

appreciably changing its carrying capacity for direct currents. The

reason its capacity for direct currents is increased at all by

flattening it, is that the surface for the radiation of heat is

increased. However, when enclosed in a tube, radiation of heat is

limited, so that for direct currents the carrying capacity of fuses

varies closely with the area of cross-section.

City-Exchange Requirements. The foregoing has set down the

requirements of good practice in an average city-exchange system.

Nothing short of the general arrangement shown in Fig. 225 meets the

usual assortment of hazards of such an exchange. It is good modern

practice to distribute lines by means of cables, supplemented in part

by short insulated drop wires twisted in pairs. Absence of bare wires

reduces electrical hazards enormously. Nevertheless, hazards remain.

Though no less than the spirit of this plan of protection should be

followed, additional hazards may exist, which may require additional

elements of protection. At the end of a cable, either aërial or

underground, long open wires may extend into the open country as rural

or long-distance circuits. If these be longer than a mile or two, in

most regions they will be subjected to lightning discharges. These may

be subjected to high-potential contacts as well.

If a specific case of such exposure indicates that the cables may be

in danger, the long open lines then are equipped with additional

air-gap arresters at the point of junction of those open lines with

the cable. Practice varies as to the type. Maintenance charges are

increased if carbon arresters separated .005 inch are used, because of

the cost of sending to the end of the long cable to clear the blocks

from carbon dust after each slight discharge. Roughened metal blocks

do not become grounded as readily as do carbon blocks. The occasions

of visit to the arresters, therefore, usually follow actual heavy

discharges through them.

The recommendations and the practice of the American Telephone and

Telegraph Company differ on this point, while the practice of other

companies varies with the temperaments of the engineers. The American

Company specifies copper-block arresters where long country lines

enter cables, if those lines are exposed to lightning discharges only.

The exposed line is called _long_ if more than one-half mile in

length. If it is exposed to high-potential hazards, carbon blocks are

specified instead of copper. Other specifications of that company have

called for the use of copper-block arresters on lines exposed to

hazards above 2,500 volts.

[Illustration: ONE OF THE FOUR WINGS OF THE OLD KELLOGG DIVIDED

MULTIPLE BOARD OF THE CUYAHOGA TELEPHONE COMPANY, CLEVELAND, OHIO

Ultimate Capacity, 24,000 Lines. One of the Two Examples in the United

States of a Multiple Switchboard Having an Ultimate Capacity over

18,000 Lines. Replaced Recently by a Kellogg Straight Multiple Board

Having an Ultimate Capacity of 18,000 Lines and a Present Capacity of

10,000 Lines.]

The freedom of metal-block arresters from dust troubles gives them a

large economical advantage over carbon. For similar separations, the

ratio of striking voltages between carbon blocks and metal blocks

respectively is as 7 to 16. In certain regions of the Pacific Coast

where the lightning hazard is negligible and the high tension hazard

is great, metal-block arresters at the outer ends of cables give

acceptable protection.

High winds which drive snow or dust against bare wires of a long line,

create upon or place upon those wires a charge of static electricity

which makes its way from the line in such ways as it can. Usually it

discharges across arresters and when this discharge takes place, the

line is disturbed in its balance and loud noises are heard in the

telephones upon it.

[Fig. 232. Drainage Coils]

A telephone line which for a long distance is near a high-tension

transmission line may have electrostatic or electromagnetic

potentials, or both, induced upon it. If the line be balanced in its

properties, including balance by transposition of its wires, the

electrostatic induction may neutralize itself. The electromagnetic

induction still may disturb it.

_Drainage Coils_. The device shown in Fig. 232, which amounts merely

to an inductive leak to earth, is intended to cure both the snowstorm

and electromagnetic induction difficulties. It is required that its

impedance be high enough to keep voice-current losses low, while being

low enough to drain the line effectively of the disturbing charges.

Such devices are termed "drainage coils."

Electrolysis. The means of protection against the danger due to

chemical action, set forth in the preceding chapter, form such a

distinct phase of the subject of guarding property against electrical

hazards as to warrant treatment in a separate chapter devoted to the

subject of electrolysis.

[Illustration: MAIN EXCHANGE, CLEVELAND, OHIO.

Largest Four-Party Selective Ringing Switchboard in the World. Kellogg

Switchboard and Supply Co.]

CHAPTER XX

GENERAL FEATURES OF THE TELEPHONE EXCHANGE

Up to this point only those classes of telephone service which could be

given between two or more stations on a single line have been

considered. Very soon after the practical conception of the telephone,

came the conception of the telephone exchange; that is, the conception

of centering a number of lines at a common point and there terminating

them in apparatus to facilitate their interconnection, so that any

subscriber on any line could talk with any subscriber on any other

line.

