SELECTIVE PARTY-LINE SYSTEMS
The problem which confronts one in the production of a system of
selective ringing on party lines is that of causing the bell of any
chosen one of the several parties on a circuit to respond to a signal
sent out from the central office without sounding any of the other
bells. This, of course, must be accomplished without interfering with
the regular functions of the telephone line and apparatus. By this is
meant that the subscribers must be able to call the central office and
to signal for disconnection when desired, and also that the
association of the selective-signaling devices with the line shall not
interfere with the transmission of speech over the line. A great many
ways of accomplishing selective ringing on party lines have been
proposed, and a large number of them have been used. All of these ways
may be classified under four different classes according to the
underlying principle involved.
Classification. (_1_) _Polarity_ systems are so called because they
depend for their operation on the use of bells or other responsive
devices so polarized that they will respond to one direction of
current only. These bells or other devices are so arranged in
connection with the line that the one to be rung will be traversed by
current in the proper direction to actuate it, while all of the others
will either not be traversed by any current at all, or by current in
the wrong direction to cause their operation.
(_2_) The _harmonic_ systems have for their underlying principle the
fact that a pendulum or elastic reed, so supported as to be capable of
vibrating freely, will have one particular rate of vibration which it
may easily be made to assume. This pendulum or reed is placed under
the influence of an electromagnet associated with the line, and owing
to the fact that it will vibrate easily at one particular rate of
vibration and with extreme difficulty at any other rate, it is clear
that for current impulses of a frequency corresponding to its natural
rate the reed will take up the vibration, while for other frequencies
it will fail to respond.
Selection on party lines by means of this system is provided for by
tuning all of the reeds on the line at different rates of vibration
and is accomplished by sending out on the line ringing currents of
proper frequency to ring the desired bell. The current-generating
devices for ringing these bells are capable of sending out different
frequencies corresponding respectively to the rates of vibration of
each of the vibrating reed tongues. To select any one station,
therefore, the current frequency corresponding to the rate of
vibration of the reed tongue at that station is sent and this, being
out of tune with the reed tongues at all of the other stations,
operates the tongue of the desired station, but fails to operate those
at all of the other stations.
(_3_) In the _step-by-step_ system the bells on the line are normally
not in operative relation with the line and the bell of the desired
party on the line is made responsive by sending over the line a
certain number of impulses preliminary to ringing it. These impulses
move step-by-step mechanisms at each of the stations in unison, the
arrangement being such that the bells at the several stations are each
made operative after the sending of a certain number of preliminary
impulses, this number being different for all the stations.
(_4_) The _broken-line_ systems are new in telephony and for certain
fields of work look promising. In these the line circuit is normally
broken up into sections, the first section terminating at the first
station out from the central office, the second section at the second
station, and so on. When the line is in its normal or inactive
condition only the bell at the first station is so connected with the
line circuit as to enable it to be rung, the line being open beyond.
Sending a single preliminary impulse will, however, operate a
switching device so as to disconnect the bell at the first station and
to connect the line through to the second station. This may be carried
out, by sending the proper number of preliminary impulses, so as to
build up the line circuit to the desired station, after which the
sending of the ringing current will cause the bell to ring at that
station only.
Polarity Method. The polarity method of selective signaling on party
lines is probably the most extensively used. The standard selective
system of the American Telephone and Telegraph Company operates on
this principle.
_Two-Party Line._ It is obvious that selection may be had between two
parties on a single metallic-circuit line without the use of biased
bells or current of different polarities. Thus, one limb of a metallic
circuit may be used as one grounded line to ring the bell at one of
the stations, and the other limb of the metallic circuit may be used
as another grounded line to ring the bell of the other station; and
the two limbs may be used together as a metallic circuit for talking
purposes as usual.
This is shown in Fig. 170, where the ringing keys at the central office
are diagrammatically shown in the left-hand portion of the figure as
_K_^{1} and _K_^{2}. The operation of these keys will be more fully
pointed out in a subsequent chapter, but a correct understanding will
be had if it be remembered that the circuits are normally maintained by
these keys in the position shown. When, however, either one of the keys
is operated, the two long springs may be considered as pressed apart so
as to disengage the normal contacts between the springs and to engage
the two outer contacts, with which they are shown in the cut to be
disengaged. The two outer contacts are connected respectively to an
ordinary alternating-current ringing generator and to ground, but the
connection is reversed on the two keys.
[Illustration: Fig. 170. Simple Two-Party Line Selection]
At Station A the ordinary talking set is shown in simplified form,
consisting merely of a receiver, transmitter, and hook switch in a
single bridge circuit across the line. An ordinary polarized bell is
shown connected in series with a condenser between the lower limb of
the line and ground. At Station B the same talking circuit is shown,
but the polarized bell and condenser are bridged between the upper
limb of the line and ground.
