Circuit.
An important feature is the method by which any receiving station
can interrupt the sending station and obtain control of the circuit.
As shown, this is accomplished if the line circuit is interrupted by
the break key during the time the sending brush is passing over the
sixth segment when a break relay at the sending station will be
energized to open the circuit of the magnet controlling the latch. The
release key will then have to be operated to permit further sending.
This printing system has been used to a considerable extent for
news distribution, where in many cases a number of sending and
receiving stations are connected to one circuit and means must be
provided to allow any station to obtain control of the circuit. Key-
board arrangements which may be used for perforating tape or for
sending directly to the line have been developed and found very
satisfactory for this kind of service. The Western Electric type-bar
printer has been found very satisfactory for news service. This is
provided with a stationary paper platen and a moving type-bar
basket. Books containing carbon paper for making a large number
703
of copies and forms holding wax stencil sheets may
be readily inserted into the machine. Means are
provided for adjusting the strength of the blow of
the type bars so that one to twenty copies can be
secured.
(c) Cipher Printing System. A. printing system
for rapidly ciphering and deciphering telegraph mes-
sages has been developed. It is thought that cipher
messages prepared by this system are absolutely un-
breakable. It was successfully applied by the U.S.
Army Signal Corps during the war and tests made
indicate that messages can be ciphered and deci-
phered by this means with greater accuracy and
many times faster than by other methods.
This system was developed by the engineers of
the American Telephone and Telegraph Company.
Its principles are illustrated in fig. 7. A message
prepared in perforated tape form by the ordinary
printer perforator passes through transmitter A.
This would ordinarily control the selecting mag-
nets of a printer or an automatic perforator. Key
tapes B and C, however, passing through ordinary
tape transmitters, control relays which interfere
with the normal operation of the selecting magnets
so that the resultant selection for any character of
the message may be any one of the characters of
the Baudot code. The characters of the key tapes
are selected at random and B is one unit less in
length than C. All tapes are stepped in unison.
Repetition in the resultant key will not occur until B
has revolved 1,000 times. The proper starting
position of the key tapes B and C for any message
may be indicated by six characters which may be
FIG. 9. Polar Relay for Cable Telegraph System.
704
TELEGRAPH
ciphered by an additional key. In deciphering the cipher message
tape is placed in transmitter A and the characters combined with
the proper key to obtain the original message.
Methods of Working Lines
(a) Simultaneous Telephone and Telegraph Working. Considerable
advance has been made in the technical knowledge of simultaneous
telephone and telegraph operation of line circuits. Hand telegraph
systems and single-channel printer systems usually operate at dot
speeds of 10 to 25 per second and it has been found that in order to
secure satisfactory service it is necessary to design line systems to
transmit a frequency band of about 100 cycles per second. Since
telephone frequencies range from about 250 to about 2,500 cycles per
second, it is possible to secure satisfactory telegraph operation
from telephone circuits by using frequencies below the lowest tele-
phone frequencies and frequencies above highest telephone fre-
quencies. More than 600,000 m. of telegraph circuits are obtained
from telephone circuits in the United States.
Permanent
Magnet
FIG. IO. Diagram of Polar Relay for Cable Telegraph System.
(V) Metallic Telegraph System. The increase in knowledge of the
fundamental requirements of simultaneous telephone and telegraph
operation has enabled a telegraph system to be developed for opera-
tion over long small-gauge telephone cable circuits. This system is
arranged for metallic circuit working using a relay operating with
a current of approximately 2 milliamperes. The general circuit
arrangements of this system are shown in fig. 8. The cable circuit
is divided by a composite set or filter into two branches, one for the
telephone and the other for the telegraph, the telegraph branch
absorbing frequencies below the telephone interval. All metallic
lines in a single office are supplied from a common battery. The type
of polar relay selected for this circuit is shown in fig. 9 and the
magnetic principle illustrated in fig. 10. The relay is provided
with a Gulstad vibrating circuit and the armature, a reed, is the
cross piece of a magnetic bridge. The line windings surround the
armature, and a current in one direction causes the armature to move
toward one pole while a current in the opposite direction causes it to
reverse, its motion. Chatter at the contacts is practically prevented
by cushioning contact springs attached to the armature. This
system has been designed for cables containing as many as 300 work-
ing circuits. A telegraph repeater is shown in fig. n. The
simultaneous operation of telephone and telegraph circuits has been
carefully worked out in connexion with the design of long interurban
cables and the equipment used in connexion with them.