The complete equipment of lines, telephone instruments, and switching

facilities by which the telephone stations of the community are given

telephone service is called a telephone exchange.

The building where a group of telephone lines center for

interconnection is called a central office, and its telephonic

equipment the central-office equipment. The terms telephone office and

telephone exchange are frequently confused. Although a telephone office

building may be properly referred to as a telephone exchange building,

it is hardly proper to refer to the telephone office as a telephone

exchange, as is frequently done. In modern parlance the telephone

exchange refers not only to the central office and its equipment but to

the lines and instruments connected therewith as well; furthermore, a

telephone exchange may embrace a number of telephone offices that are

interconnected by means of so-called trunk lines for permitting the

communication of subscribers whose lines terminate in one office with

those subscribers whose lines terminate in any other office.

Since a given telephone exchange may contain one or more central

offices, it is proper to distinguish between them by referring to an

exchange which contains but a single central office as a single office

exchange, and to an exchange which contains a plurality of central

offices as a multi-office exchange.

In telephone exchange working, three classes of lines are dealt

with--subscribers' lines, trunk lines, and toll lines.

Subscribers' Lines. The term subscriber is commonly applied to the

patron of the telephone service. His station is, therefore, referred

to as a subscriber's station, and the telephone equipment at any

subscriber's station is referred to as a subscriber's station

equipment. Likewise, a line leading from a central office to one or

more subscribers' stations is called a subscriber's line. A

subscriber's line may, as has been shown in a previous chapter, be an

individual line if it serves but one station, or a party line if it

serves to connect more than one station with the central office.

Trunk Lines. A trunk line is a line which is not devoted to the

service of any particular subscriber, but which may form a connecting

link between any one of a group of subscribers' lines which terminate

in one place and any one of a group of subscribers' lines which

terminate in another place. If the two groups of subscribers' lines

terminate in the same building or in the same switchboard, so that the

trunk line forming the connecting link between them is entirely within

the central-office building, it is called a local trunk line, or a

local trunk. If, on the other hand, the trunk line is for connecting

groups of subscribers' lines which terminate in different central

offices, it is called an inter-office trunk.

Toll Lines. A toll line is a telephone line for the use of which a

special fee or toll is charged; that is, a fee that is not included in

the charges made to the subscriber for his regular local exchange

service. Toll lines extend from one exchange district to another, more

or less remote, and they are commonly termed _local_ toll and

_long-distance_ toll lines according to the degree of remoteness. A

toll line, whether local or long-distance, may be looked upon in the

nature of an inter-exchange trunk.

Districts. The district in a given community which is served by a

single central office is called an office district. Likewise, the

district which is served by a complete exchange is called an exchange

district. An exchange district may, therefore, consist of a number of

central-office districts, just as an exchange may comprise a number of

central offices. To illustrate, the entire area served by the exchange

of the Chicago Telephone Company in Chicago, embracing the entire city

and some of its suburbs, is the Chicago exchange district. The area

served by one of the central offices, such as the Hyde Park office,

the Oakland office, the Harrison office, or any of the others, is an

office district.

Switchboards. The apparatus at the central office by which the

telephone lines are connected for conversation and afterwards

disconnected, and by which the various other functions necessary to

the giving of complete telephone service are performed, is called a

switchboard. This may be simple in the case of small exchanges, or of

vast complexity in the case of the larger exchanges.

Sometimes the switchboards are of such nature as to require the

presence of operators, usually girls, to connect and disconnect the

line and perform the other necessary functions, and such switchboards,

whether large or small, are termed _manual_.

Sometimes the switchboards are of such a nature as not to require the

presence of operators, the various functions of connection,

disconnection, and signaling being performed by the aid of special

forms of apparatus which are under the control of the subscriber who

makes the call. Such switchboards are termed _automatic_.

Of recent years there has appeared another class of switchboards,

employing in some measure the features of the automatic and in some

measure those of the manual switchboard. These boards are commonly

referred to as _semi-automatic_ switchboards, presumably because they

are supposed to be half automatic and half manual.

_Manual_. Manual switchboards may be subdivided into two classes

according to the method of distributing energy for talking purposes.

Thus we may have _magneto_ switchboards, which are those capable of

serving lines equipped with magneto telephones, local batteries being

used for talking purposes. On the other hand, we may have

_common-battery_ switchboards, adapted to connect lines employing

common-battery telephones in which all the current for both talking

and signaling is furnished from the central office. In still another

way we may classify manual switchboards if the method of distributing

the energy for talking and signaling purposes is ignored. Thus,

entirely irrespective of whether the switchboards are adapted to serve

common-battery or local-battery lines, we may have non-multiple

switchboards and multiple switchboards.