If the operator desires to call Station A, she will press key _K_^{1}
which will ground the upper side of the line and connect the lower
side of the line with the generator _G_^{1}, and this, obviously, will
cause the bell at Station A to ring. The bell at Station B will not
ring because it is not in the circuit. If, on the other hand, the
operator desires to ring the bell at Station B, she will depress key
_K_^{2}, which will allow the current from generator _G_^{2} to pass
over the upper side of the line through the bell and condenser at
Station B and return by the path through the ground. The object of
grounding the opposite sides of the keys at the central office is to
prevent cross-ringing, that is, ringing the wrong bell. Were the keys
not grounded this might occur when a ringing current was being sent
out while the receiver at one of the stations was off its hook; the
ringing current from, say, generator _G_^{1} then passing not only
through the bell at Station A as intended, but also through the bell
at Station B by way of the bridge path through the receiver that
happened to be connected across the line. With the ringing keys
grounded as shown, it is obvious that this will not occur, since the
path for the ringing current through the wrong bell will always be
shunted by a direct path to ground on the same side of the line.
In such a two-party-line selective system the two generators _G_^{1}
and _G_^{2} may be the same generator and may be of the ordinary
alternating-current type. The bells likewise may be of the ordinary
alternating-current type.
The two-party selective line just described virtually employs two
separate circuits for ringing. Now each of these circuits alone may be
employed to accomplish selective ringing between two stations by using
two biased bells oppositely polarized, and employing pulsating ringing
currents of one direction or the other according to which bell it is
desired to ring. One side of a circuit so equipped is shown in Fig.
171. In this the two biased bells are at Station A and Station B,
these being bridged to ground in each case and adapted to respond only
to positive and negative impulses respectively. At the central office
the two keys _K_^{1} and _K_^{2} are shown. A single
alternating-current generator _G_ is shown, having its brush _1_
grounded and brush _2_ connected to a commutator disk _3_ mounted on
the generator shaft so as to revolve therewith. One-half of the
periphery of this disk is of insulating material so that the brushes
_4_ and _5_, which bear against the disk, will be alternately
connected with the disk and, therefore, with the brush _2_ of the
generator. Now the brush _2_, being one terminal of an
alternating-current machine, is alternately positive and negative, and
the arrangement of the commutator is such that the disk, which is
always at the potential of the brush _2_, will be connected to the
brush _5_ only while it is positively charged and with the brush _4_
only while it is negatively charged. As a result, brush _5_ has a
succession of positive impulses and brush _4_ a succession of negative
ones. Obviously, therefore, when key _K_^{1} is depressed only the
bell at Station A will be rung, and likewise the depression of key
_K_^{2} will result only in the ringing of the bell at Station B.
[Illustration: Fig. 171. Principle of Selection by Polarity]
_Four-Party Line._ From the two foregoing two-party line systems it is
evident that a four-party line system may be readily obtained, that
is, by employing two oppositely polarized biased bells on each side of
the metallic circuit. The selection of any of the four bells may be
obtained, choosing between the pairs connected, respectively, with the
two limbs of the line, by choosing the limb on which the current is to
be sent, and choosing between the two bells of the pair on that side
of the line by choosing which polarity of current to send.
Such a four-party line system is shown in Fig. 172. In this the
generators are not shown, but the wires leading from the four keys are
shown marked plus or minus, according to the terminal of the generator
to which they are supposed to be connected. Likewise the two bells
connected with the lower side of the line are marked positive and
negative, as are the two bells connected with the upper side of the
line. From the foregoing description of Figs. 170 and 171, it is clear
that if key _K_^{1} is pressed the bell at Station A will be rung, and
that bell only, since the bells at Station C and Station _D_ are not
in the circuit and the positive current sent over the lower side of
the line is not of the proper polarity to ring the bell at Station B.
The system shown in Fig. 172 is subject to one rather grave defect. In
subsequent chapters it will be pointed out that in common-battery
systems the display of the line signal at the central office is
affected by any one of the subscribers merely taking his receiver off
its hook and thus establishing a connection between the two limbs of
the metallic circuit. Such common-battery systems should have the two
limbs of the line, normally, entirely insulated from each other. It is
seen that this is not the case in the system just described, since
there is a conducting path from one limb of the line through the two
bells on that side to ground, and thence through the other pair of
bells to the other limb of the line. This means that unless the
resistance of the bell windings is made very high, the path of the
signaling circuit will be of sufficiently low resistance to actuate
the line signal at the central office.
[Illustration: Fig. 172. Four-Party Polarity Selection]
It is not feasible to overcome this objection by the use of condensers
in series with the bells, as was done in the system shown in Fig. 170,
since the bells are necessarily biased and such bells, as may readily
be seen, will not work properly through condensers, since the placing
of a condenser in their circuit means that the current which passes
through the bell is alternating rather than pulsating, although the
original source may have been of pulsating nature only.
[Illustration: Fig 173. Standard Polarity System]
The remedy for this difficulty, therefore, has been to place in series
with each bell a very high non-inductive resistance of about 15,000 or
20,000 ohms, and also to make the windings of the bells of
comparatively high resistance, usually about 2,500 ohms. Even with
this precaution there is a considerable leakage of the central-office
battery current from one side of the line to the other through the two
paths to ground in series. This method of selective signaling has,
therefore, been more frequently used with magneto systems. An endeavor
to apply this principle to common-battery systems without the
objections noted above has led to the adoption of a modification,
wherein a relay at each station normally holds the ground connection
open. This is shown in Fig. 173 and is the standard four-party line
ringing circuit employed by the American Telephone and Telegraph
Company and their licensees.
