CHAPTER XVII

LOCK-OUT PARTY-LINE SYSTEMS

The party-line problem in rural districts is somewhat different from

that within urban limits. In the latter cases, owing to the closer

grouping of the subscribers, it is not now generally considered

desirable, even from the standpoint of economy, to place more than

four subscribers on a single line. For such a line selective ringing

is simple, both from the standpoint of apparatus and operation; and

moreover owing to the small number of stations on a line, and the

small amount of traffic to and from such subscribers as usually take

party-line service, the interference between parties on the same line

is not a very serious matter.

For rural districts, particularly those tributary to small towns,

these conditions do not exist. Owing to the remoteness of the stations

from each other it is not feasible from the standpoint of line cost to

limit the number of stations to four. A much greater number of

stations is employed and the confusion resulting is distressing not

only to the subscribers themselves but also to the management of the

company. There exists then the need of a party-line system which will

give the limited user in rural districts a service, at least

approaching that which he would get if served by an individual line.

The principal investment necessary to provide facilities for telephone

service is that required to produce the telephone line. In many cases

the cost of instruments and apparatus is small in comparison with the

cost of the line. By far the greater number of subscribers in rural

districts are those who use their instruments a comparatively small

number of times a day, and to maintain an expensive telephone line for

the exclusive use of one such subscriber who will use it but a few

minutes each day is on its face an economic waste. As a result, where

individual line service is practiced exclusively one of two things

must be true: either the average subscriber pays more for his service

than he should, or else the operating company sells the service for

less than it costs, or at best for an insufficient profit. Both of

these conditions are unnatural and cannot be permanent.

The party-line method of giving service, by which a single line is

made to serve a number of subscribers, offers a solution to this

difficulty, but the ordinary non-selective or even selective party

line has many undesirable features if the attempt is made to place on

it such a large number of stations as is considered economically

necessary in rural work. These undesirable features work to the

detriment of both the user of the telephone and the operating company.

Many attempts have been made to overcome these disadvantages of the

party line in sparsely settled communities, by producing what are

commonly called lock-out systems. These, as their name implies, employ

such an arrangement of parts that when the line is in use by any two

parties, all other parties are locked out from the circuit and cannot

gain access to it until the parties who are using it are through.

System after system for accomplishing this purpose has been announced

but for the most part these have involved such a degree of complexity

and have introduced so many undesirable features as to seriously

affect the smooth operation of the system and the reliability of the

service.

We believe, however, in spite of numerous failures, that the lock-out

selective-signaling party line has a real field of usefulness and that

operating companies as well as manufacturing companies are beginning

to appreciate this need, and as a result that the relief of the rural

subscriber from the almost intolerable service he has often had to

endure is at hand. A few of the most promising lock-out party-line

systems now before the public will, therefore, be described in some

detail.

Poole System. The Poole system is a lock-out system pure and simple,

its devices being in the nature of a lock-out attachment for

selective-signaling lines, either of the polarity or of the harmonic

type wherein common-battery transmission is employed. It will be here

described as employed in connection with an ordinary harmonic-ringing

system.

In Fig. 188 there is shown a four-station party line equipped with

Poole lock-out devices, it being assumed that the ringers at each

station are harmonic and that the keys at the central office are the

ordinary keys adapted to impress the proper frequency on the line for

ringing any one of the stations. In addition to the ordinary talking

and ringing apparatus at each subscriber's station, there is a relay

of special form and also a push-button key.

[Illustration: Fig. 188. Poole Lock-Out System]

Each of the relays has two windings, one of high resistance and the

other of low resistance. Remembering that the system to which this

device is applied is always a common-battery system, and that,

therefore, the normal condition of the line will be one in which there

is a difference of potential between the two limbs, it will be evident

that whenever any subscriber on a line that is not in use raises his

receiver from its hook, a circuit will be established from the upper

contact of the hook through the lever of the hook to the

high-resistance winding _1_ of the relay and thence to the other side

of the line by way of wire _6_. This will result in current passing

through the high-resistance winding of the relay and the relay will

pull up its armature. As soon as it does so it establishes two other

circuits by the closure of the relay armature against the contacts _4_

and _5_.

The closing of the contact _4_ establishes a circuit from the upper

side of the line through the upper contact of the switch hook, thence

through the contacts of the push button _3_, thence through the

low-resistance winding _2_ of the relay to the terminal _4_, thence

through the relay armature and the transmitter to the lower side of

the line. This low-resistance path across the line serves to hold the

relay armature attracted and also to furnish current to the

transmitter for talking. The establishment of this low-resistance path

across the line does another important thing, however; it practically

short-circuits the line with respect to all the high-resistance relay

windings, and thus prevents any of the other high-resistance relay

windings from receiving enough current to actuate them, should the

subscriber at any other station remove his receiver from the hook in

an attempt to listen in or to make a call while the line is in use. As

a subscriber can only establish the proper conditions for talking and

listening by the attraction of this relay armature at his station, it

is obvious that unless he can cause the pulling up of his relay

armature he can not place himself in communication with the line.

The second thing that is accomplished by the pulling up of the relay

armature is the closure of the contacts _5_, and that completes the

talking circuit through the condenser and receiver across the line in

an obvious fashion. The result of this arrangement is that it is the

first party who raises his receiver from its hook who is enabled to

successfully establish a connection with the line, all subsequent

efforts, by other subscribers, failing to do so because of the fact

that the line is short-circuited by the path through the

low-resistance winding and the transmitter of the station that is

already connected with the line.

A little target is moved by the action of the relay so that a visual

indication is given to the subscriber in making a call to show whether

or not he is successful in getting the use of the line. If the relay

operates and he secures control of the line, the target indicates the

fact by its movement, while if someone else is using the line and the

relay does not operate, the target, by its failure to move, indicates

that fact.

When one party desires to converse with another on the same line, he

depresses the button _3_ at his station until after the called party

has been rung and has responded. This holds the circuit of his

low-resistance winding open, and thus prevents the lock-out from

becoming effective until the called party is connected with the line.

The relay armature of the calling party does not fall back with the

establishment of the low-resistance path at the called station,

because, even though shunted, it still receives sufficient current to

hold its armature in its attracted position. After the called party

has responded, the button at the calling station is released and both

low-resistance holding coils act in multiple.

[Illustration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate

Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.

Siemens-Halske Equipment. Note Horizontal Disposal of Multiple Jack

Field.]

No induction coil is used in this system and the impedance of the

holding coil is such that incoming voice currents flow through the

condenser and the receiver, which, by reference to the figure, will be

seen to be in shunt with the holding coil. The holding coil is in

series with the local transmitter, thus making a circuit similar to

that of the Kellogg common-battery talking circuit already discussed.

A possible defect in the use of this system is one that has been common

to a great many other lock-out systems, depending for their operation

on the same general plan of action. This appears when the instruments

are used on a comparatively long line. Since the locking-out of all the

instruments that are not in use by the one that is in use depends on

the low-resistance shunt that is placed across the line by the

instrument that is in use, it is obvious that, in the case of a long

line, the resistance of the line wire will enter into the problem in

such a way as to tend to defeat the locking-out function in some cases.

Thus, where the first instrument to use the line is at the remote end

of the line, the shunting effect that this instrument can exert with

respect to another instrument near the central office is that due to

the resistance of the line plus the resistance of the holding coil at

the end instrument. The resistance of the line wire may be so high as

to still allow a sufficient current to flow through the high-resistance

coil at the nearer station to allow its operation, even though the more

remote instrument is already in use.

Coming now to a consideration of the complete selective-signaling

lock-out systems, wherein the selection of the party and the locking

out of the others are both inherent features, a single example of the

step-by-step, and of the broken-line selective lock-out systems will

be discussed.

Step-by-Step System. The so-called K.B. system, manufactured by the

Dayton Telephone Lock-out Manufacturing Company of Dayton, Ohio,

operates on the step-by-step principle. The essential feature of the

subscriber's telephone equipment in this system is the step-by-step

actuating mechanism which performs also the functions of a relay. This

device consists of an electromagnet having two cores, with a permanent

polarizing magnet therebetween, the arrangement in this respect being

the same as in an ordinary polarized bell. The armature of this magnet

works a rocker arm, which, besides stepping the selector segment

around, also, under certain conditions, closes the bell circuit and

the talking circuit, as will be described.

[Illustration: Fig. 189. K.B. Lock-Out System]

Referring first to Fig. 189, which shows in simplified form a

four-station K.B. lock-out line, the electromagnet is shown at _1_ and

the rocker arm at _2_. The ratchet _3_ in this case is not a complete

wheel but rather a segment thereof, and it is provided with a series

of notches of different depths. It is obvious that the depth of the

notches will determine the degree of movement which the upper end of

the rocker arm may have toward the left, this being dependent on the

extent to which the pawl _6_ is permitted to enter into the segment.