The term _multiple_ switchboard is applied to that class of

switchboards in which the connection terminals or jacks for all the

lines are repeated at intervals along the face of the switchboard, so

that each operator may have within her reach a terminal for each line

and may thus be able to complete by herself any connection between two

lines terminating in the switchboard.

The term _non-multiple_ switchboard is applied to that class of boards

where the provision for repeating the line terminals at intervals along

the face of the board is not employed, but where, as a consequence,

each line has but a single terminal on the face of the board.

Non-multiple switchboards have their main use in small exchanges where

not more than a few hundred lines terminate. Where such is the case, it

is an easy matter to handle all the traffic by one, two, or three

operators, and as all of these operators may reach all over the face of

the switchboard, there is no need for giving any line any more than one

connection terminal. Such boards may be called _simple_ switchboards.

There is another type of non-multiple switchboard adaptable for use in

larger exchanges than the simple switchboard. A correct idea of the

fundamental principle involved in these may be had by imagining a row

of simple switchboards each containing terminals or jacks for its own

group of lines. In order to provide for the connection of a line in

one of these simple switchboards with a line in another one, out of

reach of the operator at the first, short connecting lines extending

between the two switchboards are provided, these being called

_transfer_ or _trunk_ lines. In order that connections may be made

between any two of the simple boards, a group of transfer lines is run

from each board to every other one.

In such switchboards an operator at one of the boards or positions may

complete the connection herself between any two lines terminating at

her own board. If, however, the line called for terminates at another

one of the boards, the operator makes use of the transfer or trunk line

extending to that board, and the operator at this latter board

completes the connection, so that the two subscribers' lines are

connected through the trunk or transfer line. A distinguishing feature,

therefore, in the operation of so-called transfer switchboards, is that

an operator can not always complete a connection herself, the

connection frequently requiring the attention of two operators.

Transfer systems are not now largely used, the multiple switchboard

having almost entirely supplanted them in manual exchanges of such size

as to be beyond the limitation of the simple switchboard. At

multi-office manual exchanges, however, where there are a number of

multiple switchboards employed at various central offices, the same

sort of a requirement exists as that which was met by the provision of

trunk lines between the various simple switchboards in a transfer

system. Obviously, the lines in one central office must be connected to

those of another in order to give universal service in the community in

which the exchange operates. For this purpose inter-office trunk lines

are used, the arrangement being such that when an operator at one

office receives a call for a subscriber in another office, she will

proceed to connect the calling subscriber's line, not directly with the

line of the called subscriber because that particular line is not

within her reach, but rather with a trunk line leading to the office in

which the called-for subscriber's line terminates; having done this she

will then inform an operator at that second office of the connection

desired, usually by means of a so-called order-wire circuit. The

connection between the trunk line so used and the line of the

called-for subscriber will then be completed by the connecting link or

trunk line extending between the two offices.

In such cases the multiple switchboard at each office is divided into

two portions, termed respectively the _A_ board and the _B_ board.

Each of these boards, with the exception that will be pointed out in a

subsequent chapter, is provided with a full complement of multiple

jacks for all of the lines entering that office. At the _A_ board are

located operators, called _A_ operators, who answer all the calls from

the subscribers whose lines terminate in that office. In the case of

calls for lines in that same office, they complete the connection

themselves without the assistance of the other operators. On the other

hand, the calls for lines in another office are handled through trunk

lines leading to that other office, as before described, and these

trunk lines always terminate in the _B_ board at that office. The _B_

operators are, therefore, those operators who receive the calls over

trunk lines and complete the connection with the line of the

subscriber desired.

To define these terms more specifically, an _A_ board is a multiple

switchboard in which the subscriber's lines of a given office district

terminate. For this reason the _A_ board is frequently referred to as

a subscribers' board, and the operators who work at these boards and

who answer the calls of the subscribers are called _A_ operators or

subscribers' operators. _B_ boards are switchboards in which terminate

the incoming ends of the trunk lines leading from other offices in the

same exchange. These boards are frequently called incoming trunk

boards, or merely trunk boards, and the operators who work at them and

who receive the directions from the _A_ operators at the other boards

are called _B_ operators, or incoming trunk operators.

The circuits which are confined wholly to the use of operators and

over which the instructions from one operator to another are sent, as

in the case of the _A_ operator giving an order for a connection to a

_B_ operator at another switchboard, are designated _call circuits_ or

_order wire circuits_.

