by the Stromberg-Carlson Telephone Manufacturing Company at one time is
shown in Fig. 282. In this the clearing-out drop was not bridged but
was placed in series in the tip side of the line and was shunted by a
condenser. The resistance of the clearing-out drop was 1,000 ohms and
the capacity of the condenser was 2 microfarads. It is obvious that
this way of connecting the clearing-out drop was subject to the
_ringing-through_ difficulty, since the circuit through which the
clearing-out current necessarily passed included the telephone
instrument of the line that was not sending the clearing-out signal.
This form was also objectionable because it was necessary for the
subscriber to ring through the combined resistance of two lines, and in
case the other line happened to be open, no clearing-out signal would
be received. While this circuit, therefore, was perhaps not quite so
likely as the other to tie up the subscriber, that is, to leave him
connected without the ability to send a clearing-out signal, yet it was
sure to ring through, for the clearing-out drop could not be thrown
without the current passing through the other subscriber's station.
[Illustration: Fig. 283. Dean Non-Ring-Through Cord Circuit]
_Non-Ring-Through Type._ An early attempt at a non-ring-through cord
is shown in Fig. 283, this having once been standard with the Dean
Electric Company. It made use of two condensers of 1 microfarad each,
one in each side of the cord circuit. The clearing-out drop was of 500
ohms resistance and was connected from the answering side of the tip
conductor to the calling side of the sleeve conductor. In this way
whatever clearing-out current reached the central office passed
through at least one of the condensers and the clearing-out drop. In
order for the clearing-out current to pass on beyond the central
office it was necessary for it to pass through the two condensers in
series. This arrangement had the advantage of giving a positive
ring-off, regardless of the condition of the connected line.
Obviously, even if the line was short-circuited, the ringing currents
from the other line would still be forced through the clearing-out
drop on account of the high effective resistance of the 1-microfarad
condenser connected in series with the short-circuited line. Also the
clearing-out signal would be properly received if the connected line
were open, since the clearing-out drop would still be directly across
the cord circuit. This arrangement also largely prevented through
ringing, since the currents would pass through the 1-microfarad
condenser and the 500-ohm drop more readily than through the two
condensers connected in series.
[Illustration: Fig. 284. Monarch Non-Ring-Through Cord Circuit]
In Fig. 284 is shown the non-ring-through arrangement of cord circuit
adopted by the Monarch Company. In this system the clearing-out drop
has two windings, either of which will operate the armature. The two
windings are bridged across the cord circuit, with a 1/2-microfarad
condenser in series in the tip strand between the two winding
connections. While the low-capacity condenser will allow the
high-frequency talking current to pass readily without affecting it to
any appreciable extent, it offers a high resistance to a low-frequency
ringing current, thus preventing it from passing out on a connected
line and forcing it through one of the windings of the coil. There is
a tendency to transformer action in this arrangement, one of the
windings serving as a primary and the other as a secondary, but this
has not prevented the device from being highly successful.
A modification of this arrangement is shown in Fig. 285, wherein a
double-wound clearing-out drop is used, and a 1/2-microfarad condenser
is placed in series in each side of the cord circuit between the
winding connections of the clearing-out drop. This circuit should give
a positive ring-off under all conditions and should prevent through
ringing except as it may be provided by the transformer action between
the two windings on the same core.
[Illustration: Fig. 285. Non-Ring-Through Cord Circuit]
Another rather ingenious method of securing a positive ring-off and
yet of preventing in a certain degree the undesirable ringing-through
feature is shown in the cord circuit, Fig. 286. In this two
non-inductive coils _1_ and _2_ are shown connected in series in the
tip and sleeve strands of the coils, respectively. Between the neutral
point of these two non-inductive windings is connected the
clearing-out drop circuit. Voice currents find ready path through
these non-inductive windings because of the fact that, being
non-inductive, they present only their straight ohmic resistance. The
impedance of the clearing-out drop prevents the windings being shunted
across the two sides of the cord circuit. With this circuit a positive
ring-off is assured even though the line connected with the one
sending the clearing-out signal is short-circuited or open. If it is
short-circuited, the shunt around the clearing-out drop will still
have the resistance of two of the non-inductive windings included in
it, and thus the drop will never be short-circuited by a very
low-resistance path. Obviously, an open circuit in the line will not
prevent the clearing-out signal being received. While this is an
ingenious scheme, it is not one to be highly recommended since the
non-inductive windings, in order to be effective so far as signaling
is concerned, must be of considerable resistance and this resistance
is in series in the talking circuit. Even non-inductive resistance is
to be avoided in the talking circuit when it is of considerable
magnitude and where there are other ways of solving the problem.
[Illustration: Fig. 286. Cord Circuit with Differential Windings]
_Double Clearing-out Type. _Some people prefer two clearing-out drops
in each cord circuit, so arranged that the one will be responsive to
currents sent from the line with which the answering plug is connected
and the other responsive only to currents sent from the line with
which the calling plug is connected. Such a scheme, shown in Fig. 287,
is sometimes employed by the Dean, the Monarch, and the Kellogg
companies. Two 500-ohm clearing-out drops of ordinary construction are
bridged across the cord circuit and in each side of the cord circuit
there is included between the drop connections a 1-microfarad
condenser. Ringing currents originating on the line with which the
answering plug is connected will pass through the clearing-out drop,
which is across that side of the cord circuit, without having to pass
through any condensers. In order to reach the other clearing-out drop
the ringing current must pass through the two 1-microfarad condensers
in series, this making in effect only 1/2-microfarad. As is well
known, a 1/2-microfarad condenser not only transmits voice currents
with ease but also offers a very high apparent resistance to ringing
currents. With the double clearing-out drop system the operator is
enabled to tell which subscriber is ringing off. If both shutters fall
she knows that both subscribers have sent clearing-out signals and
she, therefore, pulls down the connection without the usual precaution
of listening to see whether one of the subscribers may be waiting for
another connection. This double clearing-out system is analogous to
the complete double-lamp supervision that will be referred to more
fully in connection with common-battery circuits. There is not the
need for double supervision in magneto work, however, that there is in
common-battery work because of the fact that in magneto work the
subscribers frequently fail to remember to ring off, this act being
entirely voluntary on their part, while in common-battery work, the
clearing-out signal is given automatically by the subscriber when he
hangs up his receiver, thus accomplishing the desired end without the
necessity of thoughtfulness on his part.
