MAGNETO SIGNALING APPARATUS
Method of Signaling. The ordinary apparatus, by which speech is
received telephonically, is not capable of making sufficiently loud
sounds to attract the attention of people at a distance from the
instrument. For this reason it is necessary to employ auxiliary
apparatus for the purpose of signaling between stations. In central
offices where an attendant is always on hand, the sense of sight is
usually appealed to by the use of signals which give a visual
indication, but in the case of telephone instruments for use by the
public, the sense of hearing is appealed to by employing an audible
rather than a visual signal.
Battery Bell. The ordinary vibrating or battery bell, such as is
employed for door bells, is sometimes, though not often, employed in
telephony. It derives its current from primary batteries or from any
direct-current source. The reason why they are not employed to a
greater extent in telephony is that telephone signals usually have to
be sent over lines of considerable length and the voltage that would
be required to furnish current to operate such bells over such lengths
of line is higher than would ordinarily be found in the batteries
commonly employed in telephone work. Besides this the make-and-break
contacts on which the, ordinary battery bell depends for its operation
are an objectionable feature from the standpoint of maintenance.
Magneto Bell. Fortunately, however, there has been developed a
simpler type of electric bell, which operates on smaller currents, and
which requires no make-and-break contacts whatever. This simpler form
of bell is commonly known as the _polarized_, or _magneto_, bell or
_ringer_. It requires for its operation, in its ordinary form, an
alternating current, though in its modified forms it may be used with
pulsating currents, that is, with periodically recurring impulses of
current always in the same direction.
Magneto Generator. In the early days of telephony there was nearly
always associated with each polarized bell a magneto generator for
furnishing the proper kind of current to ring such bells. Each
telephone was therefore equipped, in addition to the transmitter and
receiver, with a signal-receiving device in the form of a polarized
bell, and with a current generator by which the user was enabled to
develop his own currents of suitable kind and voltage for ringing the
bells of other stations.
Considering the signaling apparatus of the telephones alone,
therefore, each telephone was equipped with a power plant for
generating currents used by that station in signaling other stations,
the prime mover being the muscles of the user applied to the turning
of a crank on the side of the instrument; and also with a
current-consuming device in the form of a polarized electromagnetic
bell adapted to receive the currents generated at other stations and
to convert a portion of their energy into audible signals.
The magneto generator is about the simplest type of dynamo-electric
machine, and it depends upon the same principles of operation as the
much larger generators, employed in electric-lighting and
street-railway power plants, for instance. Instead of developing the
necessary magnetic field by means of electromagnets, as in the case of
the ordinary dynamo, the field of the magneto generator is developed
by permanent magnets, usually of the horseshoe form. Hence the name
_magneto_.
[Illustration: Fig. 68. Principles of Magneto Generator]
In order to concentrate the magnetic field within the space in which
the armature revolves, pole pieces of iron are so arranged in
connection with the poles of the permanent magnet as to afford a
substantially cylindrical space in which the armature conductors may
revolve and through which practically all the magnetic lines of force
set up by the permanent magnets will pass. In Fig. 68 there is shown,
diagrammatically, a horseshoe magnet with such a pair of pole pieces,
between which a loop of wire is adapted to rotate. The magnet _1_ is
of hardened steel and permanently magnetized. The pole pieces are
shown at _2_ and _3_, each being of soft iron adapted to make good
magnetic contact on its flat side with the inner flat surface of the
bar magnet, and being bored out so as to form a cylindrical recess
between them as indicated. The direction of the magnetic lines of
force set up by the bar magnet through the interpolar space is
indicated by the long horizontal arrows, this flow being from the
north pole (N) to the south pole (S) of the magnet. At _4_ there is
shown a loop of wire supposed to revolve in the magnetic field of
force on the axis _5-5_.