Sometimes trunk lines are so arranged that connections may be

originated at either of their ends. In other cases they are so arranged

that one group of trunk lines connecting two offices is for the traffic

in one direction only, while another group leading between the same two

offices is for handling only the traffic in the other direction. Trunk

lines are called _one-way_ or _two-way_ trunks, according to whether

they handle the traffic in one direction or in two. A trunking system,

where the same trunks handle traffic both ways, is called a

_single-track system_; and, on the other hand, a system in which there

are two groups of trunks, one handling traffic in one direction and the

other in the other, is called a _double-track system_. This

nomenclature is obviously borrowed from railroad practice.

There is still another class of manual switchboards called the _toll

board_ of which it will be necessary to treat. Telephone calls made by

one person for another within the limits of the same exchange district

are usually charged for either by a flat rate per month, or by a

certain charge for each call. This is usually regardless of the

duration of the conversation following the call. On the other hand,

where a call is made by one party for another outside of the limits of

the exchange district and, therefore, in some other exchange district,

a charge is usually made, based on the time that the connecting

long-distance line is employed. Such calls and their ensuing

conversations are charged for at a very much higher rate than the

purely local calls, this rate depending on the distance between the

stations involved. The making up of connections between a

long-distance and a local line is usually done by means of operators

other than those employed in handling the local calls, who work either

by means of special equipment located on the local board, or by means

of a separate board. Such equipments for handling long-distance or

toll traffic are commonly termed toll switchboards.

They differ from local boards (a) in that they are arranged for a very

much smaller number of lines; (b) in that they have facilities by

which the toll operator may make up the connections with a minimum

amount of labor on the part of the assisting local operators; and (c)

in that they have facilities for recording the identification of the

parties and timing the conversations taking place over the toll lines,

so that the proper charge may be made to the proper subscriber.

CHAPTER XXI

THE SIMPLE MAGNETO SWITCHBOARD

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.

[Illustration: Fig. 233. Drop Symbol]

_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.

[Illustration: Fig. 234. Spring Jack]

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.

[Illustration: Fig. 235. Plug]

[Illustration: Fig. 236. Plug and Jack]

[Illustration: Fig. 237. Jack and Plug Symbols]

_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_.

[Illustration: Fig. 238. Ringing and Listening Key]

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_.

[Illustration: Fig. 239. Ringing-and Listening-Key Symbols]

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_.

[Illustration: 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 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.

[Illustration: Fig. 241. Subscriber Calling]

[Illustration: 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.

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.

[Illustration: Fig. 242. Operator Answering]

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.

[Illustration: 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.

[Illustration: 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

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.

[Illustration: 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 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.

[Illustration: Fig. 246 Old-Style Drop]

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.

[Illustration: Fig. 247. Tubular Drop]

[Illustration: 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 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.

[Illustration: 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 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.

[Illustration: Fig. 250. Individual Jack]

[Illustration: Fig. 251. Strip of Jacks]

_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.

[Illustration: 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

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.

[Illustration: 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 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.

[Illustration: Fig. 254. Western Electric Drop and Jack]

[Illustration: Fig. 255. Western Electric Drop and Jack]

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.

[Illustration: Fig. 256. Dean Drop and Jack]

[Illustration: Fig. 257. Dean Drop and Jack]

[Illustration: Fig. 258. Details of Dean Jack]

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.

[Illustration: Fig. 259. Monarch Drop and 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.

[Illustration: Fig. 260. Code Signal Attachment]

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.

[Illustration: Fig. 261. Combined Drop and Ringer]

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.

[Illustration: Fig. 262. Electrically Restored Drop]

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.

[Illustration: Fig. 263. Electrically Restored Drop]

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.

[Illustration: Fig. 264. Switchboard Plug]

_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.

[Illustration: Fig. 265. Switchboard Cord]

_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.

[Illustration: Fig. 266. Horizontal-Spring Listening and Ringing Key]

_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.

[Illustration: Fig. 267. Vertical-Spring Listening and Ringing Key]

_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.

[Illustration: Fig. 268. Vertical Listening and Ringing Key]

[Illustration: Fig. 269. Four-Party Listening and Ringing Key]

_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.

