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Патент USA US2405599

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Aug. 13, 1946.
D. MITCHELL
2,405,599
PRIVACY SYSTEM
Filed Sept. 24, 1941
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D. MITCHELL
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Aug., 13, 1946.
D. MITCHELL
2,405,599
PRIVACY SYSTEM
Filed Sept. 24, 1941
È.
2 Sheets-Sheet 2
2,405,599
Patented Aug. 13, 1946
UNITED STATES PATENT CFFICE
2,405,599
PRIVACY SYSTEM
Deren Mitchell, Bound Brook, N. J., assignor to
Bell Telephone Laboratories, Incorporated, New
York, N. Y., a corporation of New York
Application September 24, 1941, Serial No. 412,070
7 Claims. (Cl. 179-15)
1
2
This invention relates to a transmission sys
tem and, particularly, to a method of and means
for assuring secrecy in the transmission of mes
of the system of Fig. 1.
The diagram of Fig. 1 is not an actual circuit
diagram but rather a single line layout, each line
indicating a transmission path, In this layout
a normal break in a path is indicated by sepa
sages.
An object of the invention is the assurance of
secrecy in the transmission of messages and the
prevention of intelligibie reception of the mes
sage by unauthorized listeners.
Applicant has disclosed in an earlier patent
rated arrowheads and a normal make by con
necting arrowheads.
In order to indicate that
a transmission element is to be controlled by a
device, an arrow is 'shown pointing from the de
(2,129,860, issued September 13, 1938), a privacy 10 vice towards the element. An arrow directed to
a make point indicates that the path will be dis
arrangement in accordance with which previous
abled at that point by operation of the control
ly recorded noise or spurious speech is applied to
device andan arrow directed at a break point
the transmission channel during the silent inter
indicates that the break in the path will be elimi
vals, i. e., when the regular talkers are not speak
ing, whereby thereis produced on the channel 15 nated, i. e., the path will be enabled, by operation
of the control device.
'
a practically continuous train of sound composed
Referring now to Fig. 1, the arrangement con
of the useful speech and the noise or spurious
templated by the invention is shown incorpo
speech. This train of sound is in the nature of
rated in a two-way, four-wire circuit forming a
a meaningless jargon, the useful speech being so
blended with the noise or spurious speech that it 20 part of a radio telephone system. The two ter
minals, West and East, of the system are repre
will be practically impossible for unauthorized
sented, the circuit at the West terminal compris
listeners to interpret the message.
ing a one-way transmission circuit EA for con
The arrangement contemplated by the pres
necting the output of West subscriber’s station
ent invention is of the same general type as that
disclosed by the above patent, but is an improve 25 II to the West-East radio transmitter I2 and a
ment over that prior arrangement in respects “ one-way transmission circuit WB for connecting
the East-West radio receiver I3 to the input of
that will be pointed out subsequently.
West subscriber’s station I I. Similarly, at the
In accordance with a feature of the present
East terminal, a one-way transmission circuit
invention, the operation of the necessary switch
ing devices is controlled by a small portion of 30 EB connects the West-East radio receiver I4 to
the speech frequency band. As a result, signal - » the input of East subscriber’s station I5, while
.the output of subscriber’s station I5 is connect
frequencies outside the speech band, which were
ed to the East-West radio transmitter I6.
required in the instance of the prior arrange
The West terminal transmitting circuit E‘A is
ment referred to, are not necessary in the pres
ent arrangement.
35 normally disabled (i. e., when subscriber W is
not speaking) so far as the output of subscrib
In accordance with the general method of op
er’s station Il is concerned at control point I1
eration contemplated by the present invention
by suitable means, for example, a short circuit,
the same frequency band is occupied by the spu
controlled by switching relays operating, in turn,
rious speech and the useful speech. The fre
quency spectrums of the two are caused to diiîer 40 under control of ampliñer-detector 2è. At this
time, however, spurious speech from source 22, a
in a small but distinctive manner, however, and
phonograph for example, is applied to circuit
this difference is utilized at the receiving end of
EA and transmitted thereover to radio trans
the channel to control operation of the necessary
mitter I 2; radio transmitter I2 transmits this
switching devices.
A full understanding cf the operation of the 45 spurious speech to the East terminal.