In this system the biased bells are normally disconnected from the
line, and, therefore, the leakage path through them from one side of
the line to the other does not exist. At each station there is a relay
winding adapted to be operated by the ringing current bridged across
the line in series with a condenser. As a result, when ringing current
is sent out on the line all of the relays, _i.e._, one at each
station, are energized and attract their armatures. This establishes
the connection of all the bells to line and really brings about
temporarily a condition equivalent to that of Fig. 172. As a result,
the sending of a positive current on the lower line with a ground
return will cause the operation of the bell at Station A. It will not
ring the bell at Station B because of the wrong polarity. It will not
ring the bells of Station C and Station D because they are in the
circuit between the other side of the line and ground. As soon as the
ringing current ceases all of the relays release their armatures and
disconnect all the bells from the line.
By this very simple device the trouble, due to marginal working of
the line signal, is done away with, since normally there is no leakage
from one side of the line to the other on account of the presence of
the condensers in the bridge at each station.
[Illustration: Fig. 174. Ringing-Key Arrangement]
In Fig. 174, the more complete connections of the central-office
ringing keys are shown, by means of which the proper positive or
negative ringing currents are sent to line in the proper way to cause
the ringing of any one of the four bells on a party line of either of
the types shown in Figs. 172 and 173.
In this the generator _G_ and its commutator disk _3_, with the
various brushes, _1_, _2_, _4_, and _5_, are arranged in the same
manner as is shown in Fig. 171. It is evident from what has been said
that wire _6_ leading from generator brush _2_ and commutator disk _3_
will carry alternating potential; that wire _7_ will carry positive
pulsations of potential; and that wire _8_ will carry negative
pulsations of potential. There are five keys in the set illustrated in
Fig. 174, of which four, viz, _K_^{1}, _K_^{2}, _K_^{3}, and _K_^{4},
are connected in the same manner as diagrammatically indicated in
Figs. 172 and 173, and will, obviously, serve to send the proper
current over the proper limb of the line to ring one of the bells. Key
_K_^{5}, the fifth one in the set, is added so as to enable the
operator to ring an ordinary unbiased bell on a single party line when
connection is made with such line. As the two outside contacts of this
key are connected respectively to the two brushes of the
alternating-current dynamo _G_, it is clear that it will impress an
alternating current on the line when its contacts are closed.
_Circuits of Two-Party Line Telephones._ In Fig. 175 is shown in
detail the wiring of the telephone set usually employed in connection
with the party-line selective-ringing system illustrated in Fig. 170.
In the wiring of this set and the two following, it must be borne in
mind that the portion of the circuit used during conversation might
be wired in a number of ways without affecting the principle of
selective ringing employed; however, the circuits shown are those most
commonly employed with the respective selective ringing systems which
they are intended to illustrate. In connecting the circuits of this
telephone instrument to the line, the two line conductors are
connected to binding posts _1_ and _2_ and a ground connection is made
to binding post _3_. In practice, in order to avoid the necessity of
changing the permanent wiring of the telephone set in connecting it as
an A or B Station (Fig. 170), the line conductors are connected to the
binding posts in reverse order at the two stations; that is, for
Station A the upper conductor, Fig. 170, is connected to binding post
_1_ and the lower conductor to binding post _2_, while at Station B
the upper conductor is connected to binding post _2_ and the lower
conductor to binding post _1_. The permanent wiring of this telephone
set is the same as that frequently used for a set connected to a line
having only one station, the proper ringing circuit being made by the
method of connecting up the binding posts. For example, if this
telephone set were to be used on a single station line, the binding
posts _1_ and _2_ would be connected to the two conductors of the line
as before, while binding post _3_ would be connected to post _1_
instead of being grounded.
[Illustration: Fig. 175. Circuit of Two-Party Station]
_Circuits of Four-Party-Line Telephones._ The wiring of the telephone
set used with the system illustrated in Fig. 172 is shown in detail in
Fig. 176. The wiring of this set is arranged for local battery or
magneto working, as this method of selective ringing is more frequently
employed with magneto systems, on account of the objectionable features
which arise when applied to common-battery systems. In this figure the
line conductors are connected to binding posts _1_ and _2_, and a
ground connection is made to binding post _3_. In order that all sets
may be wired alike and yet permit the instrument to be connected for
any one of the various stations, the bell is not permanently wired to
any portion of the circuit but has flexible connections which will
allow of the set being properly connected for any desired station. The
terminals of the bell are connected to binding posts _9_ and _10_, to
which are connected flexible conductors terminating in terminals _7_
and _8_. These terminals may be connected to the binding posts _4_,
_5_, and _6_ in the proper manner to connect the set as an A, B, C, or
D station, as required. For example, in connecting the set for Station
A, Fig. 172, terminal _7_ is connected to binding post _6_ and _8_ to
_5_. For connecting the set for Station B terminal _7_ is connected to
binding post _5_ and _8_ to _6_. For connecting the set for Station C
terminal _7_ is connected to binding post _6_ and _8_ to _4_. For
connecting the set for Station D terminal _7_ is connected to binding
post _4_ and _8_ to _6_.