_Self-Indicating Keys._ A complication that has given a good deal of

trouble in the matter of party-line ringing is due to the fact that it

is sometimes necessary to ring a second or a third time on a

party-line connection, because the party called may not respond the

first time. The operator is not always able to remember which one of

the four keys associated with the plug connected with the desired

party she has pressed on the first occasion and, therefore, when it

becomes necessary to ring again, she may ring the wrong party. This is

provided for in a very ingenious way in the key shown in Fig. 269, by

making the arrangement such that after a given key has been depressed

to its full extent in ringing, and then released, it does not come

quite back to its normal position but remains slightly depressed. This

always serves as an indication to the operator, therefore, as to which

key she depressed last, and in the case of a re-ring, she merely

presses the key that is already down a little way. On the next call if

she is required to press another one of the four keys, the one which

remained down a slight distance on the last call will be released and

the one that is fully depressed will be the one that remains down as

an indication.

Such keys, where the key that was last used leaves an indication to

that effect, are called _indicating_ ringing keys. In other forms the

indication is given by causing the key lever to move a little target

which remains exposed until some other key in the same set is moved.

The key shown in Fig. 266 is an example of this type.

NOTE. The matter of automatic ringing and other special forms of

ringing will be referred to and discussed at their proper places

in this work, but at this point they are not pertinent as they

are not employed in simple switchboards.

Operator's Telephone Equipment. Little need be said concerning the

matter of the operator's talking apparatus, _i.e._, the operator's

transmitter and receiver, since as transmitters and receivers they are

practically the same as those in ordinary use for other purposes. The

watch-case receiver is nearly always employed for operators' purposes

on account of its lightness and compactness. It is used in connection

with a head band so as to be held continually at the operator's ear,

allowing both of her hands to be free.

The transmitter used by operators does not in itself differ from the

transmitters employed by subscribers, but the methods by which it is

supported differ, two general practices being followed. One of these

is to suspend the transmitter by flexible conducting cords so as to be

adjustable in a vertical direction. A good illustration of this is

given in Fig. 270. The other method, and one that is coming into more

and more favor, is to mount the transmitter on a light bracket

suspended by a flexible band from the neck of the operator, a breast

plate being furnished so that the transmitter will rest on her breast

and be at all times within proper position to receive her speech. To

facilitate this, a long curved mouthpiece is commonly employed, as

shown clearly in Fig. 47.

[Illustration: Fig. 270. Operator's Transmitter Suspension]

_Cut-in Jack._ It is common to terminate that portion of the apparatus

which is worn on the operator's person--that is, the receiver only if

the suspended type of transmitter is employed, and the receiver and

transmitter if the breast plate type of transmitter is employed--in a

plug, and a flexible cord connecting the plug terminates with the

apparatus. The portions of the operator's talking circuit that are

located permanently in the switchboard cabinet are in such cases

terminated in a jack, called an operator's _cut-in jack_. This is

usually mounted on the front rail of the switchboard cabinet just

below the key shelf. Such a cut-in jack is shown in Fig. 271 and it is

merely a specialized form of spring jack adapted to receive the short,

stout plug in which the operator's transmitter, or transmitter and

receiver, terminate. By this arrangement the operator is enabled

readily to connect or disconnect her talking apparatus, which is worn

on her person, whenever she comes to the board for work or leaves it

at the end of her work. A complete operator's telephone set, or that

portion that is carried on the person of the operator, together with

the cut-in plug, is shown in Fig. 272.

[Illustration: Fig. 271. Operator's Cut-in Jack]

[Illustration: Fig. 272. Operator's Talking Set]

Circuits of Complete Switchboard. We may now discuss the circuits of

a complete simple magneto switchboard. The one shown in Fig. 273 is

typical. Before going into the details of this, it is well to inform

the student that this general form of circuit representation is one

that is commonly employed in showing the complete circuits of any

switchboard. Ordinarily two subscribers' lines are shown, these

connecting their respective subscribers' stations with two different

line equipments at the central office. The jacks and signals of these

line equipments are turned around so as to face each other, in order

to clearly represent how the connection between them may be made by

means of the cord circuit. The elements of the cord circuit are also

spread out, so that the various parts occupy relative positions which

they do not assume at all in practice. In other words it must be

remembered that, in circuit diagrams, the relative positions of the

parts are sacrificed in order to make clear the circuit connections.

However, this does not mean that it is often not possible to so locate

the pieces of apparatus that they will in a certain way indicate

relative positions, as may be seen in the case of the drop and jack in

Fig. 273, the drop being shown immediately above the jack, which is

the position in which these parts are located in practice.

[Illustration: Fig. 273. Circuit of Simple Magneto Switchboard]

Little need be said concerning this circuit in view of what has

already been said in connection with Figs. 240 to 245. It will be seen

in the particular sub-station circuit here represented, that the

talking apparatus is arranged in the usual manner and that the ringer

and generator are so arranged that when the generator is operated the

ringer will be cut out of circuit, while the generator will be placed

across the circuit; while, when the generator is idle, the ringer is

bridged across the circuit and the generator is cut out.