The first or normal notch, _i.e._, the top notch, is always of such a

depth that it will allow the rocker-arm lever _2_ to engage the

contact lever _4_, but will not permit the rocker arm to swing far

enough to the left to cause that contact to engage the bell contact

_5_. As will be shown later, the condition for the talking circuit to

be closed is that the rocker arm _2_ shall rest against the contact

_4_; and from this we see that the normal notch of each of the

segments _3_ is of such a depth as to allow the talking circuit at

each station to be closed. The next notch, _i.e._, the second one in

each disk, is always shallow, as are all of the other notches except

one. A deep notch is placed on each disk anywhere from the third to

the next to the last on the segment. This deep notch is called the

_selective notch_, and it is the one that allows of contact being made

with the ringer circuit of that station when the pawl _6_ drops into

it. The position of this notch differs on all of the segments on a

line, and obviously, therefore, the ringer circuit at any station may

be closed to the exclusion of all the others by stepping all of the

segments in unison until the deep notch on the segment of the desired

station lies opposite to the pawl _6_, which will permit the rocker

arm _2_ to swing so far to the left as to close not only the circuit

between _2_ and _4_, but also between _2_, _4_, and _5_. In this

position the talking and the ringing circuits are both closed.

The position of the deepest notch, _i.e._, the selective notch, on the

circumference of the segment at any station depends upon the number of

that station; thus, the segment of Station 4 will have a deep notch in

the sixth position; the segment for Station 9 will have a deep notch

in the eleventh position; the segment for any station will have a deep

notch in the position corresponding to the number of that station plus

two.

From what has been said, therefore, it is evident that the first, or

normal, notch on each segment is of such a depth as to allow the

moving pawl _6_ to fall to such a depth in the segment as to permit

the rocker arm _2_ to close the talking circuit only. All of the other

notches, except one, are comparatively shallow, and while they permit

the moving pawl _6_ under the influence of the rocker arm _2_ to move

the segment _3_, yet they do not permit the rocker arm _2_ to move so

far to the left as to close even the talking circuit. The exception is

the deep notch, or selective notch, which is of such depth as to

permit the pawl _6_ to fall so far into the segment as to allow the

rocker arm _2_ to close both the talking and the ringing circuits.

Besides the moving pawl _6_ there is a detent pawl _7_. This always

holds the segment _3_ in the position to which it has been last moved

by the moving pawl _6_.

The actuating magnet _1_, as has been stated, is polarized and when

energized by currents in one direction, the rocker arm moves the pawl

_6_ so as to step the segment one notch. When this relay is energized

by current in the opposite direction, the operation is such that both

the moving pawl _6_ and the detent pawl _7_ will be pulled away from

the segment, thus allowing the segment to return to its normal position

by gravity. This is accomplished by the following mechanism: An

armature stop is pivoted upon the face of the rocker arm so as to swing

in a plane parallel to the pole faces of the relay, and is adapted,

when the relay is actuated by selective impulses of one polarity, to be

pulled towards one of the pole faces where it acts, through impact with

a plate attached to the pole face of the relay, as a limiting means

for the motion of the rocker arm when the rocker arm is actuated by the

magnet. When, however, the relay is energized by current in the

opposite direction, as on a releasing impulse, the armature stop swings

upon its pivot towards the opposite pole face, in which position the

lug on the end of the armature stop registers with a hole in the plate

on the relay, thus allowing the full motion of the rocker arm when it

is attracted by the magnet. This motion of the rocker arm withdraws the

detent pawl from engagement with the segment as well as the moving

pawl, and thereby permits the segment to return to its normal position.

As will be seen from Fig. 189, each of the relay magnets _1_ is

permanently bridged across the two limbs of the line.

Each station is provided with a push button, not shown, by means of

which the subscriber who makes a call may prevent the rocker arm of

his instrument from being actuated while selective impulses are being

sent over the line. The purpose of this is to enable one party to make

a call for another on the same line, depressing his push button while

the operator is selecting and ringing the called party. The segment at

his own station, therefore, remains in its normal position, in which

position, as we have already seen, his talking circuit is closed; all

of the other segments are, however, stepped up until the ringing and

talking circuits of the desired station are in proper position, at

which time ringing current is sent over the line. The segments in Fig.

189, except at Station C, are shown as having been stepped up to the

sixth position, which corresponds to the ringing position of the

fourth station, or Station D. The condition shown in this figure

corresponds to that in which the subscriber at Station C originated

the call and pressed his button, thus retaining his own segment in its

normal position so that the talking circuits would be established with

Station D.

When the line is in normal position any subscriber may call central by

his magneto generator, not shown in Fig. 189, which will operate the

drop at central, but will not operate any of the subscribers' bells,

because all bell circuits are normally open. When a subscriber desires

connection with another line, the operator sends an impulse back on

the line which steps up and locks out all instruments except that of

the calling subscriber.

[Illustration: Fig. 190. K.B. Lock-Out Station]

A complete K.B. lock-out telephone is shown in Fig. 190. This is the

type of instrument that is usually furnished when new equipment is

ordered. If, however, it is desired to use the K.B. system in

connection with telephones of the ordinary bridging type that are

already in service, the lock-out and selective mechanism, which is

shown on the upper inner face of the door in Fig. 190, is furnished

separately in a box that may be mounted close to the regular telephone

and connected thereto by suitable wires, as shown in Fig. 191. It is

seen that this instrument employs a local battery for talking and also

a magneto generator for calling the central office.

The central-office equipment consists of a dial connected with an

impulse wheel, together with suitable keys by which the various

circuits may be manipulated. This dial and its associated mechanism

may be mounted in the regular switchboard cabinet, or it may be

furnished in a separate box and mounted alongside of the cabinet in

either of the positions shown at _1_ or _2_ of Fig. 192.

In order to send the proper number of impulses to the line to call a

given party, the operator places her finger in the hole in the dial

that bears the number corresponding to the station wanted and rotates

the dial until the finger is brought into engagement with the fixed

stop shown at the bottom of the dial in Fig. 192. The dial is then

allowed to return by the action of a spring to its normal position,

and in doing so it operates a switch within the box to make and break

the battery circuit the proper number of times.

_Operation._ A complete description of the operation may now be had in

connection with Fig. 193, which is similar to Fig. 189, but contains

the details of the calling arrangement at the central office and also

of the talking circuits at the various subscribers' stations.

[Illustration: Fig. 191. K.B. Lock-Out Station]

Referring to the central-office apparatus the usual ringing key is

shown, the inside contacts of which lead to the listening key and to

the operator's telephone set as in ordinary switchboard practice.

Between the outside contact of this ringing key and the ringing

generator there is interposed a pair of contact springs _8-8_ and

another pair _9-9_. The contact springs _8_ are adapted to be moved

backward and forward by the impulse wheel which is directly controlled

by the dial under the manipulation of the operator. When these springs

_8_ are in their normal position, the ringing circuit is continued

through the release-key springs _9_ to the ringing generator. These

springs _8_ occupy their normal position only when the dial is in its

normal position, this being due to the notch _10_ in the contact wheel.

At all other times, _i.e._, while the impulse wheel is out of its

normal position, the springs _8-8_ are either depressed so as to engage

the lower battery contacts, or else held in an intermediate position so

as to engage neither the battery contacts nor the generator contacts.

[Illustration: Fig. 192. Calling Apparatus K.B. System]

When it is desired to call a given station, the operator pulls the

subscriber's number on the dial and holds the ringing key closed,

allowing the dial to return to normal. This connects the impulse

battery to the subscriber's line as many times as is required to move

the subscriber's sectors to the proper position, and in such direction

as to cause the stepping movement of the various relays. As the

impulse wheel comes to its normal position, the springs _8_,

associated with it, again engage their upper contacts, by virtue of

the notch _10_ in the impulse wheel, and this establishes the

connection between the ringing generator and the subscriber's line,

the ringing key being still held closed. The pulling of the

transmitter dial and holding the ringing key closed, therefore, not

only sends the stepping impulses to line, but also follows it by the

ringing current. The sending of five impulses to line moves all of the

sectors to the sixth notch, and this corresponds to the position

necessary to make the fourth station operative. Such a condition is

shown in Fig. 193, it being assumed that the subscriber at Station C

originated the call and pressed his own button so as to prevent his

sector from being moved out of its normal position. As a result of

this, the talking circuit at Station C is left closed, and the talking

and the ringing circuit of Station D, the called station, are closed,

while both the talking and the ringing circuits of all the other

stations are left open. Station D may, therefore, be rung and may

communicate with Station C, while all of the other stations on the

line are locked out, because of the fact that both their talking and

ringing circuits are left open.

[Illustration: Fig. 193. Circuit K.B. System]

When conversation is ended, the operator is notified by the usual

clearing-out signal, and she then depresses the release button, which

brings the springs _9_ out of engagement with the generator contact

but into engagement with the battery contact in such relation as to

send a battery current on the line in the reverse direction from that

sent out by the impulse wheel. This sends current through all of the

relays in such direction as to withdraw both the moving and the

holding pawls from the segments and thus allow all of the segments to

return to their normal positions. Of course, in thus establishing the

release current, it is necessary for the operator to depress the

ringing key as well as the release key.

A one-half microfarad condenser is placed in the receiver circuit at

each station so that the line will not be tied up should some

subscriber inadvertently leave his receiver off its hook. This permits

the passage of voice currents, but not of the direct currents used in

stepping the relays or in releasing them.

The circuit of Fig. 193 is somewhat simplified from that in actual

practice, and it should be remembered that the hook switch, which is

not shown in this figure, controls in the usual way the continuity of

the receiver and the transmitter circuits as well as of the generator

circuits, the generator being attached to the line as in an ordinary

telephone.

Broken-Line System. The broken-line method of accomplishing

selective signaling and locking-out on telephone party lines is due to

Homer Roberts and his associates.