[Illustration: Fig. 176. Circuit of Four-Party Station without Relay]
[Illustration: Fig. 177. Circuit of Four-Party Station with Relay]
The detailed wiring of the telephone set employed in connection with
the system illustrated in Fig. 173 is shown in Fig. 177. The wiring of
this set is arranged for a common-battery system, inasmuch as this
arrangement of signaling circuit is more especially adapted for
common-battery working. However, this arrangement is frequently
adapted to magneto systems as even with magneto systems a permanent
ground connection at a subscriber's station is objectionable inasmuch
as it increases the difficulty of determining the existence or
location of an accidental ground on one of the line conductors. The
wiring of this set is also arranged so that one standard type of
wiring may be employed and yet allow any telephone set to be connected
as an A, B, C, or D station.
Harmonic Method. _Principles._ To best understand the principle of
operation of the harmonic party-line signaling systems, it is to be
remembered that a flexible reed, mounted rigidly at one end and having
its other end free to vibrate, will, like a violin string, have a
certain natural period of vibration; that is, if it be started in
vibration, as by snapping it with the fingers, it will take up a
certain rate of vibration which will continue at a uniform rate until
the vibration ceases altogether. Such a reed will be most easily
thrown into vibration by a series of impulses having a frequency
corresponding exactly to the natural rate of vibration of the reed
itself; it may be thrown into vibration by very slight impulses if
they occur at exactly the proper times.
It is familiar to all that a person pushing another in a swing may
cause a considerable amplitude of vibration with the exertion of but a
small amount of force, if he will so time his pushes as to conform
exactly to the natural rate of vibration of the swing. It is of course
possible, however, to make the swing take up other rates of vibrations
by the application of sufficient force. As another example, consider a
clock pendulum beating seconds. By gentle blows furnished by the
escapement at exactly the proper times, the heavy pendulum is kept in
motion. However, if a person grasps the pendulum weight and shakes it,
it may be made to vibrate at almost any desired rate, dependent on the
strength and agility of the individual.
The conclusion is, therefore, that a reed or pendulum may be made to
start and vibrate easily by the application of impulses at proper
intervals, and only with great difficulty by the application of
impulses at other than the proper intervals; and these facts form the
basis on which harmonic-ringing systems rest.
The father of harmonic ringing in telephony was Jacob B. Currier, an
undertaker of Lowell, Mass. His harmonic bells were placed in series
in the telephone line, and were considerably used in New England in
commercial practice in the early eighties. Somewhat later James A.
Lighthipe of San Francisco independently invented a harmonic-ringing
system, which was put in successful commercial use at Sacramento and a
few other smaller California towns. Lighthipe polarized his bells and
bridged them across the line in series with condensers, as in modern
practice, and save for some crudities in design, his apparatus closely
resembled, both in principle and construction, some of that in
successful use today.
Lighthipe's system went out of use and was almost forgotten, when
about 1903, Wm. W. Dean again independently redeveloped the harmonic
system, and produced a bell astonishingly like that of Lighthipe, but
of more refined design, thus starting the development which has
resulted in the present wide use of this system.
The signal-receiving device in harmonic-ringing systems takes the form
of a ringer, having its armature and striker mounted on a rather stiff
spring rather than on trunnions. By this means the moving parts of the
bell constitute in effect a reed tongue, which has a natural rate of
vibration at which it may easily be made to vibrate with sufficient
amplitude to strike the gongs. The harmonic ringer differs from the
ordinary polarized bell or ringer, therefore, in that its armature
will vibrate most easily at one particular rate, while the armature of
the ordinary ringer is almost indifferent, between rather wide limits,
as to the rate at which it vibrates.
As a rule harmonic party-line systems are limited to four stations on
a line. The frequencies employed are usually 16-2/3, 33-1/3, 50, and
66-2/3 cycles per second, this corresponding to 1,000, 2,000, 3,000,
and 4,000 cycles per minute. The reason why this particular set of
frequencies was chosen is that they represent approximately the range
of desirable frequencies, and that the first ringing-current machines
in such systems were made by mounting the armatures of four different
generators on a single shaft, these having, respectively, two poles,
four poles, six poles, and eight poles each. The two-pole generator
gave one cycle per revolution, the four-pole two, the six-pole three,
and the eight-pole four, so that by running the shaft of the machine
at exactly 1,000 revolutions per minute the frequencies before
mentioned were attained. This range of frequencies having proved
about right for general practice and the early ringers all having been
attuned so as to operate on this basis, the practice of adhering to
these numbers of vibrations has been kept up with one exception by all
the manufacturers who make this type of ringer.
_Tuning._ The process of adjusting the armature of a ringer to a
certain rate of vibration is called tuning, and it is customary to
refer to a ringer as being tuned to a certain rate of vibration, just
as it is customary to refer to a violin string as being tuned to a
certain pitch or rate of vibration.