The line terminates in each case in the tip and sleeve contacts of the

jack, and in the normal condition of the jack the line drop is bridged

across the line. The arrangement by which the drop is restored and at

the same time cut out of circuit when the operator plugs in the jack,

is obvious from the diagrammatic illustration. The cord circuit is the

same as that already discussed, with the exception that two ringing

keys are provided, one in connection with the calling plug, as is

universal practice, and the other in connection with the answering plug

as is sometimes practiced in order that the operator may, when occasion

requires, ring back the calling subscriber without the necessity of

changing the plug in the jack. The outer contacts of these two ringing

keys are connected to the terminals of the ringing generator and, when

either key is operated, the connection between the plug, on which the

ringing is to be done, and the rest of the cord circuit will be broken,

while the generator will be connected with the terminals of the plug.

The listening key and talking apparatus need no further explanation, it

being obvious that when the key is operated the subscriber's telephone

set will be bridged across the cord circuit and, therefore, connected

with either or both of the talking subscribers.

[Illustration: Fig. 274. Night-Alarm Circuit]

Night-Alarm Circuits. The circuit of Fig. 273, while referred to as

a complete circuit, is not quite that. The night-alarm circuit is not

shown. In order to clearly indicate how a single battery and bell, or

buzzer, may serve in connecting a number of line drops, reference is

made to Fig. 274 which shows the connection between three different

line drops and the night-alarm circuit. The night-alarm apparatus

consists in the battery _1_ and the buzzer, or bell, _2_. A switch _3_

adapted to be manually operated is connected in the circuit with the

battery and the buzzer so as to open this circuit when the night alarm

is not needed, thus making it inoperative. During the portions of the

day when the operator is needed constantly at the board it is

customary to leave this switch _3_ open, but during the night period

when she is not required constantly at the board this switch is closed

so that an audible signal will be given whenever a drop falls. The

night-alarm contact _4_ on each of the drops will be closed whenever a

shutter falls, and as the two members of this contact, in the case of

each drop, are connected respectively with the two sides of the

night-alarm circuit, any one shutter falling will complete the

necessary conditions for causing the buzzer to sound, assuming of

course that the switch _3_ is closed.

_Night Alarm with Relay._ A good deal of trouble has been caused in

the past by uncertainty in the closure of the night-alarm circuit at

the drop contact. Some of the companies have employed the form of

circuit shown in Fig. 275 to overcome this. Instead of the night-alarm

buzzer being placed directly in the circuit that is closed by the

drop, a relay _5_ and a high-voltage battery _6_ are placed in this

circuit. The buzzer and the battery for operating it are placed in a

local circuit controlled by this relay. It will be seen by reference

to Fig. 275 that when the shutter falls, it will, by closing the

contact _4_, complete the circuit from the battery _6_ through the

relay _5_--assuming switch _3_ to be closed--and thus cause the

operation of the relay. The relay, in turn, by pulling up its

armature, will close the circuit of the buzzer _2_ through the battery

_7_ and cause the buzzer to sound.

[Illustration: Fig. 275. Night-Alarm Circuit with Relay]

The advantage of this method over the direct method of operating the

buzzer is that any imperfection in the night-alarm contact at the drop

is much less likely to prevent the flow of current of the high-voltage

battery _6_ than of the low-voltage battery _1_, shown in connection

with Fig. 274. This is because the higher voltage is much more likely

to break down any very thin bit of insulation, such as might be caused

by a minute particle of dust or oxide between contacts that are

supposed to be closed by the falling of the shutter. It has been

common to employ for battery _6_ a dry-cell battery giving about 20

or 24 volts, and for the operation of the buzzer itself, a similar

battery of about two cells giving approximately 3 volts.

_Night-Alarm Contacts._ The night-alarm contact _4_ of the drop shown

diagrammatically in Figs. 274 and 275 would, if taken literally,

indicate that the shutter itself actually forms one terminal of the

circuit and the contact against which it falls, the other. This has

not been found to be a reliable way of closing the night-alarm

contacts and this method is indicated in these figures and in other

figures in this work merely as a convenient way of representing the

matter diagrammatically. As a matter of fact the night-alarm contacts

are ordinarily closed by having the shutter fall against one spring,

which is thereby pressed into engagement with another spring or

contact, as shown in Fig. 249. This method employs the shutter only as

a means for mechanically causing the one spring to press against the

other, the shutter itself forming no part of the circuit. The reason

why it is not a good plan to have the shutter itself act as one

terminal of the circuit is that this necessitates the circuit

connections being led to the shutter through the trunnions on which

the shutter is pivoted. This is bad because, obviously, the shutter

must be loosely supported on its trunnions in order to give it

sufficiently free movement, and, as is well known, loose connections

are not conducive to good electrical contacts.

