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Патент USA US2074829

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March 23, 1937.
2,074,829
Y J. M. CAGE
ELECTRON BEAM TUBE
Filed June 21, 1955
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Patented Mar. 23, 17937
UNITED STATES PATENT OFFICE
2,074,829
ELECTRON BEAM TUBE
' John M. Cage, Schenectady, N. Y., assignor to
General Electric Company, a corporation of .
New York
Application June 21, 1933, Serial No. 676,875
13 Claims. (Cl. 250-27 )
The present invention relates to electron dis
ance with my invention, it becomes possible en
charge devices, more particularly to those de
tirely to eliminate the usual focusing member
vices which utilize a beam of electrons de?ected surrounding the cathode and to rely entirely upon
electrostatically within the tube.
the focusing effect offered by the de?ecting mem
5.
Tubes of this character usually employ a source
of electrons together with a focusing arrange
ment, an anode for receiving the electrons which
is charged positively with respect to the electron
source, and an intermediately positioned elec
w trode for moving the electron beam over the face
of the anode or other electron-receiving member.
The focusing device may take the form of a hol
low cylinder surrounding the source of electrons
and is provided with a cutaway portion or slit
1;, through which the electrons move on their way
to the anode.
In the design of these prior art tubes, it was
considered necessary to perform all of the focus
ing action at a position as close to the cathode as
possible rather than at any other position on the
_ 20 theory that, if the electrons are once focused or
constrained to a beam of proper shape, they will
remain in this constrained condition throughout
their travel toward the anode. However, it was
subsequently found that notwithstanding their
25 initial focussed condition, the electrons tend to
spread or disperse when the beam is electrostat
bers, thus permitting the elimination of a mem
ber heretofore considered essential, although it >
is preferred to employ the usual form of focusing
electrode in addition to the combined focusing
and deflecting members.
An object of the present invention is to improve
the operation of/beam tubes and, in particular,
to improve the focus of the beam so that the
latter will maintain a highly concentrated form
as it sweeps across the electron receiving surface.
The manner in which this object is carried out
and the invention, itself, will be apparent from
the following description when perused in con
nection with the accompanying drawing in which
Fig. 1 is an elevational view of anelectron beam
tube improved in accordance with the present in- .
vention; Fig. 2 is a diagrammatic view of the
tube shown in Fig. 1 and connected in an elec
trical system for ampli?cation purposes; while
Fig. 3 shows in diagram, a modi?ed form of elec
tron beam tube utilizing a ?uorescent screen and 25
connected in suitable circuits.
.
Referring more particularly to Fig. 1, numeral
ically de?ected across the tube and a considerable
I designates an envelope which may be either‘
distortion is introduced into the translating prop
highly evacuated or contain a small amount of
erties of the tube. According to the prior art gas, the envelope terminating at the bottom, as
30 teachings, the cylindrical member, which sur
shown, in a reentrant stem 2 and a four-pillar
rounds the cathode, was vested solely with the ' press 3. The envelope is of the “tipless” type,
focusing function and, for this purpose, was hence, an evacuating tubulation 4, which commu
charged negatively with respect to the cathode. nicates with the interior of the envelope, is pro
In these tubes, the remaining electrodes, includ
vided within the stem 2. As will be seen from the
35 ing the de?ecting members, were either so biased drawing, the entire electrode structure is sup
with respect to the cathode or else were entirely ported in a symmetrical manner entirely from
disconnected therefrom so as to preclude‘ any the press 3. This structure includes an indirectly
concentrating e?ect on the moving electrons.
heated cathode 5, seen morev clearly in Figs. 2
Contrary to the accepted principles of design of and 3, containing a ?lamentary heater 6, prefer 40
40 the prior art tubes, I have discovered that, when ably of tungsten. The cathode conveniently
the de?ecting members are connect-ed to the takes a form of an elongated cylinder and is pref
cathode through a biasing battery of suitable erabiy coated with electron-emitting material,
polarity and potential, they perform not only such as alkaline earth oxide. concentrically
their usual function of sweeping the electron surrounding the cathode, there is a cylindrical
beam across the tube but, in addition, effectively member ‘I, also of elongated form which is pro
serve to ,focus the electrons and thus assist in
maintaining the original concentrated shape of
the beam during its transverse excursions. Thus,
an additional focusing effect is introduced into
the tube at a position intermediate the cathode
and anode, which is exercised simultaneously with
vided with a narrow slit 8 for the egress of elec
trons on their way to electron-receiving members
described ‘hereinafter. There is an electrode 9
positioned in proximity to the slit 8 for the pur
pose of accelerating the electrons. This electron
the de?ecting action, serving to eliminate the‘
accelerating member is preferably constituted of
two angular pieces of elongated'form which are
normal tendency of the electrons to disperse or
55 spread before they reachthe anode. In accord
joined together only at their ends by metal straps
It so that the longer legs of the angular pieces are
2
- '
-
'
2,074,829
'_ in the same planes, and leaving a rectangular slit
lliinlinewiththeslit?.
-
.
