RECEIVERS
The telephone receiver is the device which translates the energy of
the voice currents into the energy of corresponding sound waves. All
telephone receivers today are of the electromagnetic type, the voice
currents causing a varying magnetic pull on an armature or diaphragm,
which in turn produces the sound waves corresponding to the
undulations of the voice currents.
Early Receivers. The early forms of telephone receivers were of the
_single-pole_ type; that is, the type wherein but one pole of the
electromagnet was presented to the diaphragm. The single-pole receiver
that formed the companion piece to the old Blake transmitter and that
was the standard of the Bell companies for many years, is shown in
Fig. 49. While this has almost completely passed out of use, it may be
profitably studied in order that a comparison may be made between
certain features of its construction and those of the later forms of
receivers.
The coil of this receiver was wound on a round iron core _2_,
flattened at one end to afford means for attaching the permanent
magnet. The permanent magnet was of laminated construction, consisting
of four hard steel bars _1_, extending nearly the entire length of the
receiver shell. These steel bars were all magnetized separately and
placed with like poles together so as to form a single bar magnet.
They were laid together in pairs so as to include between the pairs
the flattened end of the pole piece _2_ at one end and the flattened
portion of the tail piece _3_ at the other end. This whole magnet
structure, including the core, the tail piece, and the permanently
magnetized steel bars, was clamped together by screws as shown. The
containing shell was of hard rubber consisting of three pieces, the
barrel _4_, the ear-piece _5_, and the tail cap _6_. The barrel and
the ear piece engaged each other by means of a screw thread and served
to clamp the diaphragm between them. The compound bar magnet was held
in place within the shell by means of a screw _7_ passing through the
hard rubber tail cap _6_ and into the tail block _3_ of the magnet.
External binding posts mounted on the tail cap, as shown, were
connected by heavy leading-in wires to the terminals of the
electromagnet.
A casual consideration of the magnetic circuit of this instrument will
show that it was inefficient, since the return path for the lines of
force set up by the bar magnet was necessarily through a very long air
path. Notwithstanding this, these receivers were capable of giving
excellent articulation and were of marvelous delicacy of action. A
very grave fault was that the magnet was supported in the shell at the
end farthest removed from the diaphragm. As a result it was difficult
to maintain a permanent adjustment between the pole piece and the
diaphragm. One reason for this was that hard rubber and steel contract
and expand under changes of temperature at very different rates, and
therefore the distance between the pole piece and the diaphragm
changed with changes of temperature. Another grave defect, brought
about by this tying together of the permanent magnet and the shell
which supported the diaphragm at the end farthest from the diaphragm,
was that any mechanical shocks were thus given a good chance to alter
the adjustment.
[Illustration: Fig. 49. Single-Pole Receiver]
Modern Receivers. Receivers of today differ from this old
single-pole receiver in two radical respects. In the first place, the
modern receiver is of the bi-polar type, consisting essentially of a
horseshoe magnet presenting both of its poles to the diaphragm. In the
second place, the modern practice is to either support all of the
working parts of the receiver, _i.e._, the magnet, the coils, and the
diaphragm, by an inner metallic frame entirely independent of the
shell; or, if the shell is used as a part of the structure, to rigidly
fasten the several parts close to the diaphragm rather than at the end
farthest removed from the diaphragm.