An unauthorized person “listening-in” at this
arrangement contemplated by the present in- t
time on 4the transmission from the West termi
vention as well as appreciation of the various ad
nal, therefore, would hear only the spurious
vantageous features thereof may be gained from
speech emanating from source 22. In order to
consideration of the following detailed descrip
tion in connection with the accompanying draw 50 make things even more confusing to an unau
thorized listener, the spurious speech may be
ings, in which:
recorded by the person ordinarily talking from
Fig. 1 illustrates schematically a two-Way,
four-wire radioI telephone system embodying
subscriber’s station I l in order that the spurious
speech and the useful speech may be “in the
features of the present invention; and
Fig. 2 illustrates in greater detail one terminal 55 same voice."
2,405,599
4
3
rThe frequency band occupied by the useful
speech and that occupied by the spurious speech
tor 43 and after rectification is applied to wind
ing 25 of relay 25.
are the same; this frequency band will be re
ferred to as fl-fio.
Band elimination filter 23 is included in the
Ampliñer-detectors t3 and 44 are so designed
that the sensitivity of the first mentioned is ap
preciably greater than that of the second so that
even though the energy of the portion fz-fa is
considerably less than that of the total band
path over which the spurious speech from source
22 is applied to circuit EA; this filter eliminates
from the frequency band of the spurious speech
a predetermined, relatively narrow, portion. The
eliminated portion of the band will be referred
(which is applied to amplifier-detector 44) the
rectified energy applied to relay winding 26 will
be substantially greatei~V than that applied to
winding 21. Relay 25 will be operated to the
to as f2-fs.
In order that subscriber E, the authorized
speaker at subscriber’s station l5, may not be
subjected to the spurious speech which is now
being?transmitte'd as described above, ‘circuit EB 15
“up” position, therefore, thereby enabling cir
cuit EB at point 2li for transmission of the useful
speech to subscriber E at subscriber’s station l5.
A portion of the speech energy is by-passed
through ampliñer-detector 52Aand causes, through
is held open at point 2d by suitable means, for
operation of a relay or other suitable means, dis
example a short circuit, applied and removed un
abling of circuit WA atv-_point 53. This prevents
der control of diñerential relay 25. Relay 25 is
undesirable operation of the transmitting con
provided with two windings-_winding 2@ ener
gization of which tends to operate the armature 20 trols of the’East terminal by echoes.
When the regular talker at subscriber’s sta
“up” and winding 2l energization of which tends
tion H ceases speaking, the circuits immediately
to voperate the armature “down” When the
return to the condition illustrated and spurious
armature is in the “down” position, as illustrated,
speech is again applied to the transmission chan
circuit EB isr open at point 2li but when the arma
ture is operated to the “up” position, circuit EB 25 nel. An unauthorized listener, therefore, upon
gaining access to the channel would hear a prac
’is closed through or enabled at point 24; these
tically continuous “jargon” comprised partly of
operations will be described in detail in connec
tion with Fig.V 2.
1
'
>
Y windings 2:6 and El are connected Vto circuit
EB through respective lines il! and e2, amplifier
detector #lâ being connected inline ¿l ê 'and' ampli
ier-detector ¿lli being connected in line e2. Band
pass filter ¿l5 is also included in line lil; this
filter is designed to pass only a small band of
useful speech
partly of spurious speech. In
terpretation or" the message by recognization of
30 the legitimate portions and mental masking of the
spurious portions would be very diñcult thereby
rendering the message highly secret so far as un
authorized listeners are concerned. Confusion
and frustration on the part of unauthorized lis
frequencies including the portion fz-f‘s of the 35 teners and persons attempting to “crack” the
privacy system are heightened by the fact that
spurious speech band which was eliminated by
>both the -useful speech band and the spurious
band elimination filter 23;
speech bandv have similar, small portions elimi
Y During the time spurious speech from source
nated therefrom and may be increased still fur
22 is being received at the East terminal, the
armature of relay 2E will be held in the “down” 40 ther if the spurious speech and the useful speech
position as winding 2l will be energized by cur
rent rectified by ampliñer-detector 134. Little or
be in the same voice.
‘Referring now to Fig. 2 a more detailed show
ing is provided of the West terminal of Fig. l.