[Illustration: Fig. 287. Double Clearing-Out Drops]
Another form of double clearing-out cord circuit is shown in Fig. 288.
In this the calling and the answering plugs are separated by repeating
coils, a condenser of 1-microfarad capacity being inserted between each
pair of windings on the two ends of the circuit. The clearing-out
drops are placed across the calling and answering cords in the usual
manner. The condenser in this case prevents the drop being
short-circuited with respect to ringing currents and yet permits the
voice currents to flow readily through it. The high impedance of the
drop forces the voice currents to take the path through the repeating
coil rather than through the drop. This circuit has the advantage of a
repeating-coil cord circuit in permitting the connection of metallic
and grounded lines without causing the unbalancing of the metallic
circuits by the connection to them of the grounded circuits.
[Illustration: Fig. 288. Double Clearing-Out Drops]
Recently there has been a growing tendency on the part of some
manufacturers to control their clearing-out signals by means of relays
associated with cord circuits, these signals sometimes being ordinary
clearing-out drops and sometimes incandescent lamps.
[Illustration: Fig. 289. Relay-Controlled Clearing-Out Drop]
In Fig. 289 is shown the cord circuit sometimes used by the L.M.
Ericsson Telephone Manufacturing Company. A high-wound relay is
normally placed across the cord and this, besides having a
high-resistance and impedance winding has a low-resistance locking
winding so arranged that when the relay pulls up its armature it will
close a local circuit including this locking winding and local battery.
When once pulled up the relay will, therefore, stay up due to the
energizing of this locking coil. Another contact operated by the relay
closes the circuit of a low-wound clearing-out drop placed across the
line, thus bridging it across the line. The condition of high impedance
is maintained across the cord circuit normally while the subscribers
are talking; but when either of them rings off, the high-wound relay
pulls up and locks, thus completing the circuit of the clearing-out
drop across the cords. The subsequent impulses sent from the
subscribers' generators operate this drop. The relay is restored or
unlocked and the clearing-out drop disconnected from the cord circuit
by means of a key which opens the locking circuit of the relay. This
key is really a part of the listening key and serves to open this
locking circuit whenever the listening key is operated. The
clearing-out drop is also automatically restored by the action of the
listening key, this connection being mechanical rather than electrical.
Recall Lamp:--The Monarch Company sometimes furnishes what it terms a
recall lamp in connection with the clearing-out drops on its magneto
switchboards. The circuit arrangement is shown in Fig. 290, wherein
the drop is the regular double-wound clearing-out drop like that of
Fig. 284. The armature carries a contact spring adapted to close the
local circuit of a lamp whenever it is attracted. The object of this
is to give the subscriber, whose line still remains connected by a
cord circuit, opportunity to recall the central office if the operator
has not restored the clearing-out drop.
[Illustration: Fig. 290. Cord Circuit with Recall Lamp]
_Lamp-Signal Type._ There has been a tendency on the part of some
manufacturing companies to advocate, instead of drop signals,
incandescent lamp signals for the cord circuits, and sometimes for the
line circuits on magneto boards. In most cases this may be looked upon
as a "frill." Where line lamps instead of drops have been used on
magneto switchboards, it has been the practice to employ, instead of a
drop, a locking relay associated with each lamp, which was so arranged
that when the relay was energized by the magneto current from the
subscriber's station, it would pull up and lock, thus closing the lamp
circuit.
The local circuit, or locking circuit, which included the lamp was
carried through a pair of contacts in the corresponding jacks so
arranged that when the plug was inserted in answer to the call, this
locking lamp circuit would be open, thereby extinguishing the lamp and
also unlocking the relay. There seems to be absolutely no good reason
why lamp signals should be substituted for mechanical drops in magneto
switchboards. There is no need for the economy in space which the lamp
signal affords, and the complications brought in by the locking
relays, and the requirements for maintaining a local battery suitable
for energizing the lamps are not warranted for ordinary cases.
[Illustration: Fig. 291. Cord Circuit with Double Lamp Signals]
In Fig. 291 is shown a cord circuit, adaptable to magneto
switchboards, provided with double lamp signals instead of
clearing-out drops. Two high-wound locking relays are bridged across
the line, the cord strands being divided by 1-microfarad condensers.
When the high-wound coil of either relay is energized by the magneto
current from the subscriber's station, the relay pulls up and closes a
locking circuit including a battery and a coil _2_, the contact _3_ of
the locking relay, and also the contact _4_ of a restoring key. This
circuit may be traced from the ground through battery, coil _2_,
contact _3_ controlled by the relay, and contact _4_ controlled by the
restoring key, and back to ground. In multiple with the locking coil
_2_ is the lamp, which is illuminated, therefore, whenever the locking
circuit is closed. Pressure on the restoring key breaks the locking
circuit of either of the lamps, thereby putting out the lamp and at
the same time restoring the locking relay to its normal position.