[Illustration: Fig. 194. Roberts Latching Relay]

To understand just how the principles illustrated in Figs. 186 and 187

are put into effect, it will be necessary to understand the latching

relay shown diagrammatically in its two possible positions in Fig. 194,

and in perspective in Fig. 195. Referring to Fig. 194, the left-hand

cut of which shows the line relay in its normal position, it is seen

that the framework of the device resembles that of an ordinary

polarized ringer. Under the influence of current in one direction

flowing through the left-hand coil, the armature of this device

depresses the hard rubber stud _4_, and the springs _1_, _2_, and _3_

are forced downwardly until the spring _2_ has passed under the latch

carried on the spring _5_. When the operating current through the coil

_6_ ceases, the pressure of the armature on the spring _1_ is relieved,

allowing this spring to resume its normal position and spring _3_ to

engage with spring _2_. The spring _2_ cannot rise, since it is held by

the latch _5_, and the condition shown in the right-hand cut of Fig.

194 exists. It will be seen that the spring _2_ has in this operation

carried out just the same function as the switch lever performed as

described in connection with Figs. 186 and 187. An analysis of this

action will show that the normal contact between the springs _1_ and

_2_, which contact controls the circuit through the relay coil and the

bell, is not broken until the coil _6_ is de-energized, which means

that the magnet is effective until it has accomplished its work. It is

impossible, therefore, for this relay to cut itself out of circuit

before it has caused the spring _2_ to engage under the latch _5_. If

current of the proper direction were sent through the coil _7_ of the

relay, the opposite end of the armature would be pulled down and the

hard rubber stud at the left-hand end of the armature would bear

against the bent portion of the spring _5_ in such manner as to cause

the latch of this spring to release the spring _2_ and thus allow the

relay to assume its normal, or unlatched, position.

A good idea of the mechanical construction of this relay may be

obtained from Fig. 195. The entire selecting function of the Roberts

system is performed by this simple piece of apparatus at each station.

[Illustration: Fig. 195. Roberts Latching Relay]

The diagram of Fig. 196 shows, in simplified form, a four-station

line, the circuits being given more in detail than in the diagrams of

Chapter XVI.

It will be noticed that the ringer and the relay coil _6_ at the

first station are bridged across the sides of the line leading to the

central office. In like manner the bell and the relay magnets are

bridged across the two limbs of the line leading into each succeeding

station, but this bridge at each of the stations beyond Station A is

ineffective because the line extension _R__{x} is open at the next

station nearest the central office.

[Illustration: Fig. 196. Simplified Circuits of Roberts System]

In order to ring Station A it is only necessary to send out ringing

current from the central office. This current is in such direction as

not to cause the operation of the relay, although it passes through

the coil _6_. If, on the other hand, it is desired to ring Station B,

a preliminary impulse would be sent over the metallic circuit from the

central office, which impulse would be of such direction as to operate

the relay at Station A, but not to operate the bell at that station.

The operation of the relay at Station A causes the spring _2_ of this

relay to engage the spring _3_, thus extending the line on to the

second station. After the spring _2_ at Station A has been forced into

contact with the spring _3_, it is caught by the latch of the spring

_5_ and held mechanically. When the impulse from the central office

ceases, the spring _1_ resumes its normal position, thus breaking the

bridge circuit through the bell at that station. It is apparent now

that the action of coil _6_ at Station A has made the relay powerless

to perform any further action, and at the same time the line has been

extended on to the second station. A second similar impulse from the

central office will cause the relay at Station B to extend the line on

to Station C, and at the same time break the circuit through the

operating coil and the bell at Station B. In this way any station may

be picked out by sending the proper number of impulses to operate the

line relays of all the stations between the station desired and the

central office, and having picked out a station it is only necessary

to send out ringing current, which current is in such direction as to

ring the bell but not to operate the relay magnet at that station.

In Fig. 197, a four-station line, such as is shown in Fig. 196, is

illustrated, but the condition shown in this is that existing when two

preliminary impulses have been sent over the line, which caused the

line relays at Station A and Station B to be operated. The bell at

Station C is, therefore, the only one susceptible to ringing current

from the central office.

[Illustration: Fig. 197. Simplified Circuits of Roberts System]

Since only one bell and one relay are in circuit at any one time, it

is obvious that all of the current that passes over the line is

effective in operating a single bell or relay only. There is no

splitting up of the current among a large number of bells as in the

bridging system of operating step-by-step devices, which method

sometimes so greatly reduces the effective current for each bell that

it is with great difficulty made to respond. All the energy available

is applied directly to the piece of apparatus at the time it is being

operated. This has a tendency toward greater surety of action, and the

adjustment of the various pieces of apparatus may be made with less

delicacy than is required where many pieces of apparatus, each having

considerable work to do, must necessarily be operated in multiple.

The method of unlatching the relays has been briefly referred to.

After a connection has been established with a station in the manner

already described, the operator may clear the line when it is proper

to do so by sending impulses of such a nature as to cause the line

relays of the stations beyond the one chosen to operate, thus

continuing the circuit to the end of the line. The operation of the

line relay at the last station brings into circuit the coil _8_, Figs.

196 and 197, of a grounding device. This is similar to the line relay,

but it holds its operating spring in a normally latched position so as

to maintain the two limbs of the line disconnected from the ground.

The next impulse following over the metallic circuit passes through

the coil _8_ and causes the operation of this grounding device which,

by becoming unlatched, grounds the limb _L_ of the line through the

coil _8_. This temporary ground at the end of the line makes it

possible to send an unlocking or restoring current from the central

office over the limb _L_, which current passes through all of the

unlocking coils _7_, shown in Figs. 194, 196, and 197, thus causing

the simultaneous unlocking of all of the line relays and the

restoration of the line to its normal condition, as shown in Fig. 196.

[Illustration: Fig. 198. Details of Latching Relay Connections]

As has been stated, the windings _7_ on the line relays are the

unlatching windings. In Figs. 196 and 197, for the purpose of

simplicity, these windings are not shown connected, but as a matter of

fact each of them is included in series in the continuous limb _L_ of

the line. This would introduce a highly objectionable feature from the

standpoint of talking over the line were it not for the balancing

coils _7_^{1}, each wound on the same core as the corresponding

winding _7_, and each included in series in the limb _R_ of the line,

and in such direction as to be differential thereto with respect to

currents passing in series over the two limbs of the line.

The windings _7_ are the true unlocking windings, while the windings

_7_^{1} have no other function than to neutralize the inductive

effects of these unlocking windings necessarily placed in series in

the talking circuit. All of these windings are of low ohmic

resistance, a construction which, as has previously been noted, brings

about the desired effect without introducing any self-induction in

the line, and without producing any appreciable effect upon the

transmission. A study of Fig. 198 will make clear the connections of

these unlocking and balancing windings at each station.

The statement of operation so far given discloses the general method

of building up the line in sections in order to choose any party and

of again breaking it up into sections when the conversation is

finished. It has been stated that the same operation which selects the

party wanted also serves to give that party the use of the line and to

lock the others off. That this is true will be understood when it is

stated that the ringer is of such construction that when operated to

ring the subscriber wanted, it also operates to unlatch a set of

springs similar to those shown in Fig. 194, this unlatching causing

the proper connection of the subscriber's talking circuit across the

limbs of the line, and also closing the local circuit through his

transmitter. The very first motion of the bell armature performs this

unlatching operation after which the bell behaves exactly as an

ordinary polarized biased ringer.

[Illustration: Fig. 199. Broken-Back Ringer]

The construction of this ringer is interesting and is shown in its two

possible positions in Fig. 199. The group of springs carried on its

frame is entirely independent of the movement of the armature during

the ringing operation. With reversed currents, however, the armature

is moved in the opposite direction from that necessary to ring the

bells, and this causes the latching of the springs into their normal

position. In order that this device may perform the double function

of ringer and relay the tapper rod of the bell is hinged on the

armature so as to partake of the movements of the armature in one

direction only. This has been called by the inventor and engineers of

the Roberts system a _broken-back ringer_, a name suggestive of the

movable relation between the armature and the tapper rod. The

construction of the ringer is of the same nature as that of the

standard polarized ringer universally employed, but a hinge action

between the armature and the tapper rod, of such nature as to make the

tapper partake positively of the movements of the armature in one

direction, but to remain perfectly quiescent when the armature moves

in the other direction, is provided.

[Illustration: Fig. 200. Details of Ringer Connection]

How this broken-back ringer controls the talking and the locking-out

conditions may best be understood in connection with Fig. 200. The

ringer springs are normally latched at all stations. Under these

conditions the receiver is short-circuited by the engagement of

springs _10_ and _11_, the receiver circuit is open between springs

_10_ and _12_, and the local-battery circuit is open between springs

_9_ and _12_. The subscribers whose ringers are latched are,

therefore, locked out in more ways than one.

When the bell is rung, the first stroke it makes unlatches the springs,

which assume the position shown in the right-hand cut of Fig. 199, and

this, it will be seen from Fig. 200, establishes proper conditions for

enabling the subscriber to transmit and to receive speech.

The hook switch breaks both transmitter and receiver circuits when down

and in raising it establishes a momentary circuit between the ground

and the limb _L_ of the line, both upper and lower hook contacts

engaging the hook lever simultaneously during the rising of the hook.