Grounded-and Metallic-Circuit Lines. When grounded circuits were the

rule rather than the exception, many of the switchboards were

particularly adapted for their use and could not be used with

metallic-circuit lines. These grounded-circuit switchboards provided

but a single contact in the jack and a single contact on the plug, the

cords having but a single strand reaching from one plug to the other.

The ringing keys and listening keys were likewise single-contact keys

rather than double. The clearing-out drop and the operator's talking

circuit and the ringing generator were connected between the single

strand of the cord and the ground as was required.

The grounded-circuit switchboard has practically passed out of

existence, and while a few of them may be in use, they are not

manufactured at present. The reason for this is that while many

grounded circuits are still in use, there are very few places where

there are not some metallic-circuit lines, and while the

grounded-circuit switchboard will not serve for metallic-circuit

lines, the metallic-circuit switchboard will serve equally well for

either metallic-circuit or grounded lines, and will interconnect them

with equal facility. This fact will be made clear by a consideration

of Figs. 276, 277, and 278.

[Illustration: Fig. 276. Connection Between Metallic Lines]

[Illustration: Fig. 277. Connection Between Grounded Lines]

_Connection between Two Similar Lines._ In Fig. 276 a common magneto

cord circuit is shown connecting two metallic-circuit lines; in Fig.

277 the same cord circuit is shown connecting two grounded lines. In

this case the line wire _1_ of the left-hand line is, when the plugs

are inserted, continued to the tip of the answering plug, thence

through the tip strand of the cord circuit to the tip of the calling

plug, then to the tip spring of the right-hand jack and out to the

single conductor of that line. The entire sleeve portion of the cord

circuit becomes grounded as soon as the plugs are inserted in the

jacks of such a line. Hence, we see that the sleeve contacts of the

plug and the sleeve conductor of the cord are connected to ground

through the permanent ground connection of the sleeve conductors of

the jack as soon as the plug is inserted into the jack. Thus, when the

cord circuit of a metallic-circuit switchboard is used to connect two

grounded circuits together, the tip strand of the cord is the

connecting link between the two conductors, while the sleeve strand of

the cord merely serves to ground one side of the clearing-out drop

and one side each of the operator's telephone set and the ringing

generator when their respective keys are operated.

_Connection between Dissimilar Lines._ Fig. 278 shows how the same

cord circuit and the same arrangement of line equipment may be used

for connecting a grounded line to a metallic-circuit line. The

metallic circuit line is shown on the left and the grounded line on

the right. When the two plugs are inserted into the respective jacks

of this figure, the right-hand conductor of the metallic circuit shown

on the left will be continued through the tip strand of the cord

circuit to the line conductor of the grounded line shown on the right.

The left-hand conductor of the metallic-circuit line will be connected

to ground because it will be continued through the sleeve strand of

the cord circuit to the sleeve contact of the calling plug and thence

to the sleeve contact of the jack of the grounded line, which sleeve

contact is shown to be grounded. The talking circuit between the two

connected lines in this case may be traced as follows: From the

subscriber's station at the left through the right-hand limb of the

metallic-circuit line, through the tip contact and tip conductor of

the cord circuit, to the single limb of the grounded-circuit line,

thence to the sub-station of that line and through the talking

apparatus there to ground. The return path from the right-hand station

is by way of ground to the ground connection at the central office,

thence to the sleeve contact of the grounded line jack, through the

sleeve conductor of the cord circuit, to the sleeve contact of the

metallic-circuit line jack, and thence by the left-hand limb of the

metallic-circuit line to the subscriber's station.

[Illustration: Fig. 278. Connection Between Dissimilar Lines]

A better way of connecting a metallic-circuit line to a grounded line

is by the use of a special cord circuit involving a repeating coil,

such a connection being shown in Fig. 279. The cord circuit in this

case differs in no respect from those already shown except that a

repeating coil is associated with it in such a way as to conductively

divide the answering side from the calling side. Obviously, whatever

currents come over the line connected with the answering plug will

pass through the windings _1_ and _2_ of this coil and will induce

corresponding currents in the windings _3_ and _4_, which latter

currents will pass out over the circuit of the line connected with the

calling plug. When a grounded circuit is connected to a metallic

circuit in this manner, no ground is thrown onto the metallic circuit.

The balance of the metallic circuit is, therefore, maintained.