‘Subscriber’s station i I is coupled to transmitting
tion of the frequency band passed by filter ‘l5 45 line 'il and receiving line ‘I2 by transformer 13,
network 'M Vproviding a proper balance in the
(f2-js) has been eliminated from the frequency
well-known manner. SpeechY energy emanating
band being transmitted.
'
'from subscriber’s station I! passes through arn
Let us assume now that the authorised talker
pliiier 'I5 after which it follows two parallel paths,
at West subscriber’s station il speaks. A por
tion of the resulting speech currents passes 50 one path leading through amplifier-detector 2l
and the other path leading through band elimi
through amplifier-detector 2! and, after rectifi
nation ñlter 5| and windings of transformer T! to
cation, causes operation of relays which (l) re
radio transmitter l2.
move the short circuit at point Il and enable
Phonograph turntable Si is provided together
circuit EA so far as the output of subscriber’s
station ll is concerned and (2) open line ‘i6 at 55 with suitable driving mechanism 32, record 83
being rotatably supported on the turntable. A
point Il? thereby removing the spurious speechV
pick-up device E34 is operatively associated with
from the circuit.
record 83 and serves to translate the mechanical
The useful speech originating at subscriber’s
undulations on the record into corresponding
station il passes, therefore, over circuit EA to
radio transmitter l2, is radiated therefrom and 60 electrical undulatio-ns for transmission over line
A5. Record 83 preferably has recorded thereon
is picked up by radio receiver ld. A small por
spurious speech and this spurious speech may, as
tion fe-fc of the useful speech band is eliminated
pointed out above, have been recorded in the
by band elimination ñlter 5i in order to heighten
voice of the subscriber who ordinarily talks from
the confusion between the useful speech and '
spurious speech and thereby increase the privacy. 65 station Il. The frequency band occupied by the
spurious speech is the same as that' occupied by
The eliminated portions ft-'ís and'fz-fs, while
the speech energy which emanates from sub
differing so far as their relative positions in the
scriberîs station Il; for convenience this band is
frequency spectrum fi--fm are concerned, are
designated as fi-fio. The output of pick-up 84
of equal width.
70 is connected to windings of transformer 11 by line
A portion of the received useful speech energy
4_6, .band elimination ñlter 23 being included in
passes through line e2 to amplifier-detector 44
this line. Filter 2S is so designed that it extracts
and after rectification is applied to winding 21
orneliminates from the frequency band fi--fio a
of relay ze. Portion f2-f3 of the useful speech
definite small portion which we will designate
band passes through iilter 45 to amplifier-detec
Í2--Í3.
-no energy will be supplied to winding 2.6 by ampli
fier-detector @3 in View of the fact that the por
2,405,599
5
6
It might be pointed out here that band elimi
path including the armature and break contact
nation filter 5|, which is included in line 1| and
through which the speech energy emanating from
subscriber’s station | | passes, is designed to elimi
nate from the frequency band fl-flu occupied by
that energy a small portion of the band which
of relay ||2 and transmission of the spurious
speech being received to subscriber’s station ||
is prevented, and the subscriber, therefore, is sub
jected to no annoyance.
Resistance pad H3 provided in receiving line
12 is arranged to prevent the short circuit applied
to the line at relay | I2 from interfering with the
transmission of received energy into the paths
tion in the frequency spectrum; we shall desig
nate the portion eliminated by filter 5| as f5-fs» 10 leading to amplifier-detectors 8&3 and |03.
Turning our attention again now to the trans
Radio receiver I3 is associated with receiving
mitting circuits of Fig. 2 and assuming that the
line 12 which, in turn, -is inductively coupled to
subscriber at station || starts talking, a portion
subscriber’s station || by transformer 13, ampli
of the speech energy applied to transmitting line
fier 85 being connected in line 12. Two paths
for speech energy arerprovided in shunt to line 15 1| is diverted through amplifier-detector 2| and,
after rectification thereby, causes operation of
12, one path leading through amplifier-detector
relays |61 and Illi.
86 to winding 81 of differential relay i0! and the
Operation of relay |01 removes the short cir
other path leading through band-pass filter |02
cuit from line 1| and enables the line for trans
and amplifier-detector |03 to winding |04 of the
differential relay. Band-pass ñlter |02 is de 20 mission of the speech energy. Operation of relay
H4 applies a short circuit to line $6 thereby dis
signed to pass only a small band of frequencies
abling the line for transmission of the spurious
including that portion of the frequency band
speech. The spurious speech is, therefore, re
Íi--fio which is removed by band elimination
moved from the transmission channel and the
filter 23, that is, the portion designated fz-fs.