The mechanism at the central office by which selection of the proper

station is made in a rapid manner is shown in Fig. 201. It has already

been stated that the selection of the proper subscriber is brought

about by the sending of a predetermined number of impulses from the

central office, these impulses passing in one direction only and over

the metallic circuit. After the proper party has been reached, the

ringing current is put on in the reverse direction.

[Illustration: Fig. 201. Central-Office Impulse Transmitter]

The operator establishes the number of impulses to be sent by placing

the pointer opposite the number on the dial corresponding to the

station wanted. The ratchet wheel is stepped around automatically by

each impulse of current from an ordinary pole changer such as is

employed in ringing biased bells. When the required number of impulses

has been sent, a projection, carried on a group of springs, drops into

a notch on the drum of the selector shaft, which operation instantly

stops the selecting current impulses and at the same time throws on

the ringing current which consists of impulses in the reverse

direction. So rapidly does this device operate that it will readily

follow the impulses of an ordinary pole changer, even when this is

adjusted to its maximum rate of vibration.

[Illustration: VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD]

_Operation._ Space will not permit a full discussion of the details of

the central-office selective apparatus, but a general resumé of the

operation of the system may now be given, with the aid of Fig. 202,

which shows a four-station line with the circuits of three of the

stations somewhat simplified. In this figure Station A, Station B,

and Station D are shown in their locked-out positions, A and B having

been passed by the selection and ringing of Station C, while Station D

is inoperative because it was not reached in the selection and the

line is still broken at Station C. Station C, therefore, has

possession of the line.

When the subscriber at Station C raised his receiver in order to call

central, a "flash" contact was made as the hook moved up, which

momentarily grounded the limb _L_ of the line. (See Fig. 200.) This

"flash" contact is produced by the arrangement of the hook which

assures that the lower contact shall, by virtue of its flexibility,

follow up the hook lever until the hook lever engages the upper

contact, after which the lower contact breaks. This results in the

momentary connection of both the upper and the lower contacts of the

hook with the lever, and, therefore, the momentary grounding of the

limb _L_ of the line. This limb always being continuous serves, when

this "flash" contact is made, to actuate the line signal at the

central office.

[Illustration: Fig. 202. Circuits of Roberts Line]

Since, however, all parties on the line are normally locked out of

talking circuits, some means must be provided whereby the operator may

place the signaling party in talking connection and leave all the

other instruments on the line in their normally locked-out position.

In fact, the operator must be able automatically to pick out the

station that signaled in, and operate the ringer to unlatch the

springs controlling the talking circuit of that station. Accordingly

the operator sends impulses on the line, from a grounded battery,

which are in the direction to operate the line relays and to continue

the line circuit to the station calling. When, after a sufficient

number of impulses, this current reaches that station it finds a path

to ground from the limb _L_. This path is made possible by the fact

that the subscriber's receiver is off its hook at that station. In

order to understand just how this ground connection is made, it must

be remembered that each of the ringer magnets is energized with each

selecting impulse, but in such a direction as not to ring the bells,

it being understood that all of the ringer mechanisms are normally

latched. When the selecting impulse for Station C arrives, it passes

through the ringer and the selecting relay coils at that station and

starts to operate the remainder of the ringers sufficiently to cause

the spring _12_ to engage the spring _13_. This establishes the ground

connection from the limb _L_ of the line, the circuit being traced

through limb _L_ through the upper contact of the switch, thence

through springs _12_ and _13_ to ground, and this, before the line

relay has time to latch, operates the quick-acting relay at the

central office, which acts to cut off further impulses, and thus

automatically stops at the calling station. Ringing current in the

opposite direction is then sent to line; this unlatches the ringer

springs and places the calling subscriber in talking circuit. When the

operator has communicated with the calling subscriber, and found, for

example, that another party on another similar line is desired, she

turns the dial pointer on the selector to the number corresponding to

the called-for party's number on that line, and presses the signal

key. Pressing this key causes impulses to "run down the line,"

selecting the proper party and ringing his bell in the manner already

described. The connection between the two parties is then established,

and no one else can in any possible way, except by permission of the

operator, obtain access to the line.

It is obvious that some means must be provided for restoring the

selecting relays to normal after a conversation is finished. By

referring to Fig. 194 it will be seen that the upper end of the latch

spring _5_ is bent over in such a manner that when the armature is

attracted by current flowing through the coil _7_, the knob on the

left-hand end of the armature on rising engages with the bent cam

surface and forces back the latch, permitting spring _2_ to return to

its normal position.

To restore the line the operator sends out sufficient additional

selective impulses to extend the circuit to the end of the line, and

thus brings the grounder into circuit. The winding of the grounder is

connected in such a manner that the next passing impulse throws off

its latch, permitting the long spring to contact with the ground

spring. The operator now sends a grounded impulse over the continuous

limb _L_ of the line which passes through the restoring coils _7_ at

all the stations and through the right-hand coil of the grounding

device to ground. The selecting relays are, therefore, simultaneously

restored to normal. The grounder is also energized and restored to its

normal position by the same current.

If a party in calling finds that his own line is busy and he cannot

get central, he may leave his receiver off its hook. When the party

who is using the line hangs up his receiver the fact that another

party desires a connection is automatically indicated to the operator,

who then locks out the instrument of the party who has just finished

conversation and passes his station by. When the operator again throws

the key, the waiting subscriber is automatically selected in the same

manner as was the first party. If there are no subscribers waiting for

service, the stop relay at central will not operate until the grounder

end of the line is unlatched, the selecting relays being then restored

automatically to normal.

The circuits are so organized that at all times whether the line is

busy or not, the movement up and down of the switch hook, at any

sub-station, operates a signal before the operator. Such a movement,

when made slowly and repeatedly, indicates to the operator that the

subscriber has an emergency call and she may use her judgment as to

taking the line away from the parties who are using it, and finding

out what the emergency call is for. If the operator finds that the

subscriber has misused this privilege of making the emergency call,

she may restore the connection to the parties previously engaged in

conversation.

One of the salient points of this Roberts system is that the operator

always has control of the line. A subscriber is not able even to use

his own battery till permitted to do so. A subscriber who leaves his

receiver off its hook in order that he may be signaled by the operator

when the line is free, causes no deterioration of the local battery

because the battery circuit is held open by the switch contacts

carried on the ringer. It cannot be denied, however, that this system

is complicated, and that it has other faults. For instance, as

described herein, both sides of the line must be looped into each

subscriber's station, thus requiring four drop, or service, wires

instead of two. It is possible to overcome this objection by placing

the line relays on the pole in a suitably protected casing, in which

case it is sufficient to run but two drop wires from the nearer line

to station. There are undoubtedly other objections to this system, and

yet with all its faults it is of great interest, and although radical

in many respects, it teaches lessons of undoubted value.

CHAPTER XVIII

ELECTRICAL HAZARDS

All telephone systems are exposed to certain electrical hazards. When

these hazards become actively operative as causes, harmful results

ensue. The harmful results are of two kinds: those causing damage to

property and those causing damage to persons. The damage to persons

may be so serious as to result in death. Damage to property may

destroy the usefulness of a piece of apparatus or of some portion of

the wire plant. Or the property damage may initiate itself as a harm

to apparatus or wiring and may result in greater and extending damage

by starting a fire.

Electrical currents which endanger life and property may be furnished

by natural or artificial causes. Natural electricity which does such

damage usually displays itself as lightning. In rare cases, currents

tending to flow over grounded lines because of extraordinary

differences of potential between sections of the earth's surface have

damaged apparatus in such lines, or only have been prevented from

causing such damage by the operation of protective devices.

Telegraph and telephone systems have been threatened by natural

electrical hazards since the beginning of the arts and by artificial

electrical hazards since the development of electric light and power

systems. At the present time, contrary to the general supposition, it

is in the artificial, and not in the natural electrical hazards that

the greater variety and degree of danger lies.

Of the ways in which artificial electricity may injure a telephone

system, the entrance of current from an external electrical power

system is a greater menace than an abnormal flow of current from a

source belonging to the telephone system itself. Yet modern practice

provides opportunities for a telephone system to inflict damage upon

itself in that way. Telephone engineering designs need to provide

means for protecting _all_ parts of a system against damage, from

external ("foreign") as well as internal ("domestic") hazards, and to

cause this protection to be inclusive enough to protect persons

against injury and property from damage by any form of overheating or

electrolytic action.

A part of a telephone system for which there is even a remote

possibility of contact with an external source of electrical power,

whether natural or artificial, is said to be _exposed_ to electrical

hazard. The degree or character of possible contact or other

interference often is referred to in relative terms of _exposure_. The

same terms are used concerning inductive relations between circuits.

The whole tendency of design, particularly of wire plants, is to

arrange the circuits in such a way as to limit the exposure as greatly

as possible, the intent being to produce a condition in which all

parts of the system will be _unexposed_ to hazards.

Methods of design are not yet sufficiently advanced for any plant to

be formed of circuits wholly unexposed, so that protective means are

required to safeguard apparatus and circuits in case the hazard,

however remote, becomes operative.

Lightning discharges between the clouds and earth frequently charge

open wires to potentials sufficiently high to damage apparatus; and

less frequently, to destroy the wires of the lines themselves.