(c) Carrier Current Multiplex System. One of the most interesting
telegraph developments is the so-called Carrier Current System in
which multiplex operation is secured by the use of a number of
alternating currents of different frequencies and of resonant circuits
for selecting them at the line terminals. This system uses vacuum
tubes for generating, amplifying and rectifying the alternating cur-
.0x3)
rents and represents a radical departure in telegraphy. The circuits
are of high signal quality, very stable in operation and free from the
duplex balance difficulties of direct current systems. This system has
enabled the following communication facilities to be obtained
commercially from a single
pair of open wires: Twen- Q
ty l-way carrier channels; g
four i-way direct current
channels; and one and one-
half telephone circuits in-
cluding the phantom. The
carrier telegraph circuit is
illustrated in fig. 12. The
terminal apparatus for one
2-way channel, which re-
peats between the carrier
circuit and the direct cur-
rent extension circuit, is
mounted upon a vertical
panel similar in appearance
to that of the metallic tele-
graph system. The same
sensitive relays are used in,
both systems.
(d) Rotary Repealers.
Success has been obtained
with the use of rotary re-
peaters in connexion with
telegraph circuits operated
by the multiplex printer sys-
tem. This type of repeater
restores distorted line sig-
nals to their original form
and has enabled printer cir-
cuits 3,000 m. in length to
be operated successfully at
high speeds.
(e) Fundamental Tele-
graph-Transmission Re-
search. Considerable atten-
tion has been given to the
telegraph-transmission
problem and improvements
have been made in the meth-
ods and means for measur-
ing distortion of telegraph
signals. The fundamental
transmission requirements
for different classes of service
have been more carefully
enumerated and advances
made in the design of artifi-
cial lines.
(/) Interference. Ad-
vances have been made in
minimizing interfering cur-
rents in telegraph circuits
both from high-tension power
lines and from neighbouring
telegraph circuits. Means
have been devised to over-
come the effect of differences
in ground potentials on
grounded telegraph circuits.
This arrangement introduces
a counter-electromotive
force which is automatically
adjusted to neutralize the
earth-potential difference
FIG. II. Telegraph Repeater for
Cable System.
between any two given
points.
(g) Codes and Sending
Machines. Codes, abbrevi- .
ations, typewriters and automatic sending machines are now widely
used by operators to increase the capacity of manually-operated
telegraph circuits. The automatic machine is merely a vibrating
reed mounted in a convenient and portable manner, adjusted to
vibrate at telegraph speeds and provided with contacts for con-
trolling the telegraph circuit. A movement of the controlling lever
in one direction causes the instrument to transmit a succession of
dots, the number depending on the length of time the lever is
thus held. A contrary movement sends a dash. This instrument
permits higher speeds than are otherwise possible to be maintained
with considerably less fatigue on the part of the operator. It may be
readily connected with any ordinary telegraph circuit.
Codes and abbreviations for shortening messages are used espe-
cially in distributing news. The Phillips Code is one that has been
generally adopted and an illustration of its use follows:
Transmitted message:
t potus wi ads cgs tsp q pip qsn.
TELEGRAPH
705
Oscillator
Jo other
Sending
Circuits
Subscribers
Apparatus
Carrier
Line
To other
Receiving
Circuits
FIG. 12. Terminal Circuit for Carrier Telegraph Channel.
This would be written by the receiving operator as follows:
The President of the United States will address Congress this
afternoon on the Philippine Question.
Many other schemes are used to save time and cost.