Theory. In order to understand how currents will be generated in
this loop of wire _4_, it is only necessary to remember that if a
conductor is so moved as to cut across magnetic lines of force, an
electromotive force will be set up in the conductor which will tend to
make the current flow through it. The magnitude of the electromotive
force will depend on the rate at which the conductor cuts through the
lines of force, or, in other words, on the number of lines of force
that are cut through by the conductor in a given unit of time. Again,
the direction of the electromotive force depends on the direction of
the cutting, so that if the conductor be moved in one direction across
the lines of force, the electromotive force and the current will be in
one direction; while if it moves in the opposite direction across the
lines of force, the electromotive force and the current will be in the
reverse direction.
It is, evident that as the loop of wire _4_ revolves in the field of
force about the axis _5-5_, the portions of the conductor parallel to
the axis will cut through the lines of force, first in one direction
and then in the other, thus producing electromotive forces therein,
first in one direction and then in the other.
Referring now to Fig. 68, and supposing that the loop _4_ is revolving
in the direction of the curved arrow shown between the upper edges of
the pole pieces, it will be evident that just as the loop stands in
the vertical position, its horizontal members will be moving in a
horizontal direction, parallel with the lines of force and, therefore,
not cutting them at all. The electromotive force and the current will,
therefore, be zero at this time.
As the loop advances toward the position shown in dotted lines, the
upper portion of the loop that is parallel with the axis will begin to
cut downwardly through the lines of force, and likewise the lower
portion of the loop that is parallel with the axis will begin to cut
upwardly through the lines of force. This will cause electromotive
forces in opposite directions to be generated in these portions of the
loop, and these will tend to aid each other in causing a current to
circulate in the loop in the direction shown by the arrows associated
with the dotted representation of the loop. It is evident that as the
motion of the loop progresses, the rate of cutting the lines of force
will increase and will be a maximum when the loop reaches a horizontal
position, or at that time the two portions of the loop that are
parallel with the axis will be traveling at right angles to the lines
of force. At this point, therefore, the electromotive force and the
current will be a maximum.
From this point until the loop again assumes a vertical position, the
cutting of the lines of force will still be in the same direction, but
at a constantly decreasing rate, until, finally, when the loop is
vertical the movement of the parts of the loop that are parallel with
the axis will be in the direction of the lines of force and,
therefore, no cutting will take place. At this point, therefore, the
electromotive force and the current in the loop again will be zero. We
have seen, therefore, that in this half revolution of the loop from
the time when it was in a vertical position to a time when it was
again in a vertical position but upside down, the electromotive force
varied from zero to a maximum and back to zero, and the current did
the same.
It is easy to see that, as the loop moves through the next half
revolution, an exactly similar rise and fall of electromotive force
and current will take place; but this will be in the opposite
direction, since that portion of the loop which was going down through
the lines of force is now going up, and the portion which was
previously going up is now going down.
The law concerning the generation of electromotive force and current
in a conductor that is cutting through lines of magnetic force, may be
stated in another way, when the conductor is bent into the form of a
loop, as in the case under consideration: Thus, _if the number of
lines of force which pass through a conducting loop be varied,
electromotive forces will be generated in the loop_. This will be true
whether the number of lines passing through the loop be varied by
moving the loop within the field of force or by varying the field of
force itself. In any case, _if the number of lines of force be
increased, the current will flow in one way, and if it be diminished
the current will flow in the other way_. The amount of the current
will depend, other things being equal, on the rate at which the lines
of force through the loop are being varied, regardless of the method
by which the variation is made to take place. One revolution of the
loop, therefore, results in a complete cycle of alternating current
consisting of one positive followed by one negative impulse.
The diagram of Fig. 68 is merely intended to illustrate the principle
involved. In the practical construction of magneto generators more
than one bar magnet is used, and, in addition, the conductors in the
armature are so arranged as to include a great many loops of wire.
Furthermore, the conductors in the armature are wound around an iron
core so that the path through the armature loops or turns, may present
such low reluctance to the passage of lines of force as to greatly
increase the number of such lines and also to cause practically all of
them to go through the loops in the armature conductor.