Lightning discharges between clouds frequently induce charges in lines

sufficient to damage apparatus connected with the lines. Heavy rushes

of current in lines, from lightning causes, occasionally induce

damaging currents in adjacent lines not sufficiently exposed to the

original cause to have been injured without this induction. The

lightning hazard is least where the most lines are exposed. In a small

city with all of the lines formed of exposed wires and all of them

used as grounded circuits, a single lightning discharge may damage

many switchboard signals and telephone ringers if there be but 100 or

200 lines, while the damage might have been nothing had there been 800

to 1,000 lines in the same area.

Means of protecting lines and apparatus against damage by lightning

are little more elaborate than in the earliest days of telegraph

working. They are adequate for the almost entire protection of life

and of apparatus.

Power circuits are classified by the rules of various governing bodies

as high-potential and low-potential circuits. The classification of

the National Board of Fire Underwriters in the United States defines

low-potential circuits as having pressures below 550 volts;

high-potential circuits as having pressures from 550 to 3,500 volts,

and extra high-potential circuits as having pressures above 3,500

volts. Pressures of 100,000 volts are becoming more common. Where

power is valuable and the distance over which it is to be transmitted

is great, such high voltages are justified by the economics of the

power problem. They are a great hazard to telephone systems, however.

An unprotected telephone system meeting such a hazard by contact will

endanger life and property with great certainty. A very common form of

distribution for lighting and power purposes is the three-wire system

having a grounded neutral wire, the maximum potential above the earth

being about 115 volts.

Telephone lines and apparatus are subject to damage by any power

circuit whether of high or low potential. The cause of property damage

in all cases is the flow of current. Personal damage, if it be death

from shock, ordinarily is the result of a high potential between two

parts of the body. The best knowledge indicates that death uniformly

results from shock to the heart. It is believed that death has

occurred from shock due to pressure as low as 100 volts. The critical

minimum voltage which can not cause death is not known. A good rule is

never willingly to subject another person to personal contact with any

electrical pressure whatever.

Electricity can produce actions of four principal kinds:

physiological, thermal, chemical, and magnetic. Viewing electricity as

establishing hazards, the physiological action may injure or kill

living things; the thermal action may produce heat enough to melt

metals, to char things which can be burned, or to cause them actually

to burn, perhaps with a fire which can spread; the chemical action may

destroy property values by changing the state of metals, as by

dissolving them from a solid state where they are needed into a state

of solution where they are not needed; the magnetic action introduces

no direct hazard. The greatest hazard to which property values are

exposed is the electro-thermal action; that is, the same useful

properties by which electric lighting and electric heating thrive may

produce heat where it is not wanted and in an amount greater than can

safely be borne.

The tendency of design is to make all apparatus capable of carrying

without overheating any current to which voltage within the telephone

system may subject it, and to provide the system so designed with

specific devices adapted to isolate it from currents originating

without. Apparatus which is designed in this way, adapted not only to

carry its own normal working currents but to carry the current which

would result if a given piece of apparatus were connected directly

across the maximum pressure within the telephone system itself, is

said to be self-protecting. Apparatus amply able to carry its maximum

working current but likely to be overheated, to be injured, or perhaps

to destroy itself and set fire to other things if subjected to the

maximum pressure within the system, is not self-protecting apparatus.

To make all electrical devices self-protecting by surrounding them

with special arrangements for warding off abnormal currents from

external sources, is not as simple as might appear. A lamp, for

example, which can bear the entire pressure of a central-office

battery, is not suitable for direct use in a line several miles long

because it would not give a practical signal in series with that line

and with the telephone set, as it is required to do. A lamp suitable

for use in series with such a line and a telephone set would burn out

by current from its own normal source if the line should become

short-circuited in or near the central office. The ballast referred to

in the chapter on "Signals" was designed for the very purpose of

providing rapidly-rising resistance to offset the tendency toward

rapidly-rising current which could burn out the lamp.

As another example, a very small direct-current electric motor can be

turned on at a snap switch and will gain speed quickly enough so that

its armature winding will not be overheated. A larger motor of that

kind can not be started safely without introducing resistance into the

armature circuit on starting, and cutting it out gradually as the

armature gains speed. Such a motor could be made self-protecting by

having the armature winding of much larger wire than really is

required for mere running, choosing its size great enough to carry the

large starting current without overheating itself and its insulation.

It is better, and for long has been standard practice, to use starting

boxes, frankly admitting that such motors are not self-protecting

until started, though they are self-protecting while running at normal

speeds. Such a motor, once started, may be overloaded so as to be

slowed down. So much more current now can pass through the armature

that its winding is again in danger. Overload circuit-breakers are

provided for the very purpose of taking motors out of circuit in cases

where, once up to speed, they are mechanically brought down again and

into danger. Such a circuit-breaker is a device for protecting against

an _internal_ hazard; that is, internal to the power system of which

the motor is a part.

Another example: In certain situations, apparatus intended to operate

under impulses of large current may be capable of carrying its normal

impulses successfully but incapable of carrying currents from the same

pressure continuously. Protective means may be provided for detaching

such apparatus from the circuit whenever the period in which the

current acts is not short enough to insure safety. This is cited as a

case wherein a current, normal in amount but abnormal in duration,

becomes a hazard.

The last mentioned example of damage from internal hazards brings us

to the law of the electrical generation of heat. _The greater the

current or the greater the resistance of the conductor heated or the

longer the time, the greater will he the heat generated in that

conductor._ But this generated heat varies directly as the resistance

and as the time and as the square of the current, that is, the law is

Heat generated = _C^{2}Rt_

in which _C_ = the current; _R_=the resistance of the conductor; and

_t_ = the time.

It is obvious that a protective device, such as an overload

circuit-breaker for a motor, or a protector for telephone apparatus,

needs to operate more quickly for a large current than for a small

one, and this is just what all well-designed protective devices are

intended to do. The general problem which these heating hazards

present with relation to telephone apparatus and circuits is: _To

cause all parts of the telephone system to be made so as to carry

successfully all currents which may flow in them because of any

internal or external pressure, or to supplement them by devices which

will stop or divert currents which could overheat them._

Electrolytic hazards depend not on the heating effects of currents but

on their chemical effects. The same natural law which enables primary

and secondary batteries to be useful provides a hazard which menaces

telephone-cable sheaths and other conductors. When a current leaves a

metal in contact with an electrolyte, the metal tends to dissolve into

the electrolyte. In the processes of electroplating and electrotyping,

current enters the bath at the anode, passes from the anode through

the solution to the cathode, removing metal from the former and

depositing it upon the latter. In a primary battery using zinc as the

positive element and the negative terminal, current is caused to pass,

within the cell, from the zinc to the negative element and zinc is

dissolved. Following the same law, any pipe buried in the earth may

serve to carry current from one region to another. As single-trolley

traction systems with positive trolley wires constantly are sending

large currents through the earth toward their power stations, such a

pipe may be of positive potential with relation to moist earth at some

point in its length. Current leaving it at such a point may cause its

metal to dissolve enough to destroy the usefulness of the pipe for its

intended purpose.

Lead-sheathed telephone cables in the earth are particularly exposed

to such damage by electrolysis. The reasons are that such cables often

are long, have a good conductor as the sheath-metal, and that metal

dissolves readily in the presence of most aqueous solutions when

electrolytic differences of potential exist. The length of the cables

enables them to connect between points of considerable difference of

potential. It is lack of this length which prevents electrolytic

damage to masses of structural metal in the earth.

Electrical power is supplied to single-trolley railroads principally

in the form of direct current. Usually all the trolley wires of a city

are so connected to the generating units as to be positive to the

rails. This causes current to flow from the cars toward the power

stations, the return path being made up jointly of the rails, the

earth itself, actual return wires which may supplement the rails, and

also all other conducting things in the earth, these being principally

lead-covered cables and other pipes. These conditions establish

definite areas in which the currents tend to leave the cables and

pipes, _i.e._, in which the latter are positive to other things. These

positive areas usually are much smaller than the negative areas, that

is, the regions in which currents tend _to enter_ the cables form a

larger total than the regions in which the currents tend _to leave_

the cables. These facts simplify the ways in which the cables may be

protected against damage by direct currents leaving them and also they

reduce the amount, complication, and cost of applying the corrective

and preventive measures.

All electric roads do not use direct current. Certain simplifications

in the use of single-phase alternating currents in traction motors

have increased the number of roads using a system of

alternating-current power supply. Where alternating current is used,

the electrolytic conditions are different and a new problem is set,

for, as the current flows in recurrently different directions, an area

which at one instant is positive to others, is changed the next

instant into a negative area. The protective means, therefore, must be

adapted to the changed requirements.

CHAPTER XIX

PROTECTIVE MEANS

Any of the heating hazards described in the foregoing chapter may

cause currents which will damage apparatus. All devices for the

protection of apparatus from such damage, operate either to stop the

flow of the dangerous current, or to send that flow over some other

path.

Protection Against High Potentials. Lightning is the most nearly

universal hazard. All open wires are exposed to it in some degree.

Damaging currents from lightning are caused by extraordinarily high

potentials. Furthermore, a lightning discharge is oscillatory; that

is, alternating, and of very high frequency. Drops, ringers,

receivers, and other devices subject to lightning damage suffer by

having their windings burned by the discharge. The impedance these

windings offer to the high frequency of lightning oscillations is

great. The impedance of a few turns of heavy wire may be negligible to

alternating currents of ordinary frequencies because the resistance of

the wire is low, its inductance small, and the frequency finite. On

the other hand, the impedance of such a coil to a lightning discharge

is much higher, due to the very high frequency of the discharge.