_Lamps vs. Drops in Cord Circuits._ So much has been said and written
about the advantages of incandescent lamps as signals in switchboards
and about the merits of the common-battery method of supplying current
to the subscribers, that there has been a tendency for people in
charge of the operation of small exchanges to substitute the lamp for
the drop in a magneto switchboard in order to give the general
appearance of common-battery operations. There has also been a
tendency to employ the common-battery system of operation in many
places where magneto service should have been used, a mistake which
has now been realized and corrected. In places where the simple
magneto switchboard is the thing to use, the simpler it is the better,
and the employment of locking relays and lamp signals and the
complications which they carry with them, is not warranted.
Switchboard Assembly. The assembly of all the parts of a simple
magneto switchboard into a complete whole deserves final
consideration. The structure in which the various parts are mounted,
referred to as the cabinet, is usually of wood.
_Functions of Cabinet._ The purpose of the cabinet is not only to form
a support for the various pieces of apparatus but also to protect them
from dust and mechanical injury, and to hold those parts that must be
manipulated by the operator in such relation that they may be most
convenient for use, and thus best adapted for carrying out their
various functions. Other points to be provided for in the design of
the cabinet and the arrangement of the various parts within are: that
all the apparatus that is in any way liable to get out of order may be
readily accessible for inspection and repairs; and that provision
shall be made whereby the wiring of these various pieces of apparatus
may be done in a systematic and simple way so as to minimize the
danger of crossed, grounded, or open circuits, and so as to provide
for ready repair in case any of these injuries do occur.
_Wall-Type Switchboards._ The simplest form of switchboard is that for
serving small communities in rural districts. Ordinarily the telephone
industry in such a community begins by a group of farmers along a
certain road building a line connecting the houses of several of them
and installing their own instruments. This line is liable to be
extended to some store at the village or settlement, thus affording
communication between these farmers and the center of their community.
Later on those residing on other roads do the same thing and connect
their lines to the same store or central point. Then it is that some
form of switchboard is established, and perhaps the storekeeper's
daughter or wife is paid a small fee for attendance.
[Illustration: Fig. 292. Wall Switchboard with Telephone]
A switchboard well-adapted for this class of service where the number
of lines is small, is shown in Fig. 292. In this the operator's
talking apparatus and her calling apparatus are embodied in an
ordinary magneto wall telephone. The switchboard proper is mounted
alongside of this, and the two line binding posts of the telephone are
connected by a pair of wires to terminals of the operator's plug,
which plug is shown hanging from the left-hand portion of the
switchboard. The various lines centering at this point terminate in
the combined drops and jacks on the switchboard, of which there are 20
shown in this illustration. Beside the operator's plug there are a
number of pairs of plugs shown hanging from the switchboard cabinet.
These are connected straight through in pairs, there being no
clearing-out drops or keys associated with them in the arrangement.
Each line shown is provided with an extra jack, the purpose of which
will be presently understood.
The method of operation is as follows: When a subscriber on a certain
line desires to get connection through the switchboard he turns his
generator and throws the drop. The operator in order to communicate
with him inserts the plug in which her telephone terminates into the
jack, and removes her receiver from its hook. Having learned that it
is for a certain subscriber on another line, she withdraws her plug
from the jack of the calling line and inserts it into the jack of the
called line, then, hanging up her receiver, she turns the generator
crank in accordance with the proper code to call that subscriber. When
that subscriber responds she connects the two lines by inserting the
two plugs of a pair into their respective jacks, and the subscribers
are thus placed in communication. The extra jack associated with each
line is merely an open jack having its terminals connected
respectively with the two sides of the line. Whenever an operator
desires to listen in on two connected lines she does so by inserting
the operator's plug into one of these extra jacks of the connected
lines, and she may thus find out whether the subscribers are through
talking or whether either one of them desires another connection. The
drops in such switchboards are commonly high wound and left
permanently bridged across the line so as to serve as clearing-out
drops. The usual night-alarm attachment is provided, the buzzer being
shown at the upper right-hand portion of the cabinet.
[Illustration: Fig. 293. Combined Telephone and Switchboard]
Another type of switchboard commonly employed for this kind of
service is shown in Fig. 293, in which the telephone and the
switchboard cabinet are combined. The operation of this board is
practically the same as that of Fig. 292, although it has
manually-restored drops instead of self-restoring drops; the
difference between these two types, however, is not material for this
class of service. For such work the operator has ample time to attend
to the restoring of the drop and the only possible advantage in the
combined drop-and-jack for this class of work is that it prevents the
operator from forgetting to restore the drops. However, she is not
likely to do this with the night-alarm circuit in operation, since the
buzzer or bell would continue to ring as long as the drop was down.
[Illustration: Fig. 294. Upright Magneto Switchboard]
[Illustration: Fig. 295. Upright Magneto Switchboard--Rear View]
_Upright Type Switchboard._ By far the most common type of magneto
switchboard is the so-called upright type, wherein the drops and
jacks are mounted on the face of upright panels rising from a
horizontal shelf, which shelf contains the plugs, the keys, and any
other apparatus which the operator must manipulate. Front and rear
views of such a switchboard, as manufactured by the Kellogg Company,
are shown in Figs. 294 and 295. This particular board is provided with
fifty combined drops and jacks and, therefore, equipped for fifty
subscribers' lines. The drops and jacks are mounted in strips of five,
and arranged in two panels. The clearing-out drops, of which there are
ten, are arranged at the bottom of the two panels in a single row and
may be seen immediately above the switchboard plugs. There are ten
pairs of cords and plugs with their associated ringing and listening
keys, the plugs being mounted on the rear portion of the shelf,
while the ringing and listening keys are mounted on the hinged portion
of the shelf in front of the plugs.