A third path for speech energy provided in 25 useful speech being produced at subscriber’s sta
tion || is applied thereto.
shunt to receiving line 12 passes through ampli
It Will be remembered that the useful speech
ñer-detector |05 to relay |06.
occupies the same frequency band (f1-fm) as
The condition illustrated in Fig. 2 is that pre
that occupied by the spurious speech. A rela
vailing when the regular talker at subscriber’s
tively small portion, f5-fs, of the useful speech
station || is not speaking. Transmitting line 1|
frequency band is removed by band elimina
is “disabled” so far as the output of station ||
tion filter 5|, this removed portion being of the
is concerned by the short-circuiting path com
same magnitude as the portion removed from
pleted through the armature and break contact
the spurious speech band; the relative positions
of relay |01. Spurious speech from record B3 is
picked up by unit 80 and applied over line liS to 35 of the two removed portions in the respective
frequency spectrums are different however. The
windings of transformer 11. It will be recalled
arrangement of the winding of transformer 11
that the spurious speech occupies the band
is such that transmission of the useful speech
fi-fm and that the portion fz-fs is removed
therethrough to radio transmitter l2' is not 1n
from the energy supplied to transformer 11 by
terfered with by the short circuit applied to line
band elimination filter 23. The spurious speech,
is of the same width as that removed by filter
23 but which occupies a different relative posi
minus the portion ier-fa, is, therefore, supplied
¿i6 by operation of relay | IG.
At this time, therefore, the useful speech
to radio transmitter l2 by which it is transmitted
produced at subscriber’s station || (minus the
to the distant receiving station. The arrange
portion fs-fs) is transmitted over line 1| to
ment of the windings of transformer 11 and net
work |08 associated therewith is such that this 45 radio transmitter |2 and transmitted thereby to
the distant terminal. In order to better visual
energy is not diverted through the short circuit
ize the operation of the receiving circuits of the
applied to line 1| at the break contact of relay
distant terminal which are not illustrated in
|01.
Fig. 2, let us again consider the receiving cir
The receiving circuits of the distant terminal,
which are not illustrated, are similar to the re 50 cuits of the West terminal illustrated in Fig. 2
and assume that useful speech energy is now
ceiving circuits of the West terminal illustrated
being received from the distant terminal and
in Fig. 2 and it is believed that the effect of the
that the spurious speech is not being transmitted.
currents transmitted by radio transmitter E2
As before, a portion of the energy supplied
upon the distant terminal circuits may be better
visualized if we transfer our attention for the 55 to receiving line 12 by radio receiver I3 is di
verted through amplifier-detector 8S and, after
moment to the receiving circuits of Fig. 2 and
rectification thereby, energizes winding 81 of
assume that spurious speech (fi-fio minus the
differential relay lili. At this time, however,
portion f2-f3) is being received from the radio
energy is also diverted through amplifier-de
transmitter of the distant terminal (correspond
ing to radio transmitter i2).
60 tector |03 as band-pass filter |02 passes a small
band of frequencies including the portion ,f2-_f3
A portion of the energy received by radio re
ceiver I3 and applied to receiving line 12 is di
of the frequency band and it will be recalled that
verted through amplifier-detector 8G and after
this portion, while removed from the spurious
rectification thereby is applied to winding 81 of
speech band is present in the band of the use
differential relay lili. Substantially no energy 65 ful speech transmitted. This latter energy,
is diverted through amplifier-detector |03 at this
after rectiñcation by amplifier-detector |03, ener
time as band-pass ñlter |02 is designed to pass
gizes Winding Wfl of differential relay |£l|.
only a small band of frequencies including the
Amplifier-detector |03 is designed for a substan
frequency portion fz--fs and it will be remem
bered that this portion is missing from the spuri 70 tially higher sensitivity than is amplifier-de
tector 86 so that, while the energy applied there
ous speech band being received. Armature |||
to may be less than that applied to amplifier
of relay |0| is operated to the up position, there
detector 8B, the rectified energy applied to wind
fore, by energization of winding 81 and the ener
ing |04 willr still be appreciably greater than that
gizing path of relay ||2 is open. Receiving line
12 is, therefore, short-circuited at this time by a 75 applied to winding 81. Armature ||| is, there
2,405,599
y
>8
fore, operated to its lower or make position at
for ,controlling the operativeness of said receiver
thistime.
means.