Were it not for the extremely high pressure of lightning discharges,

their high frequency of oscillation would enable ordinary coils to be

self-protecting against them. But a discharge of electricity can take

place through the air or other insulating medium if its pressure be

high enough. A pressure of 70,000 volts can strike across a gap in air

of one inch, and lower pressures can strike across smaller distances.

When lightning encounters an impedance, the discharge seldom takes

place through the entire winding, as an ordinary current would flow,

usually striking across whatever short paths may exist. Very often

these paths are across the insulation between the outer turns of a

coil. It is not unusual for a lightning discharge to plow its way

across the outer layer of a wound spool, melting the copper of the

turns as it goes. Often the discharge will take place from inner turns

directly to the core of the magnet. This is more likely when the core

is grounded.

_Air-Gap Arrester_. The tendency of a winding to oppose lightning

discharges and the ease with which such discharge may strike across

insulating gaps, points the way to protection against them. Such

devices consist of two conductors separated by an air space or other

insulator and are variously known as lightning arresters, spark gaps,

open-space cutouts, or air-gap arresters. The conductors between which

the gap exists may be both of metal, may be one of metal and one of

carbon, or both of carbon. One combination consists of carbon and

mercury, a liquid metal. The space between the conductors may be

filled with either air or solid matter, or it may be a vacuum.

Speaking generally, the conductors are separated by some insulator.

Two conductors separated by an insulator form a condenser. The

insulator of an open-space arrester often is called the dielectric.

[Illustration Fig. 203. Saw Tooth Arrester]

Discharge Across Gaps:--Electrical discharges across a given distance

occur at lower potentials if the discharge be between points than if

between smooth surfaces. Arresters, therefore, are provided with

points. Fig. 203 shows a device known as a "saw-tooth" arrester

because of its metal plates being provided with teeth. Such an

arrester brings a ground connection close to plates connected with the

line and is adapted to protect apparatus either connected across a

metallic circuit or in series with a single wire circuit.

Fig. 201 shows another form of metal plate air-gap arrester having the

further possibility of a discharge taking place from one line wire to

the other. Inserting a plug in the hole between the two line plates

connects the line wires directly together at the arrester. This

practice was designed for use with series lines, the plug

short-circuiting the telephone set when in place.

A defect of most ordinary types of metal air-gap lightning arresters

is that heavy discharges tend to melt the teeth or edges of the

plates and often to weld them together, requiring special attention

to re-establish the necessary gap.

Advantages of Carbon:--Solid carbon is found to be a much better

material than metal for the reasons that a discharge will not melt it

and that its surface is composed of multitudes of points from which

discharges take place more readily than from metals.

[Illustration Fig. 204. Saw-Tooth Arrester]

[Illustration Fig. 205. Carbon Block Arrester]

Carbon arresters now are widely used in the general form shown in Fig.

205. A carbon block connected with a wire of the line is separated

from a carbon block connected to ground by some form of insulating

separator. Mica is widely used as such a separator, and holes of some

form in a mica slip enable the discharge to strike freely from block

to block, while preventing the blocks from touching each other.

Celluloid with many holes is used as a separator between carbon

blocks. Silk and various special compositions also have their uses.

[Illustration Fig. 206. Arrester Separators]

Dust Between Carbons:--Discharges between the carbon blocks tend to

throw off particles of carbon from them. The separation between the

blocks being small--from .005 to .015 inch--the carbon particles may

lodge in the air-gap, on the edges of the separator, or otherwise, so

as to leave a conducting path between the two blocks. Slight moisture

on the separator may help to collect this dust, thus placing a ground

on that wire of the line. This ground may be of very high resistance,

but is probably one of many such--one at each arrester connected to

the line. In special forms of carbon arresters an attempt has been

made to limit this danger of grounding by the deposit of carbon dust.

The object of the U-shaped separator of Fig. 206 is to enable the

arrester to be mounted so that this opening in the separator is

downward, in the hope that loosened carbon particles may fall out of

the space between the blocks. The deposit of carbon on the inside

edges of the U-shaped separator often is so fine and clings so tightly

as not to fall out. The separator projects beyond the blocks so as to

avoid the collection of carbon on the outer edges.

Commercial Types:--Fig. 207 is a commercial form of the arrangement

shown in Fig. 205 and is one of the many forms made by the American

Electric Fuse Company. Line wires are attached to outside binding

posts shown in the figure and the ground wire to the metal binding

post at the front. The carbon blocks with their separator slide

between clips and a ground plate. The air-gap is determined by the

thickness of the separator between the carbon blocks.

[Illustration: Fig. 207. Carbon Block Arrester]

[Illustration: Fig. 208 Roberts "Self-Cleaning" Arrester]

The Roberts carbon arrester is designed with particular reference to

the disposal of carbon dust and is termed self-cleaning for that

reason. The arrangement of carbons and dielectric in this device is

shown in Fig. 208; mica is cemented to the line carbon and is large

enough to provide a projecting margin all around. The spark gap is not

uniform over the entire surface of the block but is made wedge-shaped

by grinding away the line carbon as shown. It is claimed that a

continuous arcing fills the wedge-shaped chamber with heated air or

gas, converting the whole of the space into a field of low resistance

to ground, and that this gas in expanding drives out every particle of

carbon that may be thrown off. It seems obvious that the wedge-shaped

space offers greater freedom for carbon dust to fall out than in the

case of the parallel arrangement of the block faces.

An outdoor arrester for metallic circuits, designed by F.B. Cook, is

shown in Fig. 209. The device is adapted to mount on a pole or

elsewhere and to be covered by a protecting cap. The carbons are large

and are separated by a special compound intended to assist the

self-cleaning feature. The three carbons being grouped together as a

unit, the device has the ability to care for discharges from one

terminal to either of the others direct, without having to pass

through two gaps. In this particular, the arrangement is the same as

that of Fig. 204.

[Illustration: Fig. 209. Cook Air-Gap Arrester]

A form of Western Electric arrester particularly adapted for outside

use on railway lines is shown with its cover in Fig. 210.

[Illustration: Fig. 210. Western Electric Air-Gap Arrester]

The Kellogg Company regularly equips its magneto telephones with

air-gap arresters of the type shown in Fig. 211. The two line plates

are semicircular and of metal. The ground plate is of carbon,

circular in form, covering both line plates with a mica separator.

This is mounted on the back board of the telephone and permanently

wired to the line and ground binding posts.

[Illustration: OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN,

SAN FRANCISCO, CALIFORNIA]

[Illustration: Fig. 211. Kellogg Air-Gap Arrester]

Vacuum Arresters:--All of the carbon arresters so far mentioned depend

on the discharge taking place through air. A given pressure will

discharge further in a fairly good vacuum than in air. The National

Electric Specialty Company mounts three conductors in a vacuum of the

incandescent lamp type, Fig. 212. A greater separation and less

likelihood of short-circuiting can be provided in this way. Either

carbon or metal plates are adapted for use in such vacuum devices. The

plates may be further apart for a given discharge pressure if the

surfaces are of carbon.

[Illustration: Fig. 212. Vacuum Arrester]

Introduction of Impedance:--It has been noted that the existence of

impedance tends to choke back the passage of lightning discharge

through a coil. Fig. 213 suggests the relation between such an

impedance and air-gap arrester. If the coil shown therein be considered

an arrangement of conductors having inductance, it will be seen that a

favorable place for an air-gap arrester is between that impedance and

the line. This fact is made known in practice by frequent damage to

aërial cables by electricity brought into them over long open wires,

the discharge taking place at the first turn or bend in the aërial

cable; this discharge often damages both core and sheath. It is well to

have such bends as near the end of the cable as possible, and turns or

goosenecks at entrances to terminals have that advantage.

[Illustration: Fig. 213. Impedance and Air-Gap]

This same principle is utilized in some forms of arresters, such as

the one shown in Fig. 214, which provides an impedance of its own

directly in the arrester element. In this device an insulating base

carries a grounded carbon rod and two impedance coils. The impedance

coils are wound on insulating rods, which hold them near, but not

touching, the ground carbon. The coils are arranged so that they may

be turned when discharges roughen the surfaces of the wires.

[Illustration: Fig. 214. Holtzer-Cabot Arrester]

Metallic Electrodes:--Copper or other metal blocks with roughened

surfaces separated by an insulating slip may be substituted for the

carbon blocks of most of the arresters previously described. Metal

blocks lack the advantage of carbon in that the latter allows

discharges at lower potentials for a given separation, but they have

the advantage that a conducting dust is not thrown off from them.

[Illustration: Fig. 215. Carbon Air-Gap Arrester]

Provision Against Continuous Arc:--For the purpose of short-circuiting

an arc, a globule of low-melting alloy may be placed in one carbon

block of an arrester. This feature is not essential in an arrester

intended solely to divert lightning discharges. Its purpose is to

provide an immediate path to ground if an arc arising from artificial

electricity has been maintained between the blocks long enough to melt

the globule. Fig. 215 is a plan and section of the Western Electric

Company's arrester used as the high potential element in conjunction

with others for abnormal currents and sneak currents; the latter are

currents too small to operate air-gap arresters or substantial fuses.

Protection Against Strong Currents. _Fuses._ A fuse is a metal

conductor of lower carrying capacity than the circuit with which it is

in series at the time it is required to operate. Fuses in use in

electrical circuits generally are composed of some alloy of lead,

which melts at a reasonably low temperature. Alloys of lead have lower

conductivity than copper. A small copper wire, however, may fuse at

the same volume of current as a larger lead alloy wire.