[Illustration: Fig. 296. Details of Drop, Jack, Plug, and Key
Arrangement]
[Illustration: Fig. 297. Cross-Section of Upright Switchboard]
A better idea of the arrangement of drops, jacks, plugs, and keys may
be had from an illustration of a Dean magneto switchboard shown in
Fig. 296. The clearing-out drops and the arrangement of the plugs and
keys are clearly shown. The portion of the switchboard on which the
plugs are mounted is always immovable, the plugs being provided with
seats through which holes are bored of sufficient size to permit the
switchboard cord to pass beneath the shelf. When one of these plugs is
raised, the cord is pulled up through this hole thus allowing the plug
to be placed in any of the jacks.
The key arrangement shown in this particular cut is instructive. It
will be noticed that the right-hand five pairs of plugs are provided
with ordinary ringing and listening keys, while the left-hand five are
provided with party-line ringing keys and listening keys. The
listening key in each case is the one in the rear and is alike for all
of the cord pairs. The right-hand five ringing keys are so arranged
that pressing the lever to the rear will ring on the answering cord,
while pressing it toward the front will cause ringing current to flow
on the calling plug. In the left-hand five pairs of cords shown in
this cut, the pressure of any one of the keys causes a ringing current
of a certain frequency to flow on the calling cord, this frequency
depending upon which one of the keys is pressed.
[Illustration: Fig. 298. Cord Weight]
An excellent idea of the grouping of the various pieces of apparatus
in a complete simple magneto switchboard may be had from Fig. 297.
While the arrangement here shown is applicable particularly to the
apparatus of the Dean Electric Company, the structure indicated is
none-the-less generally instructive, since it represents good practice
in this respect. In this drawing the stationary plug shelf with the
plug seat is clearly shown and also the hinged key shelf. The hinge of
the key shelf is an important feature and is universally found in all
switchboards of this general type. The key shelf may be raised and
thus expose all of the wiring leading to the keys, as well as the
various contacts of the keys themselves, to inspection.
[Illustration: Fig. 299. Magneto Switchboard, Target Signals]
As will be seen, the switchboard cords leading from the plugs extend
down to a point near the bottom of the cabinet where they pass through
pulley weights and then up to a stationary cord rack. On this cord
rack are provided terminals for the various conductors in the cord,
and it is at this point that the cord conductors join the other wires
leading to the other portions of the apparatus as required. A good
form of cord weight is shown in Fig. 298; and obviously the function
of these weights is to keep the cords taut at all times and to prevent
their tangling.
[Illustration: Fig. 300. Rear View of Target Signal, Magneto
Switchboard]
The drawing, Fig. 297, also gives a good idea of the method of
mounting the hand generator that is ordinarily employed with such
magneto switchboards. The shaft of the generator is merely continued
out to the front of the key shelf where the usual crank is provided,
by means of which the operator is able to generate the necessary
ringing current. Beside the hand generator at each operator's
position, it is quite common in magneto boards, of other than the
smallest sizes, to employ some form of ringing generator, either a
power-driven generator or a pole changer driven by battery current for
furnishing ringing current without effort on the part of the operator.
[Illustration: Fig. 301. Dean Two-Position Switchboard]
Switchboards as shown in Figs. 294 and 295, are called single-position
switchboards because they afford room for a single operator.
Ordinarily for this class of work a single operator may handle from
one to two hundred lines, although of course this depends on the
amount of traffic on the line, and this, in turn, depends on the
character of the subscribers served, and also on the average number of
stations on a line. Another single-position switchboard is shown in
Figs. 299 and 300, being a front and rear view of the simple magneto
switchboard of the Western Electric Company, which is provided with
the target signals of that company rather than the usual form of drop.
Where a switchboard must accommodate more lines than can be handled by
a single operator, the cabinet is made wider so as to afford room for
more than one operator to be seated before it. Sometimes this is
accomplished by building the cabinet wider, or by putting two such
switchboard sections as are shown in Figs. 294 or 299 side by side. A
two-position switchboard section is shown in front and rear views in
Figs. 301 and 302.
[Illustration: Fig. 302. Rear View of Dean Two-Position Switchboard]
_Sectional Switchboards._ The problem of providing for growth in a
switchboard is very much the same as that which confronts one in
buying a bookcase for his library. The Western Electric Company has
met this problem, for very small rural exchanges, in much the same way
that the sectional bookcase manufacturers have provided for the
possible increase in bookcase capacity. Like the sectional bookcase,
this sectional switchboard may start with the smallest of equipment--a
single sectional unit--and may be added to vertically as the
requirements increase, the original equipment being usable in its more
extended surroundings.
[Illustration: Fig. 303. Sectional Switchboard--Wall Type]
This line of switchboards is illustrated in Figs. 303 to 306. The
beginning may be made with either a wall type or an upright type of
switchboard, the former being mounted on brackets secured to the wall,
and the latter on a table. A good idea of the wall type is shown in
Fig. 303. Three different kinds of sectional units are involved in
this: first, the unit which includes the cords, plugs, clearing-out
drops, listening jacks, operator's telephone set and generator;
second, the unit containing the line equipment, including a strip of
ten magneto line signals and their corresponding jacks; third, the
finishing top, which includes no equipment except the support for the
operator's talking apparatus.