Western Electric Receiver. The standard bi-polar receiver of the
Western Electric Company, in use by practically all of the Bell
operating companies throughout this country and in large use abroad,
is shown in Fig. 50. In this the shell is of three pieces, consisting
of the barrel _1_, the ear cap _2_, and the tail cap _3_. The tail cap
and the barrel are permanently fastened together to form substantially
a single piece. Two permanently magnetized bar magnets _4-4_ are
employed, these being clamped together at their upper ends, as shown,
so as to include the soft iron block _5_ between them. The north pole
of one of these magnets is clamped to the south pole of the other, so
that in reality a horseshoe magnet is formed. At their lower ends,
these two permanent magnets are clamped against the soft iron pole
pieces _6-6_, a threaded block _7_ also being clamped rigidly between
these pole pieces at this point. On the ends of the pole pieces the
bobbins are wound. The whole magnet structure is secured within the
shell _1_ by means of a screw thread on the block _7_ which engages a
corresponding internal screw thread in the shell _1_. As a result of
this construction the whole magnet structure is bound rigidly to the
shell structure at a point close to the diaphragm, comparatively
speaking, and as a result of this close coupling, the relation between
the diaphragm and the pole piece is very much more rigid and
substantial than in the case where the magnet structure and the shell
were secured together at the end farthest removed from the diaphragm.
[Illustration: Fig. 50. Western Electric Receiver]
Although this receiver shown in Fig. 50 is the standard in use by the
Bell companies throughout this country, its numbers running well into
the millions, it cannot be said to be a strictly modern receiver,
because of at least one rather antiquated feature. The binding posts,
by which the circuit conductors are led to the coils of this
instrument, are mounted on the outside of the receiver shell, as
indicated, and are thus subject to danger of mechanical injury and
they are also exposed to the touch of the user, so that he may, in
case of the wires being charged to an abnormal potential, receive a
shock. Probably a more serious feature than either one of these is
that the terminals of the flexible cords which attach to these binding
posts are attached outside of the receiver shell, and are therefore
exposed to the wear and tear of use, rather than being protected as
they should be within the shell. Notwithstanding this undesirable
feature, this receiver is a very efficient one and is excellently
constructed.
[Illustration: Fig. 51. Kellogg Receiver]
Kellogg Receiver. In Fig. 51 is shown a bi-polar receiver with
internal or concealed binding posts. This particular receiver is
typical of a large number of similar kinds and is manufactured by the
Kellogg Switchboard and Supply Company. Two straight permanently
magnetized bar magnets _1-1_ are clamped together at their opposite
ends so as to form a horseshoe magnet. At the end opposite the
diaphragm these bars clamp between them a cylindrical piece of iron
_2_, so as to complete the magnetic circuit at the end. At the end
nearest the diaphragm they clamp between them the ends of the soft
iron pole pieces _3-3_, and also a block of composite metal _4_ having
a large circular flange _4'_ which serves as a means for supporting
the magnet structure within the shell. The screws by means of which
the disk _4'_ is clamped to the shouldered seat in the shell do not
enter the shell directly, but rather enter screw-threaded brass blocks
which are moulded into the structure of the shell. It is seen from
this construction that the diaphragm and the pole pieces and the
magnet structure itself are all rigidly secured together through the
medium of the shell at a point as close as possible to the diaphragm.
Between the magnets _1-1_ there is clamped an insulating block _5_, to
which are fastened the terminal plates _6_, one on each side of the
receiver. These terminal plates are thoroughly insulated from the
magnets themselves and from all other metallic parts by means of
sheets of fiber, as indicated by the heavy black lines. On these
plates _6_ are carried the binding posts for the receiver cord
terminals. A long tongue extends from each of the plates _6_ through a
hole in the disk _4'_, into the coil chamber of the receiver, at which
point the terminal of the magnet winding is secured to it. This tongue
is insulated from the disk _4'_, where it passes through it, by means
of insulating bushing, as shown. The other terminal of the magnet
coils is brought out to the other plate _6_ by means of a similar
tongue on the other side.
In order that the receiver terminals proper may not be subjected to
any strain in case the receiver is dropped and its weight caught on
the receiver cord, a strain loop is formed as a continuation of the
braided covering of the receiver cord, and this is tied to the
permanent magnet structure, as shown. By making this strain loop
short, it is obvious that whatever pull the cord receives will not be
taken by the cord conductors leading to the binding posts or by the
binding posts or the cord terminals themselves.