GENERAL BIBLIOGRAPHY. H. H. Harrison, The Historical Basis
Modern Printing. Telegraphy (IQIS); A. C. Booth. Machine
H. H? Ha
'ekgraphy (1914);
tarrison, The Story of the Keyboard
Perforators (1917); Maj. O'Meara, The Various Systems of Multiple
Telegraphy (1911); J. H. Bell, "Printing Telegraph Systems,"
American I. E. E. (Feb. 1920) ; H. H. Harrison, " The Principles of
Modern Printing Telegraphy," Inst. E.E. (1915); D. Murray,
" Press-the-Button-Telegraphy," Telegraph and Telephone Journal
(Nov. 1914-July 1915); E. H. Colpittsand O. B. Blackwell, " Carrier
Current Telephony and Telegraphy, " American I. E. E. (Feb. 1921).
MILES OF TELEGRAPH WIRE OF THE WORLD -JANUARY i 1910, 1914 AND 1920
(Some of the figures for the most part those for small places not shown separately are necessarily in part estimated.)
SINGLE WIRE
Jan. i 1910
3",942
8,048
379,888
388,412
126,505
13,120
40,171
16,336
826,730
Jan. i 1914
313,166
8,191
381,000
459,811
129,500
13,211
42,194
16,476
929,072
Jan. i 1920
3i8,759
8,500
715,468
486,714
208,210
16,195
53,300
19,085
930,000
Italy ....
Other countries in Europe
2,111,152
2,292,621
2,756,231
Japan
97-300
399,030
106,946
436,763
127,000
553,000
Total Asia
496,330
543,709
680,000
46,853
110,000
54,201
122,159
49,432
140,000
Total Africa
156,853
176,360
- 189,432
United States
1,480,000
153,000
78,000
1,615,000
193,277
86,805
1,700,000
214,629
95,000
Canada . . . . " "
Dther countries in North America
Total North America '
1,711,000
1,895,082
2,009,629
H3,590
251,990
333,632
92,909
20,188
14,700
108,931
25,892
19,319
147,276
29,700
24,100
Dther countries in Oceania
127,797
154,142
201,076
Total throughout world
4,746,722
5.313,904
6,170,000
NOTE: In the case of countries the boundaries of which have undergone change, the figures for each year represent the number of
les of telegraph wire within the boundaries of that year.
Telegraph wire used exclusively for railroad operation and not open to public use has been excluded wherever possible,
xxxii. 23
706
TELEPHONE
TELEPHONE (see 26.547). Various improvements were made
during 1910-21 in the mechanism and working of the telephone,
apart from the introduction of wireless telephony, which is
dealt with in the separate article under WIRELESS. Since oper-
ating practice and the application of recent inventions are not
always the same in the United States as in Great Britain, some
developments which are not common to both countries are
described in the section headed United States. At the end of
that section will be found a table showing the expansion of
telephone facilities during the years 1910-21 in all countries.
GREAT BRITAIN
Private Branch Exchanges. The increasing use of the tele-
phone as between one party and another was early followed by a
demand for the means of connecting different members of a
firm in the same building without the necessity for providing
each with a separate line to the public exchange. A further
development required that incoming calls for a large firm should
be dealt with at a central point in the initial stage so that after
ascertaining the business of the caller he could be placed in
communication with the particular department concerned. To
meet these needs switchboards similar in general principles to
those provided for main exchanges have come into use. The
call from a local extension or from the main exchange is
answered by the branch exchange operator, and the connexion
is completed by the medium of connecting cords, or, on small
systems, by circuits with which each line is associated by means
of a key. In the latter case the depression of any pair of keys
on a common connecting circuit places the two corresponding
lines into connexion with one another. On the smaller installa-
tions where it is probable that an operator is not always available
to give prompt attention to calling and clearing signals, the
extension stations signal the completion of their conversation
direct to the main exchange and simultaneously to the branch
exchange. This enables the main exchange to disconnect the
circuit promptly, releasing the main exchange circuit and any
junction circuits which may have been occupied by the connex-
ion. The prompt release of these circuits is of extreme impor-
tance in the economical working of the whole system. On larger
branch exchange systems where an operator is in continuous
attendance, the main exchange clearing signals are controlled by
the withdrawal of the cord connexion at the branch exchange.