The physical difference between the ringers of the various frequencies
consists mainly in the size of the weights at the end of the vibrating
reed, that is, of the weights which form the tapper for the bell. The
low-frequency ringers have the largest weights and the high-frequency
the smallest, of course. The ringers are roughly tuned to the desired
frequencies by merely placing on the tapper rod the desired weight and
then a more refined tuning is given them by slightly altering the
positions of the weights on the tapper rod. To make the reed have a
slightly lower natural rate of vibration, the weight is moved further
from the stationary end of the reed, while to give it a slightly
higher natural rate of vibration the weight is moved toward the
stationary. In this way very nice adjustments may be made, and the aim
of the various factories manufacturing these bells is to make the
adjustment permanent so that it will never have to be altered by the
operating companies. Several years of experience with these bells has
shown that when once properly assembled they maintain the same rate of
vibration with great constancy.
There are two general methods of operating harmonic bells. One of
these may be called the in-tune system and the other the under-tune
system. The under-tune system was the first employed.
[Illustration: OPERATING ROOM AT TOKYO, JAPAN]
_Under-Tune System._ The early workers in the field of
harmonic-selective signaling discovered that when the tapper of the
reed struck against gongs the natural rate of vibration of the reed
was changed, or more properly, the reed was made to have a different
rate of vibration from its natural rate. This was caused by the fact
that the elasticity of the gongs proved another factor in the set
of conditions causing the reeds to take up a certain rate of
vibration, and the effect of this added factor was always to
accelerate the rate of vibration which the reed had when it was not
striking the gongs. The rebound of the hammer from the gongs tended,
in other words, to accelerate the rate of vibration, which, as might
be expected, caused a serious difficulty in the practical operation of
the bells. To illustrate: If a reed were to have a natural rate of
vibration, when not striking the gongs, of 50 per second and a current
of 50 cycles per second were impressed on the line, the reed would
take up this rate of vibration easily, but when a sufficient amplitude
of vibration was attained to cause the tapper to strike the gongs, the
reed would be thrown out of tune, on account of the tendency of the
gongs to make the reed vibrate at a higher rate. This caused irregular
ringing and was frequently sufficient to make the bells cease ringing
altogether or to ring in an entirely unsatisfactory manner.
In order to provide for this difficulty the early bells of Currier and
Lighthipe were made on what has since been called the "under-tuned"
principle. The first bells of the Kellogg Switchboard and Supply
Company, developed by Dean, were based on this idea as their cardinal
principle. The reeds were all given a natural rate of vibration, when
not striking the gongs, somewhat below that of the current frequencies
to be employed; and yet not sufficiently below the corresponding
current frequency to make the bell so far out of tune that the current
frequency would not be able to start it. This was done so that when
the tapper began to strike the gongs the tapper would be accelerated
and brought practically into tune with the current frequency, and the
ringing would continue regularly as long as the current flowed. It
will be seen that the under-tuned system was, therefore, one involving
some difficulty in starting in order to provide for proper regularity
while actually ringing.
Ringers of this kind were always made with but a single gong, it being
found difficult to secure uniformity of ringing and uniformity of
adjustment when two gongs were employed. Although no ringers of this
type are being made at present, yet a large number of them are in use
and they will consequently be described. Their action is interesting
in throwing better light on the more improved types, if for no other
reason.
Figs. 178 and 179 show, respectively, side and front views of the
original Kellogg bell. The entire mechanism is self-contained, all
parts being mounted on the base plate _1_. The electromagnet is of the
two-coil type, and is supported on the brackets _2_ and _3_. The
bracket _2_ is of iron so as to afford a magnetic yoke for the field of
the electromagnet, while the bracket _3_ is of brass so as not to
short-circuit the magnetic lines across the air-gap. The reed
tongue--consisting of the steel spring _5_, the soft-iron armature
pieces _6_, the auxiliary spring _7_, and the tapper ball _8_, all of
which are riveted together, as shown in Fig. 178--constitutes the only
moving part of the bell. The steel spring _5_ is rigidly mounted in the
clamping piece _9_ at the upper part of the bracket _3_, and the reed
tongue is permitted to vibrate only by the flexibility of this spring.
The auxiliary spring _7_ is much lighter than the spring _5_ and has
for its purpose the provision of a certain small amount of flexibility
between the tapper ball and the more rigid portion of the armature
formed by the iron strips _6-6_. The front ends of the magnet pole
pieces extend through the bracket _3_ and are there provided with
square soft-iron pole pieces _10_ set at right angles to the magnet
cores so as to form a rather narrow air-gap in which the armature may
vibrate.
[Illustration: Fig. 178. Under-Tuned Ringer]
The cores of the magnet and also the reed tongue are polarized by
means of the =L=-shaped bar magnet _4_, mounted on the iron yoke _2_
at one end in such manner that its other end will lie quite close to
the end of the spring _5_, which, being of steel, will afford a path
for the lines of force to the armature proper. We see, therefore, that
the two magnet cores are, by this permanent magnet, given one
polarity, while the reed tongue itself is given the other polarity,
this being exactly the condition that has already been described in
connection with the regular polarized bell or ringer.
The electromagnetic action by which this reed tongue is made to
vibrate is, therefore, exactly the same as that of an ordinary
polarized ringer, but the difference between the two is that, in this
harmonic ringer, the reed tongue will respond only to one particular
rate of vibrations, while the regular polarized ringer will respond to
almost any.