Armature. The iron upon which the armature conductors are wound is
called the _core_. The core of an ordinary armature is shown in Fig.
69. This is usually made of soft gray cast iron, turned so as to form
bearing surfaces at _1_ and _2_, upon which the entire armature may
rotate, and also turned so that the surfaces _3_ will be truly
cylindrical with respect to the axis through the center of the shaft.
The armature conductors are put on by winding the space between the
two parallel faces _4_ as full of insulated wire as space will admit.
One end of the armature winding is soldered to the pin _5_ and,
therefore, makes contact with the frame of the generator, while the
other end of the winding is soldered to the pin _6_, which engages the
stud _7_, carried in an insulating bushing in a longitudinal hole in
the end of the armature shaft. It is thus seen that the frame of the
machine will form one terminal of the armature winding, while the
insulated stud _7_ will form the other terminal.
[Illustration: Fig. 69. Generator Armature]
Another form of armature largely employed in recent magneto
generators is illustrated in Fig. 70. In this the shaft on which the
armature revolves does not form an integral part of the armature core
but consists of two cylindrical studs _2_ and _3_ projecting from the
centers of disks _4_ and _5_, which are screwed to the ends of the
core _1_. This =H= type of armature core, as it is called, while
containing somewhat more parts than the simpler type shown in Fig. 69,
possesses distinct advantages in the matter of winding. By virtue of
its simpler form of winding space, it is easier to insulate and easier
to wind, and furthermore, since the shaft does not run through the
winding space, it is capable of holding a considerably greater number
of turns of wire. The ends of the armature winding are connected, one
directly to the frame and the other to an insulated pin, as is shown
in the illustration.
[Illustration: Fig. 70. Generator Armature]
[Illustration: Fig. 71. Generator Field and Armature]
The method commonly employed of associating the pole pieces with each
other and with the permanent magnets is shown in Fig. 71. It is very
important that the space in which the armature revolves shall be truly
cylindrical, and that the bearings for the armature shall be so
aligned as to make the axis of rotation of the armature coincide with
the axis of the cylindrical surface of the pole pieces. A rigid
structure is, therefore, required and this is frequently secured, as
shown in Fig. 71, by joining the two pole pieces _1_ and _2_ together
by means of heavy brass rods _3_ and _4_, the rods being shouldered
and their reduced ends passed through holes in flanges extending from
the pole pieces, and riveted. The bearing plates in which the armature
is journaled are then secured to the ends of these pole pieces, as
will be shown in subsequent illustrations. This assures proper
rigidity between the pole pieces and also between the pole pieces and
the armature bearings.
The reason why this degree of rigidity is required is that it is
necessary to work with very small air gaps between the armature core
and its pole pieces and unless these generators are mechanically well
made they are likely to alter their adjustment and thus allow the
armature faces to scrape or rub against the pole pieces. In Fig. 71
one of the permanent horseshoe magnets is shown, its ends resting in
grooves on the outer faces of the pole pieces and usually clamped
thereto by means of heavy iron machine screws.
With this structure in mind, the theory of the magneto generator
developed in connection with Fig. 68 may be carried a little further.
When the armature lies in the position shown at the left of Fig. 71,
so that the center position of the core is horizontal, a good path is
afforded for the lines of force passing from one pole to the other.
Practically all of these lines will pass through the iron of the core
rather than through the air, and, therefore, practically all of them
will pass through the convolutions of the armature winding.
When the armature has advanced, say 45 degrees, in its rotation in the
direction of the curved arrow, the lower right-hand portion of the
armature flange will still lie opposite the lower face of the
right-hand pole piece and the upper left-hand portion of the armature
flange will still lie opposite the upper face of the left-hand pole
piece. As a result there will still be a good path for the lines of
force through the iron of the core and comparatively little change in
the number of lines passing through the armature winding. As the
corners of the armature flange pass away from the corners of the pole
pieces, however, there is a sudden change in condition which may be
best understood by reference to the right-hand portion of Fig. 71. The
lines of force now no longer find path through the center portion of
the armature core--that lying at right angles to their direction of
flow. Two other paths are at this time provided through the now
horizontal armature flanges which serve almost to connect the two pole
pieces. The lines of force are thus shunted out of the path through
the armature coils and there is a sudden decrease from a large number
of lines through the turns of the winding to almost none. As the
armature continues in its rotation the two paths through the flanges
are broken, and the path through the center of the armature core and,
therefore, through the coils themselves, is reëstablished.