The advantage to be gained under this scheme lies in the fact
that the extension station can call in the branch exchange opera-
tor during a conversation and get an established call from the
main exchange transferred to another extension station when
required. When a branch exchange is used in conjunction with
a common battery or automatic switching system, the current
for speaking purposes is fed over the main exchange lines on
exchange to extension station connexions, and by means of a
power lead from the main exchange in the case of extension to
extension connexions.
When associated with automatic switching systems the branch
exchange operator is provided with a calling dial, so that on each
exchange connexion she can dial the number required by any
extension station.
Trunk Line Working. Trunk or long distance working is
complicated by the necessity for recording the particulars of all
calls, and because instantaneous connexion cannot always be
effected owing to the prohibitive cost of providing lines with such
liberality as Would ensure a no-delay service at all times.
The system of the British Post Office is worked as follows: A
subscriber desiring a trunk connexion calls up his local exchange
and notifies his requirements. If circuits are available to the
town required on such a basis as to afford a no-delay service,
the connexion is effected at once and the signalling and control
arrangements are similar to those described for junction work-
ing. The operator records the particulars of the call on a ticket
which is used for future accounting purposes. Should a no-delay
service not be available, the operator records particulars of the
requirements herself, or, in cases where lines to the town re-
quired terminate at a separate trunk exchange, she extends the
subscriber's circuit to the trunk exchange and obtains a direct
connexion to a special record operator whose sole business it is
to note the particulars of the required trunk connexion. The
subscriber is informed that he will be called later and the con-
nexion is then severed. Meanwhile, the ticket is conveyed to the
switchboard position where the lines to the town wanted are
terminated. Calls at this point are dealt with in order of priority
as recorded by the time on the ticket when the demand was
initiated. Particulars of the connexion wanted are passed to the
distant operator, who extends the circuit direct to the line of the
" wanted " subscriber, when such lines terminate in the same
exchange, or extends the circuit to a junction, when the " wanted ";
subscriber is connected to another local exchange, and requests
the operator at that exchange to effect the connexion. The
trunk operator at the originating town simultaneously effects
the connexion direct, or where another local exchange is con-j
cerned, by the medium of a junction, to the initiating subscriber,;
and when both subscribers are on the line, she completes the:
connexion. The call is controlled by the trunk operators, thei
junction circuit being equipped in such a manner that the sub-
scriber's signals appear at the trunk exchanges, from which point
disconnecting signals are sent automatically to the local ex-
changes when the connexions between the trunk and the junction
circuits are removed.
Trunk exchanges are equipped with relays and lamps for!
signalling purposes. " Calculagraphs " are employed for stamp-]
ing the time of commencement and completion of conversation
on the tickets. There is also associated with each trunk connex-l
ion a device which lights a lamp as soon as the scheduled limit
of the period of conversation is reached.
Where the volume of traffic over any route is considerable, the
requirements to the distant town may be notified over a sepa-j
rate circuit reserved for the purpose, and the local connexions:
involving the use of junction circuits to other local exchanges
can be established in advance, thus minimizing the time of
occupation of the main circuits and securing the greatest possi-
ble effective use of the trunk lines.
Manual Exchanges. The main features of the manual tele-
phone system remained in 1921 what they were in 1910. Im-j
provements had been introduced to some extent, to diminish i
or eliminate altogether portions of the operator's work, but !
common battery transmission and signalling, connecting by
means of plugs, flexible cord conductors and jacks, lamp calling ;
and supervisory signals all these remained unaltered.
Among the improvements referred to may be mentioned:
Keyless ringing; automatic listening; secret service; ringing
tone; automatic ringing cut-off; traffic distribution; ancillary
answering jacks.
1. Keyless Ringing is the feature of a cord circuit which provides [
that the ringing of the required subscriber's bell commences auto-
matically on the operator connecting to his line and ceases auto-
matically on the removal of the receiver from its rest by this sub-
scriber. This renders unnecessary the provision of a key for ringing
purposes hence the term " keyless " ringing.