To ground one side of a metallic circuit frequently so unbalances it

as to cause it to become noisy, that is, to have currents flowing in

it, by induction or from other causes, other than the currents which

are supposed to be there for the purpose of conveying speech.

[Illustration: Fig. 279. Connection of Dissimilar Lines through

Repeating Coil]

_Convertible Cord Circuits._ The consideration of Fig. 279 brings us

to the subject of so-called convertible cord circuits. Some

switchboards, serving a mixture of metallic and grounded lines, are

provided with cord circuits which may be converted at will by the

operator from the ordinary type shown in Fig. 276 to the type shown in

Fig. 279. The advantage of this will be obvious from the following

consideration. When a call originates on any line, either grounded or

metallic, the operator does not know which kind of a line is to be

called for. She, therefore, plugs into this line with any one of her

answering plugs and completes the connection in the usual way. If the

call is for the same kind of a circuit as that over which the call

originated, she places the converting key in such a position as will

connect the conductors of the cord circuit straight through; while if

the connection is for a different kind of a line than that on which

the call originated she throws the converting key into such a

position as to include the repeating coil. A study of Fig. 280 will

show that when the converting key, which is commonly referred to as

the repeating-coil key, is in one position, the cord conductors will

be cut straight through, the repeating coil being left open in both

its windings; and when it is thrown to its other position, the

connection between the answering and calling sides of the cord circuit

will be severed and the repeating coil inserted so as to bring about

the same effects and circuit arrangements as are shown in Fig. 279.

[Illustration: Fig. 280. Convertible Cord Circuit]

Cord-Circuit Considerations. _Simple Bridging Drop Type._ The matter

of cord circuits in magneto switchboards is deserving of much

attention. So far as talking requirements are concerned, the ordinary

form of cord circuit with a clearing-out drop bridged across the two

strands is adequate for nearly all conditions except those where a

grounded-and a metallic-circuit line are connected together, in which

case the inclusion of a repeating coil has some advantages.

[Illustration: Fig. 281. Bridging Drop-Cord Circuit]

From the standpoint of signaling, however, this type of cord circuit

has some disadvantages under certain conditions. In order to simplify

the discussion of this and other cord-circuit matters, reference will

be made to some diagrams from which the ringing and listening keys and

talking apparatus have been entirely omitted. In Fig. 281 the regular

bridging type of clearing-out drop-cord circuit is shown, this being

the type already discussed as standard. For ordinary practice it is

all right. Certain difficulties are experienced with it, however,

where lines of various lengths and various types of sub-station

apparatus are connected. For instance, if a long bridging line be

connected with one end of this cord circuit and a short line having a

low-resistance series ringer be connected with the other end, then a

station on the long line may have some difficulty in throwing the

clearing-out drop, because of the low-resistance shunt that is placed

around it through the short line and the low-resistance ringer. In

other words, the clearing-out drop is shunted by a comparatively

low-resistance line and ringer and the feeble currents arriving from a

distant station over the long line are not sufficient to operate the

drop thus handicapped. The advent of the various forms of party-line

selective signaling and the use of such systems in connection with

magneto switchboards has brought in another difficulty that sometimes

manifests itself with this type of cord circuit. If two ordinary

magneto telephones are connected to the two ends of this cord circuit,

it is obvious that when one of the subscribers has hung up his

receiver and the other subscriber rings off, the bell of the other

subscriber will very likely be rung even though the clearing-out drop

operates properly; it would be better in any event not to have this

other subscriber's bell rung, for he may understand it to be a recall

to his telephone. When, however, a party line is connected through

such a cord circuit to an ordinary line having bridging instruments,

for instance, the difficulty due to ringing off becomes even greater.

When the subscriber on the magneto line operates his generator to give

the clearing-out signal, he is very likely to ring some of the bells

on the other line and this, of course, is an undesirable thing. This

may happen even in the case of harmonic bells on the party line, since

it is possible that the subscriber on the magneto line in turning his

generator will, at some phase of the operation, strike just the proper

frequency to ring some one of the bells on the harmonic party line. It

is obvious, therefore, that there is a real need for a cord circuit

that will prevent _through ringing._

One way of eliminating the through-ringing difficulty in the type of

cord circuit shown in Fig. 281 would be to use such a very low-wound

clearing-out drop that it would practically short-circuit the line

with respect to ringing currents and prevent them from passing on to

the other line. This, however, is not a good thing to do, since a

winding sufficiently low to shunt the effective ringing current would

also be too low for good telephone transmission.

[Illustration: Fig. 282. Series Drop-Cord Circuit]