As a result of this consideration it will be seen that in actual
practice the change in the number of lines passing through the
armature winding is not of the gradual nature that would be indicated
by a consideration of Fig. 68 alone, but rather, is abrupt, as the
corners of the armature flanges leave the corners of the pole pieces.
This abrupt change produces a sudden rise in electromotive force just
at these points in the rotation, and, therefore, the electromotive
force and the current curves of these magneto generators is not
usually of the smooth sine-wave type but rather of a form resembling
the sine wave with distinct humps added to each half cycle.
[Illustration: Fig. 72. Generator with Magnets Removed]
As is to be expected from any two-pole alternating generator, there is
one cycle of current for each revolution of the armature. Under
ordinary conditions a person is able to turn the generator handle at
the rate of about two hundred revolutions a minute, and as the ratio
of gearing is about five to one, this results in about one thousand
revolutions per minute of the generator, and, therefore, in a
current of about one thousand cycles per minute, this varying
widely according to the person who is doing the turning.
[Illustration: HOWARD OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO
An All-Concrete Building Serving the District South of
Market Street.]
The end plates which support the bearings for the armature are usually
extended upwardly, as shown in Fig. 72, so as to afford bearings for
the crank shaft. The crank shaft carries a large spur gear which
meshes with a pinion in the end of the armature shaft, so that the
user may cause the armature to revolve rapidly. The construction shown
in Fig. 72 is typical of that of a modern magneto generator, it being
understood that the permanent magnets are removed for clearness of
illustration.
Fig. 73 is a view of a completely assembled generator such as is used
for service requiring a comparatively heavy output. Other types of
generators having two, three, or four permanent magnets instead of
five, as shown in this figure, are also standard.
[Illustration: Fig. 73. Five-Bar Generator]
Referring again to Fig. 69, it will be remembered that one end of the
armature winding shown diagrammatically in that figure, is terminated
in the pin _5_, while the other terminates in the pin _7_. When the
armature is assembled in the frame of the generator it is evident that
the frame itself is in metallic connection with one end of the
armature winding, since the pin _5_ is in metallic contact with the
armature casting and this is in contact with the frame of the
generator through the bearings. The frame of the machine is,
therefore, one terminal of the generator. When the generator is
assembled a spring of one form or another always rests against the
terminal pin _7_ of the armature so as to form a terminal for the
armature winding of such a nature as to permit the armature to rotate
freely. Such spring, therefore, forms the other terminal of the
generator.
Automatic Shunt. Under nearly all conditions of practice it is
desirable to have the generator automatically perform some switching
function when it is operated. As an example, when the generator is
connected so that its armature is in series in a telephone line, it is
quite obvious that the presence of the resistance and the impedance of
the armature winding would be objectionable if left in the circuit
through which the voice currents had to pass. For this reason, what is
termed an _automatic shunt_ is employed on generators designed for
series work; this shunt is so arranged that it will automatically
shunt or short-circuit the armature winding when it is at rest and
also break this shunt when the generator is operated, so as to allow
the current to pass to line.
[Illustration: Fig 74. Generator Shunt Switch]
A simple and much-used arrangement for this purpose is shown in Fig.
74, where _1_ is the armature; _2_ is a wire leading from the frame of
the generator and forming one terminal of the generator circuit; and
_3_ is a wire forming the other terminal of the generator circuit,
this wire being attached to the spring _4_, which rests against the
center pin of the armature so as to make contact with the opposite end
of the armature winding to that which is connected with the frame. The
circuit through the armature may be traced from the terminal wire _2_
through the frame; thence through the bearings to the armature _1_ and
through the pin to the right-hand side of the armature winding.