A number of other manufacturers have gone even a step further than
this in securing permanency of adjustment between the receiver
diaphragm and pole pieces. They have done this by not depending at all
on the hard rubber shell as a part of the structure, but by enclosing
the magnet coil in a cup of metal upon which the diaphragm is mounted,
so that the permanency of relation between the diaphragm and the pole
pieces is dependent only upon the metallic structure and not at all
upon the less durable shell.
Direct-Current Receiver. Until about the middle of the year 1909, it
was the universal practice to employ permanent magnets for giving the
initial polarization to the magnet cores of telephone receivers. This
is still done, and necessarily so, in receivers employed in connection
with magneto telephones. In common-battery systems, however, where the
direct transmitter current is fed from the central office to the local
stations, it has been found that this current which must flow at any
rate through the line may be made to serve the additional purpose of
energizing the receiver magnets so as to give them the necessary
initial polarity. A type of receiver has come into wide use as a
result, which is commonly called the _direct-current receiver_,
deriving its name from the fact that it employs the direct current
that is flowing in the common-battery line to magnetize the receiver
cores. The Automatic Electric Company, of Chicago, was probably the
first company to adopt this form of receiver as its standard type.
Their receiver is shown in cross-section in Fig. 52, and a photograph
of the same instrument partially disassembled is given in Fig. 53. The
most noticeable thing about the construction of this receiver is the
absence of permanent magnets. The entire working parts are contained
within the brass cup _1_, which serves not only as a container for the
magnet, but also as a seat for the diaphragm. This receiver is
therefore illustrative of the type mentioned above, wherein the
relation between the diaphragm and the pole pieces is not dependent
upon any connection through the shell.
[Illustration: Fig. 52. Automatic Electric Company Direct-Current
Receiver]
[Illustration: Fig. 53. Automatic Electric Company Direct-Current
Receiver]
The coil of this instrument consists of a single cylindrical spool
_2_, mounted on a cylindrical core. This bobbin lies within a soft
iron-punching _3_, the form of which is most clearly shown in Fig. 53,
and this punching affords a return path to the diaphragm for the
lines of force set up in the magnet core. Obviously a magnetizing
current passing through the winding of the coil will cause the end of
the core toward the diaphragm to be polarized, say positively, while
the end of the enclosing shell will be polarized in the other
polarity, negatively. Both poles of the magnet are therefore presented
to the diaphragm and the only air gap in the magnetic circuit is that
between the diaphragm and these poles. The magnetic circuit is
therefore one of great efficiency, since it consists almost entirely
of iron, the only air gap being that across which the attraction of
the diaphragm is to take place.
The action of this receiver will be understood when it is stated that
in common-battery practice, as will be shown in later chapters, a
steady current flows over the line for energizing the transmitter. On
this current is superposed the incoming voice currents from a distant
station. The steady current flowing in the line will, in the case of
this receiver, pass through the magnet winding and establish a normal
magnetic field in the same way as if a permanent magnet were employed.
The superposed incoming voice currents will then be able to vary this
magnetic field in exactly the same way as in the ordinary receiver.
An astonishing feature of this recent development of the so-called
direct-current receiver is that it did not come into use until after
about twenty years of common-battery practice. There is nothing new in
the principles involved, as all of them were already understood and
some of them were employed by Bell in his original telephone; in fact,
the idea had been advanced time and again, and thrown aside as not
being worth consideration. This is an illustration of a frequent
occurrence in the development of almost any rapidly growing art. Ideas
that are discarded as worthless in the early stages of the art are
finally picked up and made use of. The reason for this is that in some
cases the ideas come in advance of the art, or they are proposed
before the art is ready to use them. In other cases the idea as
originally proposed lacked some small but essential detail, or, as is
more often the case, the experimenter in the early days did not have
sufficient skill or knowledge to make it fit the requirements as he
saw them.
Monarch Receiver. The receiver of the Automatic Electric Company
just discussed employs but a single electromagnet by which the initial
magnetization of the cores and also the variable magnetization
necessary for speech reproduction is secured. The problem of the
direct-current receiver has been attacked in another way by Ernest E.