[Illustration: Fig. 301. Sectional Switchboard--Wall Type]
The first of the units in Fig. 303 forms the foundation on which the
others are built. Two of the line-equipment units are shown; these
provide for a total of twenty lines. The top rests on the upper
line-equipment unit, and when it becomes necessary to add one or more
line-equipment units as the switchboard grows, this top is merely
taken off, the other line-equipment units put in place on top of those
already existing, and the top replaced. The wall type of sectional
switchboard is so arranged that the entire structure may be swung out
from the wall, as indicated in Fig. 304, exposing all of the apparatus
and wiring for inspection. Each of the sectional units is provided
with a separate door, as indicated, so that the rear door equipment is
added to automatically as the sections are added. In the embodiment of
the sectional switchboard idea shown in these two figures just
referred to, no ringing and listening keys are provided, but the
operator's telephone and generator terminate in a special plug--the
left-hand one shown in Fig. 303--and when the operator desires to
converse with the connected subscribers, she does so by inserting the
operator's plug into one of the jacks immediately below the
clearing-out drop corresponding to the pair of plugs used in making
the connection. The arrangement in this case is exactly the same in
principle as that described in Fig. 292. The operator's generator is
so arranged in connection with this left-hand operator's plug that the
turning of the generator crank automatically switches the operator's
telephone set off and switches the generator on, just the same as a
switch hook may do in a subscriber's series telephone.
[Illustration: Fig. 305. Sectional Switchboard--Table Type]
[Illustration: Fig. 306. Sectional Switchboard--Table Type]
The upright type of sectional switchboard is shown in Figs. 305 and
306, which need no explanation in view of the foregoing, except to say
that, in the particular instrument illustrated, ringing and listening
keys are provided instead of the jack-and-plug arrangement of the wall
type. In this case also, the top section carries an arm for supporting
a swinging transmitter instead of the hook support for the combined
transmitter and receiver.
REVIEW QUESTIONS
[Blank Page]
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 11--62
* * * * *
1. When was the telephone invented and by whom?
2. State the velocity of sound in air. Is it higher in air than in a
denser medium?
3. State and define the characteristics of sound.
4. Make sketch of Bell's original magneto telephone without permanent
magnets.
5. Describe and sketch Hughes' microphone.
6. Which is, at present, the best material for varying the resistance
in transmitters?
7. Give the fundamental differences between the magneto transmitter
and the carbon transmitter.
8. What is the function of the induction coil in the telephone
circuit?
9. Describe and sketch the different kinds of visible signals.
10. What should be the diameter of hard drawn copper wire in order to
allow economical spacing of poles?
11. State the four principal properties of a telephone line.
12. If in testing a line the capacity is changed what are the results
found on the receiver and transmitter end?
13. Why is paper used as an insulator of telephone cables?
14. How does a conductor behave in connection with direct current and
how with alternating current?
15. What influence has inductance on the telephone?
16. Define impedance and give the formula for it.
17. What is the usual specification for insulation of resistance in
telephone cables?
18. If 750 feet of cable have an insulation resistance of 9,135
megohms, how great is the insulation resistance for 7 miles and 1,744
feet of cable?
19. What is the practical limiting conversation distance for No. 10 B.
and S. wire?
20. Describe Professor Pupin's method of inserting inductance into the
telephone line.
21. What does _mho_ denote?
22. Why are Pupin's coils not so successful on open wires?
23. What is a repeater?
24. Define _reactive interference_.
25. State the frequencies of the pitches of the human voice.
26. What is the office of a diaphragm in a telephone apparatus?
27. What transmitter material has greatly increased the ranges of
speech?
28. Describe the different methods of measurements of telephone
circuits.
29. What are the two kinds of _electric calls_?
30. How many conductors has a telephone line?
31. Give formula for capacity reactance and the meaning of the
symbols.
32. Which American cities are joined by underground lines at present?
33. State the two practical ways of improving telephone transmission.
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 63--141
* * * * *
1. On what general principle are most of the telephone transmitters of
today constructed?
2. Make sketch of the new Western Electric transmitter and describe
its working.
3. Make sketch and describe the Kellogg transmitter.
4. What troubles were encountered in the earlier forms of granular
carbon transmitters and how were they overcome?
5. What limits the current-carrying capacity of the transmitter? How
may this capacity be increased?
6. State in what kind of transmitters a maximum degree of
sensitiveness is desirable.
7. Show the conventional symbols for transmitters.
8. Describe a telephone receiver.
9. Sketch a Western Electric receiver and point out its deficiencies.
10. Make a diagram of the Kellogg receiver.
11. Describe the direct-current receiver of the Automatic Electric
Company.
12. Describe and sketch the Dean receiver.
13. Show the conventional symbols of a receiver.
14. Describe exactly how, in a cell composed of a tin and a silver
plate with dilute sulphuric acid as electrolyte, the current inside
and outside of the cell will flow.
15. Describe the phenomenon of polarization.
16. What is _local action_ of a cell? How may it be prevented?
17. Into how many classes may cells be divided? Which class is most
used in telephony?
18. Describe the LeClanché cell.
19. Sketch and describe an excellent form of dry cell.
20. Show the conventional symbols for batteries.
21. Sketch and describe the generator shunt switch and the generator
cut-in switch.
22. How may a pulsating current be derived from a magneto generator?
23. Show conventional symbols for magneto generators.
24. Sketch and describe the Western Electric polarized bell.
25. Give conventional ringer symbols.
26. What is the purpose of the hook switch?
27. Make sketch and give description of Kellogg's long lever hook
switch.
28. Describe and sketch the Western Electric short lever hook switch.
29. Point out the principal difference between the desk stand hook
switches of the Western Electric Company and of the Kellogg
Switchboard and Supply Company.
30. Give conventional symbols of hook switches.
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 143--225
* * * * *
1. Describe an electromagnet and its function in telephony.
2. Sketch an iron-clad electromagnet.
3. What is a differential electromagnet? Sketch and describe one type.
4. State the desirable characteristics of good enamel insulation for
magnet wire.
5. If you have a coil of No. 23 double cotton B. and S. wire of 115
ohms resistance and you have to rewind it for 1,070 ohms resistance
with double cotton wire, what number of wire would you take? Show
calculation.