Continuing the circuit through the winding itself, it passes to the
center pin projecting from the left-hand end of the armature shaft;
thence to the spring _4_ which rests against this pin; and thence to
the terminal wire _3_.
Normally, this path is shunted by what is practically a short circuit,
which may be traced from the terminal _2_ through the frame of the
generator to the crank shaft _5_; thence to the upper end of the
spring _4_ and out by the terminal wire _3_. This is the condition
which ordinarily exists and which results in the removal of the
resistance and the impedance on the armature winding from any circuit
in which the generator is placed, as long as the generator is not
operated.
An arrangement is provided, however, whereby the crank shaft _5_ will
be withdrawn automatically from engaging with the upper end of the
spring _4_, thus breaking the shunt around the armature circuit,
whenever the generator crank is turned. In order to accomplish this
the crank shaft _5_ is capable of partial rotation and of slight
longitudinal movement within the hub of the large gear wheel. A spring
7 usually presses the crank shaft toward the left and into engagement
with the spring _4_. A pin _8_ carried by the crank shaft, rests in a
V-shaped notch in the end of the hub _6_ and as a result, when the
crank is turned the pin rides on the surface of this notch before the
large gear wheel starts to turn, and thus moves the crank shaft _5_ to
the right and breaks the contact between it and the spring _4_. Thus,
as long as the generator is being operated, its armature is connected
in the circuit of the line, but as soon as it becomes idle the
armature is automatically short-circuited. Such devices as this are
termed _automatic shunts_.
In still other cases it is desirable to have the generator circuit
normally open so that it will not affect in any way the electrical
characteristics of the line while the line is being used for talking.
In this case the arrangement is made so that the generator will
automatically be placed in proper circuit relation with the line when
it is operated.
[Illustration: Fig. 75. Generator Cut-in Switch]
A common arrangement for doing this is shown in Fig. 75, wherein the
spring _1_ normally rests against the contact pin of the armature and
forms one terminal of the armature circuit. The spring _2_ is adapted
to form the other terminal of the armature circuit but it is normally
insulated from everything. The circuit of the generator is, therefore,
open between the spring _2_ and the shaft _3_, but as soon as the
generator is operated the crank shaft is bodily moved to the left by
means of the =V=-shaped notch in the driving collar _4_ and is thus
made to engage the spring _2_. The circuit of the generator is then
completed from the spring _1_ through the armature pin to the armature
winding; thence to the frame of the machine and through shaft _3_ to
the spring _2_. Such devices as this are largely used in connection
with so-called "bridging" telephones in which the generators and bells
are adapted to be connected in multiple across the line.
A better arrangement for accomplishing the automatic switching on the
part of the generator is to make no use of the crank shaft as a part
of the conducting path as is the case in both Figs. 74 and 75, but to
make the crank shaft, by its longitudinal movement, impart the
necessary motion to a switch spring which, in turn, is made to engage
or disengage a corresponding contact spring. An arrangement of this
kind that is in common use is shown in Fig. 76. This needs no further
explanation than to say that the crank shaft is provided on its end
with an insulating stud _1_, against which a switching spring _2_
bears. This spring normally rests against another switch spring _3_,
but when the generator crank shaft moves to the right upon the turning
of the crank, the spring _2_ disengages spring _3_ and engages spring
_4_, thus completing the circuit of the generator armature. It is seen
that this operation accomplishes the breaking of one circuit and the
making of another, a function that will be referred to later on in
this work.
[Illustration: Fig. 76. Generator Cut-in Switch]
Pulsating Current. Sometimes it is desirable to have a generator
capable of developing a pulsating current instead of an alternating
current; that is, a current which will consist of impulses all in one
direction rather than of impulses alternating in direction. It is
obvious that this may be accomplished if the circuit of the generator
be broken during each half revolution so that its circuit is completed
only when current is being generated in one direction.