Yaxley, of the Monarch Telephone Manufacturing Company, with the
result shown in Fig. 54. The construction in this case is not unlike
that of an ordinary permanent-magnet receiver, except that in the
place of the permanent magnets two soft iron cores _1-1_ are employed.
On these are wound two long bobbins of insulated wire so that the
direct current flowing over the telephone line will pass through these
and magnetize the cores to the same degree and for the same purpose as
in the case of permanent magnets. These soft iron magnet cores _1-1_
continue to a point near the coil chamber, where they join the two
soft iron pole pieces _2-2_, upon which the ordinary voice-current
coils are wound. The two long coils _4-4_, which may be termed the
direct-current coils, are of somewhat lower resistance than the two
voice-current coils _3-3_. They are, however, by virtue of their
greater number of turns and the greater amount of iron that is
included in their cores, of much higher impedance than the
voice-current coils _3-3_. These two sets of coils _4-4_ and _3-3_ are
connected in multiple. As a result of their lower ohmic resistance the
coils _4-4_ will take a greater amount of the steady current which
comes over the line, and therefore the greater proportion of the
steady current will be employed in magnetizing the bar magnets. On
account of their higher impedance to alternating currents, however,
nearly all of the voice currents which are superposed on the steady
currents, flowing in the line will pass through the voice-current
coils _3-3_, and, being near the diaphragm, these currents will so
vary the steady magnetism in the cores _2-2_ as to produce the
necessary vibration of the diaphragm.
[Illustration: Fig. 54. Monarch Direct-Current Receiver]
This receiver, like the one of the Automatic Electric Company, does
not rely on the shell in any respect to maintain the permanency of
relation between the pole pieces and the diaphragm. The cup _5_, which
is of pressed brass, contains the voice-current coils and also acts as
a seat for the diaphragm. The entire working parts of this receiver
may be removed by merely unscrewing the ear piece from the hard rubber
shell, thus permitting the whole works to be withdrawn in an obvious
manner.
Dean Receiver. Of such decided novelty as to be almost revolutionary
in character is the receiver recently put on the market by the Dean
Electric Company and shown in Fig. 55. This receiver is of the
direct-current type and employs but a single cylindrical bobbin of
wire. The core of this bobbin and the return path for the magnetic
lines of force set up in it are composed of soft iron punchings of
substantially =E= shape. These punchings are laid together so as to
form a laminated soft-iron field, the limbs of which are about square
in cross-section. The coil is wound on the center portion of this _E_
as a core, the core being, as stated, approximately square in
cross-section. The general form of magnetic circuit in this instrument
is therefore similar to that of the Automatic Electric Company's
receiver, shown in Figs. 52 and 53, but the core is laminated instead
of being solid as in that instrument.
[Illustration: Fig. 55. Dean Steel Shell Receiver]
The most unusual feature of this Dean receiver is that the use of hard
rubber or composition does not enter into the formation of the shell,
but instead a shell composed entirely of steel stampings has been
substituted therefor. The main portion of this shell is the barrel
_1_. Great skill has evidently been exercised in the forming of this
by the cold-drawn process, it presenting neither seams nor welds. The
ear piece _2_ is also formed of steel of about the same gauge as the
barrel _1_. Instead of screw-threading the steel parts, so that they
would directly engage each other, the ingenious device has been
employed of swaging a brass ring _3_ in the barrel portion and a
similar brass ring _4_ in the ear cap portion, these two being slotted
and keyed, as shown at _8_, so as to prevent their turning in their
respective seats. The ring _3_ is provided with an external screw
thread and the ring _4_ with an internal screw thread, so that the
receiver cap is screwed on to the barrel in the same way as in the
ordinary rubber shell. By the employment of these brass screw-threaded
rings, the rusting together of the parts so that they could not be
separated when required--a difficulty heretofore encountered in steel
construction of similar parts--has been remedied.