NOTE. No. 23 d. c wire has res. 1.772 ohms per cubic inch; for
the core, 115 ohms. There are required in the coil 1,070 ohms,
that is, 9.3 times as much. 1.772 x 9.3 = 16.47 ohms, which must
be the resistance per cu. in. This resistance gives, according to
Table IV, No. 29 wire.
6. What is an impedance coil? State how it differs from an
electromagnet coil.
7. Describe the different kinds of impedance coils.
8. Give symbol of impedance coil.
9. What are the principal parts of an induction coil?
10. What is the function of an induction coil in telephony?
11. What is a repeating coil and how does it differ from an induction
coil?
12. Give conventional symbols of induction coils and repeating coils.
13. Enumerate the different types of non-inductive resistance devices
and give a short description of each.
14. Define condenser.
15. What is the meaning of the word _dielectrics_?
16. State what you understand by the specific inductive capacity of a
dielectric.
17. Upon what factors does the capacity of a condenser depend?
18. What is the usual capacity of condensers in telephone practice?
19. Give conventional condenser symbols.
20. By what two methods may the current be supplied to a telephone
transmitter?
21. Make sketch of local-battery stations with metallic circuit.
22. Sketch common-battery circuit in series with two lines.
23. State the objections against the preceding arrangement.
24. Make sketch of the standard arrangement of the Western Electric
Company in bridging the common battery with repeating coils.
25. Sketch the arrangement of bridging the battery with impedance
coils and state the purpose of the coils.
26. Make diagram of a common-source current supply for many lines with
repeating coils and point out the travel of the voice currents.
27. Name the different parts which comprise a telephone set.
28. What is a magneto telephone?
29. Make diagram of the circuit of a series magneto set with receiver
on the hook and explain how the different currents are flowing.
30. Show diagram of the Stromberg-Carlson magneto desk telephone
circuit and describe its working.
31. Give sketch of the Stromberg-Carlson common-battery wall set
circuit.
32. Describe briefly the microtelephone set.
33. Make sketch of the Monarch common-battery wall set.
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 227--286
* * * * *
1. What is a party line?
2. What is usually understood by private lines?
3. What problem is there to overcome in connection with party lines?
4. State the two general classes of party-line systems.
5. Point out the defects of the series system.
6. Make sketch of a metallic bridging line and show the circuit for
the voice currents.
7. What is a signal code?
8. Give classification of selective party-line systems with short
definitions.
9. Describe the principle of selection by polarity and make sketch
illustrating this principle.
10. Make diagram of the circuit of a four-party station with relay.
11. Describe the process of tuning in the harmonic system.
12. What is the difference between the under-tune and in-tune systems?
13. Sketch circuit of Kellogg's harmonic system.
14. Illustrate the principle of a broken-line system by a sketch.
15. In what particulars does the party-line system in rural districts
differ from that within urban limits?
16. Describe and sketch Pool's lock-out system.
17. Make diagram of the K.B. lock-out system.
18. What is the object of the ratchet in this system?
19. Make diagram of simplified circuits of Roberts system.
20. Sketch and describe Roberts latching key and connections.
21. Sketch circuits of bridging station for non-selective party line.
22. How would you arrange the signal code for six stations on a
non-selective party line?
23. What is the limit of number of stations on a non-selective party
line under ordinary circumstances?
24. State the objections against the party polarity system as shown in
Fig. 172.
25. What are the advantages of the harmonic party-line system?
26. To how many frequencies is the harmonic system usually limited?
27. What can you say about the commercial success of the step-by-step
method?
28. State the principles of a lock-out party line.
29. For what purpose is a condenser placed in the receiver circuit of
each station in the K.B. lock-out system?
30. How are the selecting relays in Roberts line restored to their
normal position after a conversation is finished?
31. What are the objections against the Roberts system?
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 287--315
* * * * *
1. What are electrical hazards?
2. When is the lightning hazard least?
3. What actions can electricity produce? Which involves the greater
hazard to the value of property?
4. When is a piece of apparatus called "self-protecting"?
5. Why must a protector for telephone apparatus work more quickly for
a large current than for a small one?
6. State the general problem which heating hazards present with
relation to telephone apparatus.
7. What is the most nearly universal electrical hazard?
8. Sketch and describe the saw-tooth lightning arrester.
9. Make diagram of the carbon-block arrester and state its advantages.
10. Describe a vacuum arrester.
11. Explain the reason for placing an impedance in connection with the
lightning arrester.
12. What is the purpose of the globule of low-melting alloy in the
Western Electric Company's arrester?
13. Why are not fuses good lightning arresters?
14. What is the proper function of a fuse?
15. Make sketch of a mica slip fuse.
16. Define _sneak currents_.
17. Make a diagram of a sneak-current arrester and describe its
principles and working.
18. Describe a heat coil.
19. Sketch a complete line protection.
20. Where is the proper position of the fuse?
21. Which wires are considered exposed and which unexposed?
22. Why is it not necessary to install sneak-current arresters in
central-battery subscribers' stations?
23. Sketch and describe the action of a combined sneak-current and
air-gap arrester, as widely used by Bell companies.
24. Describe the self-soldering heat-coil arrester.
25. What is the purpose of ribbon fuses?
26. What is a drainage coil?
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 317--386
* * * * *
1. What is a central office?
2. What are (_a_) subscriber's lines? (_b_) Trunk lines? (_c_) Toll
lines?
3. For what purpose is the switchboard?
4. Give short descriptions of the different classes of switchboards.
5. How are manual switchboards subdivided? Describe briefly the
different types.