Such an arrangement is indicated diagrammatically in Fig. 77. Instead
of having one terminal of the armature winding brought out through the
frame of the generator as is ordinarily done, both terminals are
brought out to a commuting device carried on the end of the armature
shaft. Thus, one end of the loop representing the armature winding is
shown connected directly to the armature pin _1_, against which bears
a spring _2_, in the usual manner. The other end of the armature
winding is carried directly to a disk _3_, mounted _on_ but insulated
_from_ the shaft and revolving therewith. One-half of the
circumferential surface of this disk is of insulating material _4_ and
a spring _5_ rests against this disk and bears alternately upon the
conducting portion _3_ or the insulating portion _4_, according to the
position of the armature in its revolution. It is obvious that when
the generator armature is in the position shown the circuit through it
is from the spring _2_ to the pin _1_; thence to one terminal of the
armature loop; thence through the loop and back to the disk _3_ and
out by the spring _5_. If, however, the armature were turned slightly,
the spring _5_ would rest on the insulating portion _4_ and the
circuit would be broken.
[Illustration: Fig. 77. Pulsating-Current Commutator]
[Illustration: Fig. 78. Generator Symbols]
It is obvious that if the brush _5_ is so disposed as to make contact
with the disk _3_ only during that portion of the revolution while
positive current is being generated, the generator will produce
positive pulsations of current, all the negative ones being cut out.
If, on the other hand, the spring _5_ may be made to bear on the
opposite side of the disk, then it is evident that the positive
impulses would all be cut out and the generator would develop only
negative impulses. Such a generator is termed a "direct-current"
generator or a "pulsating-current" generator.
The symbols for magneto or hand generators usually embody a simplified
side view, showing the crank and the gears on one side and the
shunting or other switching device on the other. Thus in Fig. 78 are
shown three such symbols, differing from each other only in the
details of the switching device. The one at the left shows the simple
shunt, adapted to short-circuit the generator at all times save when
it is in operation. The one in the center shows the cut-in, of which
another form is described in connection with Fig. 75; while the symbol
at the right of Fig. 78 is of the make-and-break device, discussed in
connection with Fig. 76. In such diagrammatic representations of
generators it is usual to somewhat exaggerate the size of the
switching springs, in order to make clear their action in respect to
the circuit connections in which the generator is used.
Polarized Ringer. The polarized bell or ringer is, as has been
stated, the device which is adapted to respond to the currents sent
out by the magneto generator. In order that the alternately opposite
currents may cause the armature to move alternately in opposite
directions, these bells are polarized, _i.e._, given a definite
magnetic set, so to speak; so the effect of the currents in the coils
is not to create magnetism in normally neutral iron, but rather to
alter the magnetism in iron already magnetized.
_Western Electric Ringer._ A typical form of polarized bell is shown
in Fig. 79, this being the standard bell or ringer of the Western
Electric Company. The two electromagnets are mounted side by side, as
shown, by attaching their cores to a yoke piece _1_ of soft iron. This
yoke piece also carries the standards _2_ upon which the gongs are
mounted. The method of mounting is such that the standards may be
adjusted slightly so as to bring the gongs closer _to_ or farther
_from_, the tapper.
The soft iron yoke piece _1_ also carries two brass posts _3_ which,
in turn, carry another yoke _4_ of brass. In this yoke _4_ is pivoted,
by means of trunnion screws, the armature _5_, this extending on each
side of the pivot so that its ends lie opposite the free poles of the
electromagnets. From the center of the armature projects the tapper
rod carrying the ball or striker which plays between the two gongs.
In order that the armature and cores may be normally polarized, a
permanent magnet _6_ is secured to the center of the yoke piece _1_.