Proper Functions:--A fuse is not a good lightning arrester. As

lightning damage is caused by current and as it is current which

destroys a fuse, a lightning discharge _can_ open a circuit over which

it passes by melting the fuse metal. But lightning may destroy a fuse

and at the same discharge destroy apparatus in series with the fuse.

There are two reasons for this: One is that lightning discharges act

very quickly and may have destroyed apparatus before heating the fuse

enough to melt it; the other reason is that when a fuse is operated

with enough current even to vaporize it, the vapor serves as a

conducting path for an instant after being formed. This conducting

path may be of high resistance and still allow currents to flow

through it, because of the extremely high pressure of the lightning

discharge. A comprehensive protective system may include fuses, but it

is not to be expected that they always will arrest lightning or even

assist other things in arresting lightning. They should be considered

as of no value for that purpose. Furthermore, fuses are best adapted

to be a part of a general protective system when they do all that they

must do in stopping abnormal currents and yet withstand lightning

discharges which may pass through them. Other things being equal, that

system of protection is best in which all lightning discharges are

arrested by gap arresters and in which no fuses ever are operated by

lightning discharges.

Mica Fuse:--A convenient and widely used form of fuse is that shown in

Fig. 216. A mica slip has metal terminals at its ends and a fuse wire

joins these terminals. The fuse is inserted in the circuit by clamping

the terminals under screws or sliding them between clips as in Figs.

217 and 218. Advantages of this method of fuse mounting for protecting

circuits needing small currents are that the fuse wire can be seen, the

fuses are readily replaced when blown, and their mountings may be made

compact. As elements of a comprehensive protective system, however, the

ordinary types of mica-slip fuses are objectionable because too short,

and because they have no means of their own for extinguishing an arc

which may follow the blowing of the fuses. As protectors for use in

distributing low potential currents from central-office power plants

they are admirable. By simple means, they may be made to announce

audibly or visibly that they have operated.

[Illustration: Fig. 216. Mica Slip Fuse]

[Illustration: Fig. 217. Postal Type Mica Fuse]

[Illustration: Fig. 218. Western Union Type Mica Fuse]

Enclosed Fuses:--If a fuse wire within an insulating tube be made to

connect metal caps on that tube and the space around the tube be

filled with a non-conducting powder, the gases of the vaporized fuse

metal will be absorbed more quickly than when formed without such

imbedding in a powder. The filling of such a tubular fuse also muffles

the explosion which occurs when the fuse is vaporized.

[Illustration: Fig. 219. Pair of Enclosed Fuses]

Fuses of the enclosed type, with or without filling, are widely used

in power circuits generally and are recommended by fire insurance

bodies. Fig. 219 illustrates an arrester having a fuse of the enclosed

type, this example being that of the H. W. Johns-Manville Company.

[Illustration Fig. 220. Bank of Enclosed Fuses]

In telephony it is frequently necessary to mount a large number of

fuses or other protective devices together in a restricted space. In

Fig. 220 a group of Western Electric tubular fuses, so mounted, is

shown. These fuses have ordinarily a carrying capacity of 6 or 7

amperes. It is not expected that this arrester will blow because 6 or

7 amperes of abnormal currents are flowing through it and the

apparatus to be protected. What is intended is that the fuse shall

withstand lightning discharges and when a foreign current passes

through it, other apparatus will increase that current enough to blow

the fuse. It will be noticed that the fuses of Fig. 220 are open at

the upper end, which is the end connected to the exposed wire of the

line The fuses are closed at the lower end, which is the end connected

to the apparatus. When the fuse blows, its discharge is somewhat

muffled by the lining of the tube, but enough explosion remains so

that the heated gases, in driving outward, tend to break the arc which

is established through the vaporized metal.

A pair of Cook tubular fuses in an individual mounting is shown in

Fig. 221. Fuses of this type are not open at one end like a gun, but

opportunity for the heated gases to escape exists at the caps. The

tubes are made of wood, of lava, or of porcelain.

Fig. 222 is another tubular fuse, the section showing the arrangement

of asbestos lining which serves the two purposes of muffling the sound

of the discharge and absorbing and cooling the resulting gases.

[Illustration: Fig. 221. Pair of Wooden Tube Fuses]

_Air-Gap vs. Fuse Arresters._ It is hoped that the student grasps

clearly the distinction between the purposes of air-gap and fuse

arresters. The air-gap arrester acts in response to high voltages,

either of lightning or of high-tension power circuits. The fuse acts in

response to a certain current value flowing through it and this minimum

current in well-designed protectors for telephone lines is not very

small. Usually it is several times larger than the maximum current

apparatus in the line can safely carry. Fuses _can_ be made so delicate

as to operate on the very smallest current which could injure apparatus

and the earlier protective systems depended on such an arrangement. The

difficulty with such delicate fuses is that they are not robust enough

to be reliable, and, worse still, they change their carrying capacity

with age and are not uniform in operation in different surroundings and

at different temperatures. They are also sensitive to lightning

discharges, which they have no power to stop or to divert.

Protection Against Sneak Currents. For these reasons, a system

containing fuses and air-gap arresters only, does not protect against

abnormal currents which are continuous and small, though large enough

to injure apparatus _because_ continuous. These currents have come to

be known as sneak currents, a term more descriptive than elegant.

Sneak currents though small, may, when allowed to flow for a long time

through the winding of an electromagnet for instance, develop enough

heat to char or injure the insulation. They are the more dangerous

because insidious.

[Illustration: Fig. 222. Tubular Fuse with Asbestos Filling]

_Sneak-Current Arresters._ As typical of sneak-current arresters,

Fig. 223 shows the principle, though not the exact form, of an

arrester once widely used in telephone and signal lines. The normal

path from the line to the apparatus is through a small coil of fine

wire imbedded in sealing wax. A spring forms a branch path from the

line and has a tension which would cause it to bear against the ground

contact if it were allowed to do so. It is prevented from touching

that contact normally by a string between itself and a rigid support.

The string is cut at its middle and the knotted ends as thus cut are

imbedded in the sealing wax which contains the coil.

[Illustration: Fig. 223. Principle of Sneak-Current Arrester]

A small current through the little coil will warm the wax enough to

allow the string to part. The spring then will ground the line. Even

so simple an apparatus as this operates with considerable accuracy.

All currents below a certain critical amount may flow through the

heating coil indefinitely, the heat being radiated rapidly enough to

keep the wax from softening and the string from parting. All currents

above this critical amount will operate the arrester; the larger the

current, the shorter the time of operating. It will be remembered that

the law of these heating effects is that the heat generated =

_C^{2}Rt_, so that if a certain current operates the arrester in, say

40 seconds, twice as great a current should operate the arrester in 10

seconds. In other words, the time of operation varies inversely as the

square of the current and inversely as the resistance. To make the

arrester more sensitive for a given current--_i.e._, to operate in a

shorter time--one would increase the resistance of the coil in the wax

either by using more turns or finer wire, or by making the wire of a

metal having higher specific resistance.

The present standard sneak-current arrester embodies the two elements

of the devices of Fig. 223: a _resistance_ material to transform the

dangerous sneak current into localized heat; and a _fusible_ material

softened by this heat to release some switching mechanism.

The resistance material is either a resistance wire or a bit of

carbon, the latter being the better material, although both are good.

The fusible material is some alloy melting at a low temperature. Lead,

tin, bismuth, and cadmium can be combined in such proportions as will

enable the alloy to melt at temperatures from 140° to 180° F. Such an

alloy is a solder which, at ordinary temperatures, is firm enough to

resist the force of powerful springs; yet it will melt so as to be

entirely fluid at a temperature much less than that of boiling water.

[Illustration: Fig. 224. Heat Coil]

_Heat Coil._ Fig. 224 shows a practical way of bringing the heating and

to-be-heated elements together. A copper spool is wound with resistance

wire. A metal pin is soldered in the bore of the spool by an easily

melting alloy. When current heats the spool enough, the pin may slide

or turn in the spool. It may slide or turn in many ways and this

happily enables many types of arresters to result. For example, the pin

may pull out, or push in, or push through, or rotate like a shaft in a

bearing, or the spool may turn on it like a hub on an axle. Messrs.

Hayes, Rolfe, Cook, McBerty, Kaisling, and many other inventors have

utilized these combinations and motions in the production of

sneak-current arresters. All of them depend on one action: the

softening of a low-melting alloy by heat generated in a resistance.

When a heat coil is associated with the proper switching springs, it

becomes a sneak-current arrester. The switching springs always are

arranged to ground the line wire. In some arresters, the line wire is

cut off from the wire leading toward the apparatus by the same

movement which grounds it. In others, the line is not broken at all,

but merely grounded. Each method has its advantages.

Complete Line Protection. Fig. 225 shows the entire scheme of protectors

in an exposed line and their relation to apparatus in the central-office

equipment and at the subscriber's telephone. The central-office

equipment contains heat coils, springs, and carbon arresters. At some

point between the central office and the subscriber's premises, each

wire contains a fuse. At the subscriber's premises each wire contains

other fuses and these are associated with carbon arresters. The figure

shows a central battery equipment, in which the ringer of the telephone

is in series with a condenser. A sneak-current arrester is not required

at the subscriber's station with such equipment.