Operation of armature Hl to the lower posi
tion completes an operating path for relay H2
traced from battery H5, make contact of relay
HEI, winding of relay H2 to ground HG. Oper
2. In a speech transmission system, a source
of useful speech waves, a source of `spurious
speech waves, said useful speech Waves and
ation of relay I l2 removes the short circuit from
respective sources occupying the same frequency
band, a transmission channel, means for apply
ing spurious speech waves from said source to
said transmission channel, means for removing
a certain definite portion of the frequency band
from'the spurious speech energy applied to said
channel, means for applying useful speech from
said source to said transmission channel, means
for removing a certain definite portion of the
frequency band from the useful speech energy
applied to said channel, means operating in re
sponse to application of said useful speech energy
to said channel to remove said spurious speech
from said channel, receiver means and means at
a receiving point of the system selectively oper
said spurious speech waves as produced by their
receiving line 'i2 and enables the line for trans
mission of the useful speech to subscriber’s sta
tion li. A portion of the energy is diverted
through amplifier-detector 195 and, after recti
flcation thereby, causes operation of relay i536
which operation is effective to short-circuit trans
mitting line 'il and prevent undesirable opera
tion of the Vtransmitting controls by echoes.
Again considering now the transmitting cir
cuits of Fig. 2, as soon as the subscriber at sta
tion ll ceases talking, the circuits return to the
condition illustrated and spurious speech is
again` applied to the channel. In accordance
with the general features of, operation, there
heightened if the spurious speech be recorded ..
ated by Wave components having frequencies in
said deñnite portion removed from the frequency
band of the spurious speech for controlling the
operativeness of said receiver means, the por
tion of the frequency band removed from the
spurious speech energy and the portion of the
frequency band removed from the useful speech
energy being of equal widths but occupying differ
ent relative positions in- the respective frequency
in theY voiceY of the subscriber who ordinarily
spectrums.
fore, the regular authorized subscribers hear only
the useful speech and are not subjected to the
spurious speech. Unauthorized persons “listen
ing in” on the channel, however, hear a mean
ingless, substantially continuous jargon Vcom
prising snatches of useful speech, interspaced
during what-would otherwise be the “silent”
periods, with the kspurious speech. Confusion is
talks from station il while “cracking” is ren
dered much more difficult Vby the fact that blank
or “no-energy” portion are to be found in both
the useful and the spurious speech bands due to
the action of the band eliminationv filters.
If desired, the pass bandsof the different
filters may be changed from time to time there
by,.in effect, varying the “combinaticn”Y of the
system and it will be understood that other
types of sharply tuned circuits may be used in
stead of the filters. It will be understood, fur
ther, that the arrangement may be operated in
conjunction with other types of privacy devices
now well known, for example the arrangements
described respectively by R. W. Chestnut et al.
Patent 1,829,783 and Patent 1,981,114 Vof appli
cant and another.
’
While certain specific embodiments of the
invention have been selected for detailedv de
scription, the invention is not, of course, limited
in its application to the embodiments described.
These embodiments should be taken as illustra
tive of the invention and not as restrictive
thereof.
.
Y
What is claimed is:
l. In a speech transmission system, a source
'
3. In a speech transmission system, including
a West terminal and an East terminal, a source
of useful speech waves -at said West terminal,
a source of spurious speech waves at said West
terminal, said spurious speech waves and said
useful speech waves as produced by said sources
occupying the same definite frequency band, a
transmission channel between said terminals, a
line at said West terminal for applying spurious
speech waves from said last-mentioned source
to said channel, means included in said line for
excluding a certain definite portion of the fre
quency band occupied by said spurious speech
waves, means at said East terminal for dis
abling said transmission channel at a point at
said East terminal, and means operating in
response to application of said excluded portion
of said frequency band to enable said channel
at said point.