As shown in Fig. 178, the tapper ball strikes on the inside surface of
the single gong. The function of the auxiliary spring _7_ between the
ball and the main portion of the armature is to allow some resilience
between the ball and the balance of the armature so as to counteract
in some measure the accelerating influence of the gong on the
armature. In these bells, as already stated, the natural rate of
vibration of the reed tongue was made somewhat lower than the rate at
which the bell was to be operated, so that the reed tongue had to be
started by a current slightly out of tune with it, and then, as the
tapper struck the gong, the acceleration due to the gong would bring
the vibration of the reed tongue, as modified by the gong, into tune
with the current that was operating it. In ether words, in this system
the ringing currents that were applied to the line had frequencies
corresponding to what may be called the _operative rates of vibration_
of the reed tongues, which operative rates of vibration were in each
case the resultant of the natural pitch of the reed as modified by the
action of the bell gong when struck.
[Illustration: Fig. 179. Under-Tuned Ringer]
_In-Tune System._ The more modern method of tuning is to make the
natural rate of vibration of the reed tongue, that is, the rate at
which it naturally vibrates when not striking the gongs, such as to
accurately correspond to the rate of vibration at which the bells are
to be operated--that is, the natural rate of vibration of the reed
tongues is made the same as the operative rate. Thus the bells are
attuned for easy starting, a great advantage over the under-tuned
system. In the under-tuned system, the reeds being out of tune in
starting require heavier starting current, and this is obviously
conducive to cross-ringing, that is, to the response of bells to other
than the intended frequency.
Again, easy starting is desirable because when the armature is at
rest, or in very slight vibration, it is at a maximum distance from
the poles of the electromagnet, and, therefore, subject to the weakest
influence of the poles. A current, therefore, which is strong enough
to start the vibration, will be strong enough to keep the bell ringing
properly.
[Illustration: Fig. 180. Dean In-Tune Ringer]
When with this "in-tune" mode of operation, the armature is thrown
into sufficiently wide vibration to cause the tapper to strike the
gong, the gong may tend to accelerate the vibration of the reed
tongue, but the current impulses through the electromagnet coils
continue at precisely the same rates as before. Under this condition
of vibration, when the reed tongue has an amplitude of vibration wide
enough to cause the tapper to strike the gongs, the ends of the
armature come closest to the pole pieces, so that the pole pieces have
their maximum magnetic effect on the armature, with the result that
even if the accelerating tendency of the gongs were considerable, the
comparatively large magnetic attractive impulses occurring at the same
rate as the natural rate of vibration of the reed tongue, serve wholly
to prevent any actual acceleration of the reed tongue. The magnetic
attractions upon the ends of the armature, continuing at the initial
rate, serve, therefore, as a check to offset any accelerating
tendency which the striking of the gong may have upon the vibrating
reed tongue.
It is obvious, therefore, that in the "in-tune" system the
electromagnetic effect on the armature should, when the armature is
closest to the pole pieces, be of such an overpowering nature as to
prevent whatever accelerating tendency the gongs may have from
throwing the armature out of its "stride" in step with the current.
For this reason it is usual in this type to so adjust the armature
that its ends will actually strike against the pole pieces of the
electromagnet when thrown into vibration. Sufficient flexibility is
given to the tapper rod to allow it to continue slightly beyond the
point at which it would be brought to rest by the striking of the
armature ends against the pole pieces and thus exert a whipping action
so as to allow the ball to continue in its movement far enough to
strike against the gongs. The rebound of the gong is then taken up by
the elasticity of the tapper rod, which returns to an unflexed
position, and at about this time the pole piece releases the armature
so that it may swing over in the other direction to cause the tapper
to strike the other gong.
[Illustration: Fig. 181. Tappers for Dean Ringers]
The construction of the "in-tune" harmonic ringer employed by the Dean
Electric Company, of Elyria, Ohio, is illustrated in Figs. 180, 181,
and 182. It will be seen from Fig. 180 that the general arrangement of
the magnet and armature is the same as that of the ordinary polarized
ringer; the essential difference is that the armature is
spring-mounted instead of pivoted. The armature and the tapper rod
normally stand in the normal central position with reference to the
pole pieces of the magnet and the gongs. Fig. 181 shows the complete
vibrating parts of four ringers, adapted, respectively, to the four
different frequencies of the system. The assembled armature, tapper
rod, and tapper are all riveted together and are non-adjustable. All
of the adjustment that is done upon them is done in the factory and
is accomplished, first, by choosing the proper size of weight, and
second, by forcing this weight into the proper position on the tapper
rod to give exactly the rate of vibration that is desired.
[Illustration: Fig. 182. Dean In-Tune Ringer]
An interesting feature of this Dean harmonic ringer is the gong
adjustment. As will be seen, the gongs are mounted on posts which are
carried on levers pivoted to the ringer frame. These levers have at
their outer end a curved rack provided with gear teeth adapted to
engage a worm or screw thread mounted on the ringer frame. Obviously,
by turning this worm screw in one direction or the other, the gongs
are moved slightly toward or from the armature or tapper. This affords
a very delicate means of adjusting the gongs, and at the same time one
which has no tendency to work loose or to get out of adjustment.