This bends around back of the electromagnets and comes into close
proximity to the armature _5_. By this means one end of each of the
electromagnet cores is given one polarity--say north--while the
armature is given the other polarity--say south. The two coils of the
electromagnet are connected together in series in such a way that
current in a given direction will act to produce a north pole in one
of the free poles and a south pole in the other. If it be assumed that
the permanent magnet maintains the armature normally of south polarity
and that the current through the coils is of such direction as to make
the left-hand core north and the right-hand core south, then it is
evident that the left-hand end of the armature will be attracted and
the right-hand end repelled. This will throw the tapper rod to the
right and sound the right-hand bell. A reversal in current will
obviously produce the opposite effect and cause the tapper to strike
the left-hand bell.
An important feature in polarized bells is the adjustment between the
armature and the pole pieces. This is secured in the Western Electric
bell by means of the nuts _7_, by which the yoke _4_ is secured to the
standards _3_. By moving these nuts up or down on the standards the
armature may be brought closer _to_ or farther _from_ the poles, and
the device affords ready means for clamping the parts into any
position to which they may have been adjusted.
[Illustration: Fig. 79. Polarized Bell]
_Kellogg Ringer._ Another typical ringer is that of the Kellogg
Switchboard and Supply Company, shown in Fig. 80. This differs from
that of the Western Electric Company mainly in the details by which
the armature adjustment is obtained. The armature supporting yoke _1_
is attached directly to the cores of the magnets, no supporting side
rods being employed. Instead of providing means whereby the armature
may be adjusted toward or from the poles, the reverse practice is
employed, that is, of making the poles themselves extensible. This is
done by means of the iron screws _2_ which form extensions of the
cores and which may be made to approach or recede from the armature by
turning them in such direction as to screw them in or out of the core
ends.
[Illustration: Fig. 80. Polarized Bell]
[Illustration: Fig. 81. Biased Bell]
_Biased Bell._ The pulsating-current generator has already been
discussed and its principle of operation pointed out in connection
with Fig. 77. The companion piece to this generator is the so-called
biased ringer. This is really nothing but a common alternating-current
polarized ringer with a light spring so arranged as to hold the
armature normally in one of its extreme positions so that the tapper
will rest against one of the gongs. Such a ringer is shown in Fig. 81
and needs no further explanation. It is obvious that if a current
flows in the coils of such a ringer in a direction tending to move the
tapper toward the left, then no sound will result because the tapper
is already moved as far as it can be in that direction. If, however,
currents in the opposite direction are caused to flow through the
windings, then the electromagnetic attraction on the armature will
overcome the pull of the spring and the tapper will move over and
strike the right-hand gong. A cessation of the current will allow the
spring to exert itself and throw the tapper back into engagement with
the left-hand gong. A series of such pulsations in the proper
direction will, therefore, cause the tapper to play between the two
gongs and ring the bell as usual. A series of currents in a wrong
direction will, however, produce no effect.
Conventional Symbols. In Fig. 82 are shown six conventional symbols
of polarized bells. The three at the top, consisting merely of two
circles representing the magnets in plan view, are perhaps to be
preferred as they are well standardized, easy to draw, and rather
suggestive. The three at the bottom, showing the ringer as a whole in
side elevation, are somewhat more specific, but are objectionable in
that they take more space and are not so easily drawn.
[Illustration: Fig. 82. Ringer Symbols]
Symbols _A_ or _B_ may be used for designating any ordinary polarized
ringer. Symbols _C_ and _D_ are interchangeably used to indicate a
biased ringer. If the bell is designed to operate only on positive
impulses, then the plus sign is placed opposite the symbol, while a
minus sign so placed indicates that the bell is to be operated only by
negative impulses.
Some specific types of ringers are designed to operate only on a given
frequency of current. That is, they are so designed as to be
responsive to currents having a frequency of sixty cycles per second,
for instance, and to be unresponsive to currents of any other
frequency. Either symbols _E_ or _F_ may be used to designate such
ringers, and if it is desired to indicate the particular frequency of
the ringer this is done by adding the proper numeral followed by a
short reversed curve sign indicating frequency. Thus 50~ would
indicate a frequency of fifty cycles per second.