2. Automatic Listening is a feature which eliminates the listening
key. The operator's telephone is automatically connected to the
calling subscriber's line when the answering plug of the cord circuit
is inserted in the answering jack of that line. The operator's tele-
phone is later automatically disconnected when, after ascertaining
the number required by the calling subscriber, the operator connects
the calling plug to the line of the required subscriber.
3. Secret Service follows from 2. Automatic listening involves the
feature that while conditions suitable for conversation are established
the operator's telephone is disconnected and without the aid of a
listening key it is impossible for the operator to listen to a con-
versation.
4. Ringing Tone is a tone (distinctive from that intimating to a
calling subscriber that the line he requires is engaged) applied to the
calling subscriber's line while the bell of the called subscriber is being
rung. Hearing this tone, which intimates that the bell of the called
subscriber is being rung, and receiving no reply after a reasonable
period, the calling subscriber infers that his correspondent is not
available and restores his receiver. This facility reduces the time
spent on " no reply " calls, by both operator and subscriber.
5. Automatic Ringing Cut-Off provides that the ringing of the
required subscriber's bell automatically ceases when the calling
subscriber, receiving no reply, decides to abandon the call and
restores his receiver to its rest.
TELEPHONE
707
6. Traffic Distribution. By means of selecting mechanism the
line of a calling subscriber is automatically connected to a dis-
engaged operator's position. This avoids the overloading of any
operator and at the same time gives all operators a fair load during
busy periods.
7. Ancillary Jacks. Additional calling lamp signals and answer-
ing jacks associated therewith placed at different parts of the
switchboard. When a subscriber calls, signals are displayed at two
or more operators' positions, thus increasing the number of operators
who may answer. The object of the arrangement is to provide for
the more even distribution of traffic and thus to reduce the answering
time of the operators during the rush periods.
Automatic Systems. The idea of automatic telephony is to
substitute for the operator of the manual exchange an electro-
mechanical or other switching system, which, controlled in its
movement by the action of the subscriber, will automatically
select, connect and disconnect circuits as desired. The process
of machine switching consists of successive group selection.
Exchange switching machines are provided and are placed under
the control of subscribers or operators. Considering only the
former case, the machines are operated by impulses originating
m a subscriber's telephone. The standard impulse now in
general use is a disconnexion of the subscriber's loop (the loop
having been closed in the first instance by the lifting of the
subscriber's receiver). The train of impulses corresponds to the
digit signalled. For instance, the signalling of the digit 7 would
constitute a series of seven disconnexions of the subscriber's
telephone loop as indicated in fig. i. It will be seen that a com-
plete impulse is 57% of the total cycle.
ETC.
When the impulses are passing at the standard rate of 10 per
second, this gives an electrical condition at the exchange of
sufficient duration to effect reliable operation of the equipment.
On the base of the automatic telephone instrument a circular
device, known as the dial, is provided for transmitting the im-
pulses. Fig. 2 shows a full view of the face of a dial that has
FIG. 2.
designed for use in large cities. It will be seen that there
are 10 holes each of which corresponds to a digit, also that
eight of the holes contain letters. Dealing first with the digits,
1 the method of operation is for the subscriber to lift his receiver
Und then if he requires to call " 7146 " he will operate these
iigits in turn. To call 7 he will place his finger in the hole
corresponding to that digit, rotate the front plate of the dial
which is free to move in a clockwise direction until his finger
and the finger hole for 7 reach the finger stop shown on the right-
hand side of fig. 2. He will then withdraw his finger and the
free plate of the dial will return to normal under the influence of
a main spring, in doing which the mechanism of the dial will
disconnect the subscriber's telephone loop seven times. He will
now insert his finger in the hole corresponding to the digit i and
proceed as before. The impulses passing from the dial will, as
has been stated, be transmitted at the rate of 10 a second, and,
as will be seen from fig. i, the short current between the impulses,
is only 43 % of the cycle. An appreciable interval, by comparison,
must occur between the trains corresponding to each digit be-
cause the process of manipulating the dial cannot be performed
rapidly enough for it to be otherwise.
In practically all automatic systems the impulse circuit is as
indicated in fig. 3, from which it will be seen that the operation
Dial on Telephone
Subscribers Line
FIG. 3.
To Switching