6. Define A and B boards.
7. What is a call circuit?
8. What kind of calls are handled on a toll switchboard?
9. Give drop symbol and describe its principles.
10. What is a jack?
11. Make a sketch of a plug inserted into a jack.
12. Give jack and plug symbols.
13. What are ringing and listening keys?
14. Show symbols for ringing and listening keys.
15. State the parts of which a cord equipment consists.
16. Show step by step the various operations of a telephone system
wherein the lines center in a magneto switchboard. Make all the
necessary diagrams and give brief descriptions to show that you
understand each operation.
17. On what principle does a drop with night-alarm contact operate?
18. What is the advantage of associating jacks and drops?
19. Describe the mechanical restoration as employed in the Miller
drop and jack.
20. Describe the electrical restoration of drop shutters as
manufactured by the Western Electric Company.
21. What complications arise in ringing of party lines and how are
they overcome?
22. Give diagram of the complete circuit of a simple magneto
switchboard.
23. Sketch night-alarm circuit with relay.
24. What is a convertible cord circuit?
25. State what disadvantages may be encountered under certain
conditions with a bridging drop-cord circuit.
26. Are lamps in cord circuits to be advocated on magneto
switchboards?
27. What is the function of the cabinet?
28. Give cross-section of upright switchboard as used in the magneto
system.
29. What is the purpose of a sectional switchboard?
30. Give a short description of the essential parts of a sectional
switchboard.
INDEX
INDEX
_The page numbers of this volume will be found at the bottom of the
pages; the numbers at the top refer only to the section._
A
Acousticon transmitter
Acoustics
characteristics of sound
loudness
pitch
timbre
human ear
human voice
propagation of sound
Air-gap vs. fuse arresters
Amalgamated zincs
Arrester separators
Audible signals
magneto bell
telegraph sounder
telephone receiver
vibrating bell
Automatic Electric Company
direct-current receiver
transmitter
Automatic shunt
B
Bar electromagnet
Battery bell
Battery symbols
Blake single electrode
Brazed bell
Broken-back ringer
Broken-line method of selective signaling
C
Capacity reactance
Carbon
adaptability
limitations
preparation of
superiority
Carbon air-gap arrester
Carbon-block arrester
Carrying capacity of transmitter
Central-office protectors
Characteristics of sound
loudness
pitch
timbre
Chloride of silver cell
Closed-circuit cells
Closed-circuit impedance coil
Common-battery telephone sets
Condensers
capacity
charge
conventional symbols
definition of
dielectric
dielectric materials
functions
means for assorting current
sizes
theory
Conductivity of conductors
Conductors, conductivity of
Conventional symbols
Cook
air-gap arrester
arrester
arrester for magneto stations
Crowfoot cell
Current supply to transmitters
common battery
advantages
bell substation arrangement
bridging battery with impedance coils
bridging battery with repeating coil
current supply from distant point
current supply over limbs of line in parallel
Dean substation arrangement
double battery with impedance coil
Kellogg substation arrangement
North Electric Company system
series battery
series substation arrangement
Stromberg-Carlson system
supply many lines from common source
repeating coil
retardation coil
local battery
D
Dean
drop and jack
receiver
wall telephone hook
Desk stand hooks
Kellogg
Western Electric
Dielectric
Dielectric materials
dry paper
mica
Differential electromagnet
Direct-current receiver
Drainage coils
E
Electric lamp signal
Electrical hazards
Electrical reproduction of speech
carbon
conversion from sound waves to vibration of diaphragm
conversion from vibration to voice currents
conversion from voice currents to vibration
cycle of conversion
detrimental effects of capacity
early conceptions
electrostatic telephone
induction coil
limitations of magneto transmitter
loose contact principle
magneto telephone
measurements of telephone currents
variation of electrical pressure
variation of resistance
Electrical signals
audible
magneto-bell
telegraph sounder
telephone receiver
vibrating bell
visible
electric lamp signal
electromagnetic signal
Electrodes
arrangement of
carbon preparation
multiple
single
Electrolysis
Electromagnetic method of measuring telephone currents
Electromagnetic signal
Electromagnets and inductive coils
conventional symbols
differential electromagnet
direction of armature motion
direction of lines of force
electromagnets
low-resistance circuits
horseshoe form
iron-clad form
special horseshoe form
impedance coils
kind of iron
number of turns
types
closed-circuit
open-circuit
toroidal
induction coil
current and voltage ratios
design
functions
use and advantage
magnet wire
enamel
silk and cotton insulation
space utilization
wire gauges
magnetic flux
magnetization curves
magnetizing force
mechanical details
permeability
reluctance
repeating coil
winding methods
winding calculations
winding data
winding terminals
Electrostatic capacity
unit of
Electrostatic telephone
Enamel
F
Five-bar generator
Fuller cell
G
Galvani
Generator armature
Generator cut-in switch
Generator shunt switch
Generator symbols
Granular carbon
Gravity cell
H
Hand receivers
Harmonic method of selective signaling
advantages
circuits
in-tune system
limitations
principles
tuning
under-tune system
Head receivers
Heat coil
Holtzer-Cabot arrester
Hook switch
automatic operation
contact material
design
desk stand hooks
Kellogg
Western Electric
purpose
symbols
wall telephone hooks
Dean
Kellogg
Western Electric
Horseshoe electromagnet
Human ear
Human voice
I
Impedance coils
kind of iron
number of turns
symbols of
types
closed-circuit
open-circuit
toroidal
Inductance vs. capacity
Induction coil
current and voltage ratios
design
functions
use and advantage
Inductive neutrality
Inductive reactance
Insulation of conductors
Introduction to telephony
Iron-clad electromagnet
Iron wire ballast
K
Kellogg
air-gap arrester
desk stand hook