[Illustration: Fig. 56. Working Parts of Dean Receiver]
The entire working parts of this receiver are contained within the cup
_5_, the edge of which is flanged outwardly to afford a seat for the
diaphragm. The diaphragm is locked in place on the shell by a
screw-threaded ring _6_, as is clearly indicated. A ring _7_ of
insulating material is seated within the enlarged portion of the
barrel _1_, and against this the flange of the cup _5_ rests and is
held in place by the cap _2_ when it is screwed home. The working
parts of this receiver partially disassembled are shown in Fig. 56,
which gives a clear idea of some of the features not clearly
illustrated in Fig. 55.
It cannot be denied that one of the principal items of maintenance of
subscribers' station equipment has been due to the breakage of
receiver shells. The users frequently allow their receiver to fall and
strike heavily against the wall or floor, thus not only subjecting the
cords to great strain, but sometimes cracking or entirely breaking the
receiver shell. The innovation thus proposed by the Dean Company of
making the entire receiver shell of steel is of great interest. The
shell, as will be seen, is entirely insulated from the circuit of the
receiver so that no contact exists by which a user could receive a
shock. The shell is enameled inside and out with a heavy black
insulating enamel baked on, and said to be of great durability. How
this enamel will wear remains to be seen. The insulation of the
interior portions of the receiver is further guarded by providing a
lining of fiber within the shell at all points where it seems possible
that a cross could occur between some of the working parts and the
metal of the shell. This type of receiver has not been on the market
long enough to draw definite conclusions, based on experience in use,
as to what its permanent performance will be.
Thus far in this chapter only those receivers which are commonly
called _hand receivers_ have been discussed. These are the receivers
that are ordinarily employed by the general public.
[Illustration: Fig. 57. Operator's Receiver]
Operator's Receiver. At the central office in telephone exchanges
the operators are provided with receivers in order that they may
communicate with the subscribers or with other operators. In order
that they may have both of their hands free to set up and take down
the connections and to perform all of the switching operations
required, a special form of receiver is employed for this purpose,
which is worn as a part of a head-gear and is commonly termed a _head
receiver_. These are necessarily of very light construction, in
order not to be burdensome to the operators, and obviously they
must be efficient. They are ordinarily held in place at the ear by a
metallic head band fitting over the head of the operator.
[Illustration: GRANT AVENUE OFFICE OF HOME TELEPHONE COMPANY, SAN
FRANCISCO, CAL. A Type of Central-Office Buildings in Down-Town
Districts of Large Cities.]
Such a receiver is shown in cross-section in Fig. 57, and completely
assembled with its head band in Fig. 58. Referring to Fig. 57 the
shell _1_ of the receiver is of aluminum and the magnets are formed of
steel rings _2_, cross-magnetized so as to present a north pole on one
side of the ring and a south pole on the other. The two L-shaped pole
pieces _3_ are secured by screws to the poles of these ring magnets,
and these pole pieces carry the magnet coils, as is clearly indicated.
These poles are presented to a soft iron diaphragm in exactly the same
way as in the larger hand receivers, the diaphragm being clamped in
place by a hard rubber ear piece, as shown. The head bands are
frequently of steel covered with leather. They have assumed numerous
forms, but the general form shown in Fig. 58 is the one commonly
adopted.
[Illustration: Fig. 58. Operator's Receiver and Cord]
[Illustration: Fig. 59. Receiver Symbols]
Conventional Symbols. The usual diagrammatic symbols for hand and
head receivers are shown in Fig. 59. They are self-explanatory. The
symbol at the left in this figure, showing the general outline of the
receiver, is the one most commonly used where any sort of a receiver
is to be indicated in a circuit diagram, but where it becomes
desirable to indicate in the diagram the actual connections with the
coil or coils of the receiver, the symbol shown at the right is to be
preferred, and obviously it may be modified as to number of windings
and form of core as desired.