[Illustration: Fig. 183. Kellogg In-Tune Ringer]
In Fig. 183 is shown a drawing of the "in-tune" harmonic ringer
manufactured by the Kellogg Switchboard and Supply Company. This
differs in no essential respect from that of the Dean Company, except
in the gong adjustment, this latter being affected by a screw passing
through a nut in the gong post, as clearly indicated.
In both the Kellogg and the Dean in-tune ringers, on account of the
comparative stiffness of the armature springs and on account of the
normal position of the armature with maximum air gaps and consequent
minimum magnetic pull, the armature will practically not be affected
unless the energizing current is accurately attuned to its own natural
rate. When the proper current is thrown on to the line, the ball will
be thrown into violent vibration, and the ends of the armature brought
into actual contact with the pole pieces, which are of bare iron and
shielded in no way. The armature in this position is very strongly
attracted and comes to a sudden stop on the pole pieces. The gongs are
so adjusted that the tapper ball will have to spring about one
thirty-second of an inch in order to hit them. The armature is held
against the pole piece while the tapper ball is engaged in striking
the gong and in partially returning therefrom, and so strong is the
pull of the pole piece on the armature in this position that the
accelerating influence of the gong has no effect in accelerating the
rate of vibration of the reed.
[Illustration: Fig. 184. Circuits of Dean Harmonic System]
_Circuits_. In Fig. 184 are shown in simplified form the circuits of a
four-station harmonic party line. It is seen that at the central
office there are four ringing keys, adapted, respectively, to impress
on the line ringing currents of four different frequencies. At the
four stations on the line, lettered A, B, C, and D, there are four
harmonic bells tuned accordingly. At Station A there is shown the
talking apparatus employing the Wheatstone bridge arrangement. The
talking apparatus at all of the other stations is exactly the same,
but is omitted for the sake of simplicity. A condenser is placed in
series with each of the bells in order that there may be no
direct-current path from one side of the line to the other when all of
the receivers are on their hooks at the several stations.
In Fig. 185 is shown exactly the same arrangement, with the exception
that the talking apparatus illustrated in detail at Station A is that
of the Kellogg Switchboard and Supply Company. Otherwise the circuits
of the Dean and the Kellogg Company, and in fact of all the other
companies manufacturing harmonic ringing systems, are the same.
_Advantages_. A great advantage of the harmonic party-line system is
the simplicity of the apparatus at the subscriber's station. The
harmonic bell is scarcely more complex than the ordinary polarized
ringer, and the only difference between the harmonic-ringing telephone
and the ordinary telephone is in the ringer itself. The absence of all
relays and other mechanism and also the absence of the necessity for
ground connections at the telephone are all points in favor of the
harmonic system.
[Illustration: Fig. 185. Circuits of Kellogg Harmonic System]
_Limitations_. As already stated, the harmonic systems of the various
companies, with one exception, are limited to four frequencies. The
exception is in the case of the North Electric Company, which sometimes
employs four and sometimes five frequencies and thus gets a selection
between five stations. In the four-party North system, the frequencies,
unlike those in the Dean and Kellogg systems, wherein the higher
frequencies are multiples of the lower, are arranged so as to be
proportional to the whole numbers 5, 7, 9, and 11, which, of course,
have no common denominator. The frequencies thus employed in the North
system are, in cycles per second, 30.3, 42.4, 54.5, and 66.7. In the
five-party system, the frequency of 16.7 is arbitrarily added.
While all of the commercial harmonic systems on the market are
limited to four or five frequencies, it does not follow that a greater
number than four or five stations may not be selectively rung. Double
these numbers may be placed on a party line and selectively actuated,
if the first set of four or five is bridged across the line and the
second set of four or five is connected between one limb of the line
and ground. The first set of these is selectively rung, as already
described, by sending the ringing currents over the metallic circuit,
while the second set may be likewise selectively rung by sending the
ringing currents over one limb of the line with a ground return. This
method is frequently employed with success on country lines, where it
is desired to place a greater number of instruments on a line than
four or five.
Step-by-Step Method. A very large number of step-by-step systems
have been proposed and reduced to practice, but as yet they have not
met with great success in commercial telephone work, and are nowhere
near as commonly used as are the polarity and harmonic systems.
_Principles_. An idea of the general features of the step-by-step
systems may be had by conceiving at each station on the line a ratchet
wheel, having a pawl adapted to drive it one step at a time, this pawl
being associated with the armature of an electromagnet which receives
current impulses from the line circuit. There is thus one of these
driving magnets at each station, each bridged across the line so that
when a single impulse of current is sent out from the central office
all of the ratchet wheels will be moved one step. Another impulse will
move all of the ratchet wheels another step, and so on throughout any
desired number of impulses. The ratchet wheels, therefore, are all
stepped in unison.
Let us further conceive that all of these ratchet wheels are provided
with a notch or a hole or a projection, alike in all respects at all
stations save in the position which this notch or hole or projection
occupies on the wheel. The thing to get clear in this part of the
conception is that all of these notches, holes, or projections are
alike on all of the wheels, but they occupy a different position on
the wheel for each one of the stations.