drop and jack
receiver
ringer
transmitter
wall telephone hook
L
Lalande cell
Lamp filament
Le Clanché cell
Lenz law
Line signals
Lines of force, direction of
Loading coils
Lock-out party-line systems
broken-line method
operation
Poole system
step-by-step system
Loudness of sound
Low-reluctance circuits
horseshoe form
iron-clad form
M
Magnetic flux
Magnetization curves
Magnetizing force
Magneto bell
Magneto operator
Magneto signaling apparatus
armature
automatic shunt
battery bell
generator symbols
magneto bell
magneto generator
method of signaling
polarized ringer
pulsating current
ringer symbols
theory
Magneto switchboard
automatic restoration
mechanical
Dean type
Kellogg type
Monarch type
Western Electric type
circuits of complete switchboard
code signaling
commercial types of drops and jacks
early drops
jack mounting
manual vs. automatic restoration
methods of associating
night alarm
tubular drops
component parts
jacks and plugs
keys
line and cord equipments
line signal
operators' equipment
cord-circuit considerations
double clearing-out type
lamp-signal type
non-ring through type
series drop type
simple bridging drop type
definitions
electrical restoration
grounded and metallic-circuit lines
mode of operation
night-alarm circuits
operation in detail
clearing out
essentials of operation
normal condition of line
operator answering
operator calling
subscriber calling
subscribers conversing
operator's telephone equipment
cut-in jack
ringing and listening keys
horizontal spring type
party-line ringing keys
self-indicating keys
vertical spring type
switchboard assembly
functions of cabinet
sectional switchboards
upright type of switchboard
wall type switchboard
switchboard cords
concentric conductors
parallel tinsel conductors
steel spiral conductors
switchboard plugs
Magneto telephone
Magneto telephone sets
Mica card resistance
Mica slip fuse
Microtelephone set
Monarch drop and jack
Monarch receiver
Monarch transmitter
Multiple electrode
Mutual induction
N
Non-inductive resistance devices
inductive neutrality
provisions against heating
temperature coefficient
types
differentially-wound unit
iron wire ballast
lamp filament
mica card unit
Non-selective party-line systems
bridging
limitations
series
signal code
O
Open-circuit cells
Open-circuit impedance coil
Operator's receiver
P
Packing of transmitters
Permeability
Pitch
Doppler's principle
vibration of diaphragms
Polarity method of selective signaling
Polarization of cells
Polarized ringer
brazed bell
Kellogg
Western Electric
Poole lock-out system
Primary cells
conventional symbol
series and multiple connections
simple voltaic
types of
closed-circuit
Fuller
gravity
Lalande
prevention of creeping
setting up
open-circuit
Le Clanché
standard
chloride of silver
Propagation of sound
Protective means
against high potentials
air-gap arrester
advantages of carbon
commercial types
continuous arcs
discharge across gaps
dust between carbons
introduction of impedance
metallic electrodes
vacuum arresters
against sneak currents
heat coil
sneak-current arresters
against strong currents
fuses
enclosed
mica
proper functions
central-office protectors
self-soldering heat coils
sneak-current and air-gap arrester
city exchange requirements
complete line protection
electrolysis
subscribers' station protectors
ribbon fuses
Pulsating-current commutator
R
Receivers
Dean
direct-current
early
Kellogg
modern
Monarch
operator's
single-pole
symbols
Western Electric
Reluctance
Repeating coil
Ribbon fuses
Ringer symbols
Ringing and listening key
Robert's latching relay
Robert's self-cleansing arrester
Rolled condenser
S
Saw-tooth arrester
Selective party-line systems
broken-line method
classification
broken-line systems
harmonic systems
polarity systems
step-by-step systems
harmonic method
polarity method
step-by-step method
Self-induction
Signal code
Signaling, method of
Silk and cotton insulation
Single electrode
Single-pole receiver
Sneak-current arresters
Solid-back transmitter
Sound
characteristics of
loudness
pitch
timbre
Standard cell
Step-by-step lock-out system
Step-by-step method of selective signaling
Subscribers' station protectors
Switchboard cords
Switchboard plugs
Switchboard transmitter
Symbols
battery
condenser
generator
hook switch
impedance coil
induction coil
receiver
repeating coil
ringer
ringing and listening key
transmitter
T
Table
condenser data
copper wire
German silver wire--18 per cent
German silver wire--30 per cent
metals, behavior of, in different electrolysis
signal code
specific inductive capacities
temperature coefficients
transmission distances, limiting
winding data for insulating wires
Tandem differential electromagnet
Telegraph sounder
Telephone currents, measurements of
electromagnetic method
thermal method
Telephone exchange, features of
districts
subscribers' lines
switchboards
toll lines
trunk lines
Telephone lines
conductivity of conductors
electrostatic capacity
inductance of circuit
inductance vs. capacity
insulation of conductors
transmission
Telephone sets
classification of
common-battery telephone
magneto telephone
wall and desk telephones
common-battery
desk
hotel
wall
magneto
circuits of
bridging
series
desk
wall
Temperature coefficients
Thermal method of measuring telephone currents
Timbre
Toroidal impedance coil
Toroidal repeating coil
Transmission, ways of improving
Transmitters
acousticon
Automatic Electric Company
carrying capacity
conventional diagram
electrode
arrangement of
multiple
single
granular carbon
Kellogg
materials
Monarch
packing
sensitiveness
switchboard
symbols
variable resistance
Western Electric solid-back
U
Under-tuned ringer
V
Vacuum arrester
Variable resistance
Vibrating bell
Visible signals
electric lamp
electromagnetic
Volta
Voltaic cell
amalgamated zincs
difference of potential
local action
polarization
theory
W
Wall telephone hooks
Dean
Kellogg
Western Electric
Western Electric
air-gap arrester
desk stand hook
drop and jack
receiver
ringer
solid-back transmitter
station arrester
wall telephone hook
White transmitter
Wire gauges