Assume the line to meet an electrical hazard at the point _X_. If this

be lightning, it will discharge to ground at the central office or at

the subscriber's instrument or at both through the carbon arresters

connected to that side of the line. If it be a high potential from a

power circuit and of more than 350 volts, it will strike an arc at the

carbon arrester connected to that wire of the line in the central

office or at the subscriber's telephone or at both, if the separation

of the carbons in those arresters is .005 inch or less. If the carbon

arresters are separated by celluloid, it will burn away and allow the

carbons to come together, extinguishing the arc. If they are separated

by mica and one of the carbons is equipped with a globule of

low-melting alloy, the heat of the arc will melt this,

short-circuiting the gap and extinguishing the arc. The passage of

current to ground at the arrester, however, will be over a path

containing nothing but wire and the arrester. The resulting current,

therefore, may be very large. The voltage at the arrester having been

350 volts or more, in order to establish the arc, short-circuiting the

gap will make the current 7 amperes or more, unless the applied

voltage miraculously falls to 50 volts or less. The current through

the fuse being more than 7 amperes, it will blow promptly, opening the

line and isolating the apparatus. It will be noted that this

explanation applies to equipment at either end of the line, as the

fuse lies between the point of contact and the carbon arrester.

[Illustration: Fig. 225. Complete Line Protection]

Assume, on the other hand, that the contact is made at the point _Y_.

The central-office carbon arrester will operate, grounding the line and

increasing the amount of current flowing. There being no fuse to blow,

a worse thing will befall, in the overheating of the line wire and the

probable starting of a fire in the central office. It is obvious,

therefore, that a fuse must be located between the carbon arrester and

any part of the line which is subject to contact with a potential which

can give an abnormal current when the carbon arrester acts.

Assume, as a third case, that the contact at the point _X_ either is

with a low foreign potential or is so poor a contact that the

difference of potential across the gap of the carbon arrester is lower

than its arcing point. Current will tend to flow by the carbon

arrester without operating it, but such a current must pass through

the winding of the heat coil if it is to enter the apparatus. The

sneak current may be large enough to overheat the apparatus if allowed

to flow long enough, but before it has flowed long enough it will have

warmed the heat-coil winding enough to soften its fusible alloy and to

release springs which ground the line, just as did the carbon arrester

in the case last assumed. Again the current will become large and will

blow the fuse which lies between the sneak-current arrester and the

point of contact with the source of foreign current. In this case,

also, contact at the point _Y_ would have operated mechanism to ground

the line at the central office, and, no fuse interposing, the wiring

would have been overheated.

_Exposed and Unexposed Wiring._ Underground cables, cables formed of

rubber insulated wires, and interior wiring which is properly done,

all may be considered to be wiring which is unexposed, that is, not

exposed to foreign high potentials, discharges, sneak, or abnormal

currents. _All other wiring_, such as bare wires, aërial cables, etc.,

should be considered as _exposed_ to such hazards and a fuse should

exist in each wire between its exposed portion and the central office

or subscriber's instrument. The rule of action, therefore, becomes:

_The proper position of the fuse is between exposed and unexposed

wiring._

It may appear to the student that wires in an aërial cable with a lead

sheath--that sheath being either grounded or ungrounded--are not

exposed to electrical hazards; in the case of the grounded sheath,

this would presume that a contact between the cable and a high

potential wire would result merely in the foreign currents going to

ground through the cable sheath, the arc burning off the

high-potential wire and allowing the contact to clear itself by the

falling of the wire. If the assumption be that the sheath is not

grounded, then the student may say that no current at all would flow

from the high-potential wire.

Both assumptions are wrong. In the case of the grounded sheath, the

current flows to it at the contact with the high-potential wire; the

lead sheath is melted, arcs strike to the wires within, and currents

are led directly to the central office and to subscribers' premises.

In the case of the ungrounded sheath, the latter charges at once

through all its length to the voltage of the high-potential wire; at

some point, a wire within the cable is close enough to the sheath for

an arc to strike across, and the trouble begins. All the wires in the

cable are endangered if the cross be with a wire of the primary

circuit of a high-tension transmission line. Any series arc-light

circuit is a high-potential menace. Even a 450-volt trolley wire or

feeder can burn a lead-covered cable entirely in two in a few seconds.

The authors have seen this done by the wayward trolley pole of a

street car, one side of the pole touching the trolley wire and the

extreme end just touching the telephone cable.

The answer lies in the foregoing rule. Place the fuse between the wires

which _can_ and the wires which _can not_ get into contact with high

potentials. In application, the rule has some flexibility. In the case

of a cable which is aërial as soon as it leaves the central office,

place the fuses in the central office; in a cable wholly underground,

from central office to subscriber--as, for example, the feed for an

office building--use no fuses at all; in a cable which leaves the

central office underground and becomes aërial, fuse the wires just

where they change from underground to aërial. The several branches of

an underground cable into aërial ones should be fused as they branch.

Wires properly installed in subscribers' premises are considered

unexposed. The position of the fuse thus is at or near the point of

entrance of the wires into that building if the wires of the

subscriber's line outside the premises are exposed, as determined by

the definitions given. If the line is unexposed, by those definitions,

no protector is required. If one is indicated, it should be used, as

compliance with the best-known practice is a clear duty. Less than

what is known to be best is not honest practice in a matter which

involves life, limb, and indefinite degrees of property values.

Protectors in central-battery subscribers' equipments need no

sneak-current arresters, as the condenser reduces that hazard to a

negligible amount. Magneto subscribers' equipments usually lack

condensers in ringer circuits, though they may have them in talking

circuits on party lines. The ringer circuit is the only path through

the telephone set for about 98 per cent of the time. Sneak-current

arresters, therefore, should be a part of subscribers' station

protectors in magneto equipment, except in such rural districts as may

have no lighting or power wires. When sneak-current arresters are so

used the arrangement of the parts then is the same as in the

central-office portion of Fig. 225.

Types of Central-Office Protectors. A form of combined heat coil and

air-gap arrester, widely used by Bell companies for central-office

protection, is shown in Fig. 226. The two inner springs form the

terminals for the two limbs of the metallic-circuit line, while the

two outside springs are terminals for the continuation of the line

leading to the switchboard. The heat coils, one on each side, are

supported between the inner and outer springs. High-tension currents

jump to ground through the air-gap arrester, while sneak currents

permit the pin of the heat coil to slide within the sleeve, thus

grounding the outside line and the line to the switchboard.

[Illustration: Fig. 226. Sneak-Current and Air-Gap Arrester]

_Self-Soldering Heat Coils._ Another form designed by Kaisling and

manufactured by the American Electric Fuse Company is shown in Fig.

227. In this the pin in the heat coil projects unequally from the ends

of the coil, and under the action of a sneak current the melting of

the solder which holds it allows the outer spring to push the pin

through the coil until it presses the line spring against the ground

plate and at the same time opens the path to the switchboard. When the

heat-coil pin assumes this new position it cools off, due to the

cessation of the current, and _resolders_ itself, and need only be

turned end for end by the attendant to be reset. Many are the

variations that have been made on this self-soldering idea, and there

has been much controversy as to its desirability. It is certainly a

feature of convenience.

[Illustration: Fig. 227. Self-Soldering Heat-Coil Arrester]

Instead of using a wire-wound resistance element in heat-coil

construction some manufacturers employ a mass of high-resistance

material, interposed in the path of the current. The Kellogg Company

has long employed for its sneak-current arrester a short graphite rod,

which forms the resistance element. The ends of this rod are

electroplated with copper to which the brass terminal heads are

soldered. These heads afford means for making the connection with the

proper retaining springs.

[Illustration: Fig. 228. Cook Arrester]

Another central-office protector, which uses a mass of special metal

composition for its heat producing element is that designed by Frank B.

Cook and shown in Fig. 228. In this the carbon blocks are cylindrical

in form and specially treated to make them "self-cleaning." Instead of

employing a self-soldering feature in the sneak-current arrester of

this device, Cook provides for electrically resoldering them after

operation, a clip being designed for holding the elements in proper

position and passing a battery current through them to remelt the

solder.

In small magneto exchanges it is not uncommon to employ combined fuse

and air-gap arresters for central-office line protection, the fuses

being of the mica-mounted type already referred to. A group of such

arresters, as manufactured by the Dean Electric Company, is shown in

Fig. 229.

[Illustration: Fig. 229. Mica Fuse and Air-Gap Arresters]

Types of Subscribers' Station Protectors. Figs. 230 and 231 show types

of subscribers' station protectors adapted to the requirements of

central-battery and magneto systems. These, as has been said, should be

mounted at or near the point of entrance of the subscriber's line into

the premises, if the line is exposed outside of the premises. It is

possible to arrange the fuses so that they will be safe and suitable

for their purposes if they are mounted out-of-doors near the point of

entrance to the premises. The sneak-current arrester, if one exists,

and the carbon arrester also, must be mounted inside of the premises or

in a protecting case, if outside, on account of the necessity of

shielding both of these devices from the weather. Speaking generally,

the wider practice is to put all the elements of the subscriber's

station protector inside of the house. It is nearer to the ideal

arrangement of conditions if the protector be placed immediately at the

point of entrance of the outside wires into the building.

[Illustration: Fig. 230. Western Electric Station Arrester]

[Illustration: Fig. 231. Cook Arrester for Magneto Stations]