Consider further that the bell circuit at each of the stations is
normally open, but that in each case it is adapted to be closed when
the notch, hole, or projection is brought to a certain point by the
revolution of the wheel.
Let us conceive further that this distinguishing notch, hole, or
projection is so arranged on the wheel of the first station as to
close the bell circuit when one impulse has been sent, that that on
the second station will close the bell circuit after the second
impulse has been sent, and so on throughout the entire number of
stations. It will, therefore, be apparent that the bell circuits at
the various stations will, as the wheels are rotated in unison, be
closed one after the other. In order to call a given station,
therefore, it is only necessary to rotate all of the wheels in unison,
by sending out the proper stepping impulses until they all occupy such
a position that the one at the desired station is in such position as
to close the bell circuit at that station. Since all of the notches,
holes, or projections are arranged to close the bell circuits at their
respective stations at different times, it follows that when the bell
circuit at the desired station is closed those at all of the other
stations will be open. If, therefore, after the proper number of
stepping impulses has been sent to the line to close the bell circuit
of the desired station, ringing current be applied to the line, it is
obvious that the bell of that one station will be rung to the
exclusion of all others. It is, of course, necessary that provision be
made whereby the magnets which furnish the energy for stepping the
wheels will not be energized by the ringing current. This is
accomplished in one of several ways, the most common of which is to
have the stepping magnets polarized or biased in one direction and the
bells at the various stations oppositely biased, so that the ringing
current will not affect the stepping magnet and the stepping current
will not affect the ringer magnets.
After a conversation is finished, the line may be restored to its
normal position in one of several ways. Usually so-called release
magnets are employed, for operating on the releasing device at each
station. These, when energized, will withdraw the holding pawls from
the ratchets and allow them all to return to their normal positions.
Sometimes these release magnets are operated by a long impulse of
current, being made too sluggish in their action to respond to the
quick-stepping impulses; sometimes the release magnets are tapped from
one limb of the line to ground, so as not to be affected by the
stepping or ringing currents sent over the metallic circuit; and
sometimes other expedients are used for obtaining the release of the
ratchets at the proper time, a large amount of ingenuity having been
spent to this end.
As practically all step-by-step party-line systems in commercial use
have also certain other features intended to assure privacy of
conversation to the users, and, therefore, come under the general
heading of lock-out party-line systems, the discussion of commercial
examples of these systems will be left for the next chapter, which is
devoted to such lock-out systems.
Broken-Line Method. The broken-line system, like the step-by-step
system, is also essentially a lock-out system and for that reason only
its general features, by which the selective ringing is accomplished,
will be dealt with here.
_Principles_. In this system there are no tuned bells, no positively
and negatively polarized bells bridged to ground on each side of the
line, and no step-by-step devices in the ordinary sense, by which
selective signaling has ordinarily been accomplished on party lines.
Instead of this, each instrument on the line is exclusively brought
into operative relation with the line, and then removed from such
operative relation until the subscriber wanted is connected, at which
time all of the other instruments are locked out and the line is not
encumbered by any bridge circuits at any of the instruments that are
not engaged in the conversation. Furthermore, in the selecting of a
subscriber or the ringing of his bell there is no splitting up of
current among the magnets at the various stations as in ordinary
practice, but the operating current goes straight to the station
desired and to that station alone where its entire strength is
available for performing its proper work.
In order to make the system clear it may be stated at the outset that
one side of the metallic circuit line is continued as in ordinary
practice, passing through all of the stations as a continuous
conductor. The other side of the line, however, is divided into
sections, its continuity being broken at each of the subscriber's
stations. Fig. 186 is intended to show in the simplest possible way
how the circuit of the line may be extended from station to station in
such manner that only the ringer of one station is in circuit at a
time. The two sides of the line are shown in this figure, and it will
be seen that limb _L_ extends from the central office on the left to
the last station on the right without a break. The limb _R_, however,
extends to the first station, at which point it is cut off from the
extension _R_{x}_ by the open contacts of a switch. For the purpose of
simplicity this switch is shown as an ordinary hand switch, but as a
matter of fact it is a part of a relay, the operating coil of which is
shown at _6_, just above it, in series with the ringer.
[Illustration: Fig. 186. Principle of Broken-Line System]
Obviously, if a proper ringing current is sent over the metallic
circuit from the central office, only the bell at Station A will
operate, since the bells at the other stations are not in the circuit.
If by any means the switch lever _2_ at Station A were moved out of
engagement with contact _1_ and into engagement with contact _3_, it
is obvious that the bell of Station A would no longer be in circuit,
but the limb _R_ of the line would be continued to the extension
_R_{x}_ and the bell of Station B would be in circuit. Any current
then sent over the circuit of the line from the central office would
ring the bell of this station. In Fig. 187 the switches of both
Station A and Station B have been thus operated, and Station C is thus
placed in circuit. Inspection of this figure will show that the bells
of Station A, Station B, and Station D are all cut out of circuit, and
that, therefore, no current from the central office can affect them.
This general scheme of selection is a new-comer in the field, and for
certain classes of work it is of undoubted promise.
[Illustration: Fig. 187. Principle of Broken-Line System]