'
4. In a speech transmission system including
a West terminalr and an East terminal, a source
of useful speech Waves at said West terminal, a
source of spurious speech waves at said’ West
55 terminal, said useful speech waves and said
>spurious'spc-aech waves as produced by their re
spective sources being of confusingly similar
sounds, a transmission channel between said
terminals, means for# applying useful speech
of useful speech Waves, a source of spurious
speech waves, said useful speech waves and said
spurious speech Waves as produced by their re
waves from said source to said channel, and
receiving point of the system selectively operated
by wave> components having frequencies in said
certain definite portion of said frequency band
tween the respective frequency spectrums.
5. In a speech privacy system of the type
whichtincludes a'> transmitting terminal, are
spective sources occupying the same frequency eo means for applying spurious speech waves from
said Vsource to said channel, means for remov
band, a transmissionV channel, means for ap
ing a’certain definite portion of the frequency
plying spurious speech waves from said source
band from the spurious speech energy applied to
to said transmission channel, means for apply
ing useful. speech from said source to said trans 65 said channel whereby the frequency spectrum
of the spurious speech waves transmitted over
mission channel, means for removing a certain
said channel is distinguishable from the fre
definite portion of the frequency band from the
quency spectrum of the useful speech waves
speech energy applied to said channel from one
transmitted thereover, and switching means at
of said sources, means operating in response to
application of said useful speech energy to said 70 the East terminal operating to'distinguish be
tween the useful speech waves and the spurious
channel to remove said spurious speech from
said channel, receiver means and means at a
speech waves on the Ybasis of the difference be
2,405,599
9
ceiving terminal, a channel between said termi
nals and means at the transmitting terminal
for impressing spurious speech on the channel
during each pause in the useful speech, the
method of discriminating between the spurious
speech and the useful speech at the receiving
terminal which includes producing a distinguish
able difference between the frequency spectrum
of the useful speech and the frequency spectrum
of the spurious speech at the transmitting termi
nal and utilizing this difference to control the
operation of switching means at the receiving
terminal.
6. In a speech transmission system including
a West terminal and an East terminal, and hav
ing a telephone transmitter, a radio transmitter
and a line for connecting said telephone trans
mitter and said radio transmitter at said West
termina1 and a radio receiver, a telephone re
ceiver and a line for connecting said radio re
portion of the frequency band eliminated by
said ñlter to enable said East terminal line at
said previously mentioned point.
’7. In a speech transmission system including
a West terminal and an East terminal, and hav
ing a telephone transmitter, a radio transmitter
and a line for connecting said telephone trans
mitter and said radio transmitter at said West
terminal and a radio receiver, a telephone re
ceiver and a line for connecting said radio re
ceiver and said telephone receiver at said East
terminal, means for disabling said East terminal
line at a definite point therein, a source of
spurious speech at said West terminal, the
spurious speech waves as produced by said source
occupying the same frequency band as that oc
cupied by useful speech'waves as produced by
said telephone transmitter, means for applying
useful speech waves produced by said telephone
ceiver and said telephone receiver at said East
terminal, means for disabling said East termi
transmitter to said West terminal line, a second
line at said West terminal for applying spurious
speech waves produced by said source to said ñrst
nal line at a definite point therein, a source of
West terminal line, a Wave filter in said second
line for eliminating a definite, relatively small
spurious speech at said West terminal, the
spurious speech Waves as produced by said source 25 portion from the frequency band of the spurious
speech waves applied thereover to said ñrst
occupying the same frequency band as that oc
West terminal line, a two-position switching de
cupied by useful speech Waves as produced by
vice at said East terminal, said device operating
said telephone transmitter, means for applying
to a ñrst position in response to application to
useful speech Waves produced by said telephone
transmitter to said West terminal line, a second 30 said East terminal line of the spurious speech
waves after removal of said portion by said
line at said West terminal for applying spurious
filter and operating to a second postion in re
speech waves produced by said source to said
sponse to application to said East terminal line
ñrst West terminal line, a wave ñlter in said
of the useful speech waves, and means operating
second line for eliminating a deñnite, relatively
small portion from the frequency band of the 35 in response to operation of said device to said
second position for enabling said East terminal
spurious speech Waves applied thereover to said
line at said previously mentioned point.
ñrst West terminal line, and switching means
DOREN MITCHELL.
at said East terminal operating only in response
to application to said East terminal line of the
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