On the opposite side of the electron-accelerat
ing electrode 9 from the cathode 5, there are a
pair of parallelly disposed metal plates 12 of a
?at rectangular con?guration, arranged sym
metrically with respect to the slits 8 and H, the
purpose of which is to de?ect the electron beam
in a controllable manner, also to focus the elec
10 trons during de?ection, as will be described here
inafter. For receiving the electrons, ‘there may
be provided a pair of physically separate auxil
iary anodes l3 which are also conveniently made
of angular pieces spaced apart to leave a slit It
15 in line with the slits of the other electrodes. Di
rectly in back of the slit l4, there may be .pro
vided the main electrode or anode l 5 for receiving
the electrons under certain conditions. The elec
trode l5 conveniently takes the form of an elon
20 gated hollow cylinder provided with a slit 16
which faces the slit M in the auxiliary anodes l3.
3 through a conductor 23, which is connected,
to one end of the heater. The other end of the
heater conveniently makes contact with the
cathode cylinder 5 so that a return for the heater '
energy is provided through the cathode ‘wire 22.
The extreme positive terminal of the battery 20
is connected to the electron-accelerating elec-,
trade 9.
>
‘
The combined electron-de?ecting and electron
focusing members l2 are- bridged by a high re
10
sistance 24 and the mid-tap on this resistance is
connected by means of a conductor 25 to an in
termediate terminal on the battery, as shown.
Separate conductors 26'are taken from both ends
of the resistance 24 so that these conductors are 15
connected directly to the respective electrodes I2.
The physically separate auxiliary anodes l8 are
likewise bridged by a high resistance 21 and a
the arrangements of the supporting members are
conductor 28 taken from the mid-tap on the re
sistance back to a terminal on the battery which 20
is less positive than the terminal connected to
the electrode 9. A conductor 29 is also provided
between the battery 20 and the main anode l5,
theterminal on the battery towhich the conduc
tor 29 is connected being preferably more positive 25
than the terminal to which the conductor 28 is
multifarious and will readily occur to those skilled
connected.
in the tube manufacturing’ art.
A source of controlling voltage is applied across
the conductors 26 which may conveniently be
The manner in which these electrodes are sup
ported from the four-pillar stem is clearly shown
in Fig. 1, and a detailed description of the elec
25 trode supports is believed to be unnecessary since
Furthermore,
the manner in which these electrodes are sup
30 ported within the tube does not constitute a part
of the present invention. It is suf?cient to state
that the electrodes and their supports are mount
ed in a symmetrical and rigid manner-within
the tube, utilizing rods of glass H, where neces
35 sary, to insulatingly support one electrode from
another. Each electrode is preferably supported
at two places so as to lend rigidity to the struc
'
considered the input circuit, and a current indi 30
cating or measuring device connected across the
conductors 30 which may be considered the out
put circuit of the device.
The operation of‘ the tube is clear from the
circuit relations shown in Fig. 2. The ?lament
6 serves to heat the cathode 5 which, on account
of being electronically active, emits a profuse
ture as a whole and suitable leading-in conduc
tors are taken through the stem to the exterior.
stream of electrons which move through the slits
40 A getter cup l8 containing magnesium or similar
depending upon the amount of de?ection given
material is conveniently secured to one of the
support rods in a position as to be heated, by a
high frequency coil placed around the envelope
during evacuation so that the magnesium may be
45 ?ashed and the envelope relieved of deleterious
remnant gases.
The electrodes are given the usual gas-denud
ing and eat treatments prior to placing within
the envelope and the tube evacuated to a high
50 degree, for example, less than a few microns of
mercury, by attaching a vacuum pump to the
tube 4. When the proper degree of vacuum has
been obtained, the tube 4 may be sealed OE With
in the stem 2, as is well known in the art. It will
55 be understood that, if desired, instead of being
operated as a high vacuum tube, the envelope
may contain one or more inert gases, such as
argon or neon, at reduced pressure. A base, not
shown, of the usual construction is usually pro
60 vided about the lower end of the tube. A number
of contact plugs may be molded in the base and
connected to the various leading-in conductors;
these plugs register with terminals in a socket
for conveniently making electrical connection
65 with the electrodes within the tube.
Fig. 2 shows one form of circuit which may
8, H, and either strike the auxiliary anodes I3,
the‘electron beam en route, or else pass through
the slit it between the auxiliary anodes and
through the slit it into the anode l5. Due to the
elongated con?guration of the various slits
through which the electrons pass, the electrons
take the shape of a beam in relatively thin sheet
form.
.
It is apparent that the electrode ‘I, being con
nected to the negative end of the battery 20, in
troducesa ?eldwhich acts symmetrically along the 50
length of the cathode and on all of the electrons
because the cathode is of an equipotential char
acter. This electrode therefore serves to focus
the electrons, i. e. to constrain them to a desired
shape, depending upon the shape of the slit 8. 55
It is also evident that the electrode 9, being con
nected to the positive end of the battery 20, pro
duces a positive ?eld in the region of the cathode
which serves to give to the electron beam, as it
emerges from the slit 8, an extremely high veloc 60
ity. Thus, it may properly be said that the
cathode 5, the negatively-charged focusing mem
ber 'l, and the positively-charged accelerating
electrode 9, together constitute an “electron gun"
which causes a fast-moving and highly concen
trated beam of electrons to emerge from the
be employed with the improved tube. As shown
slit ll.
therein, there is a source of electromotiveforce'20
This electron beam, after leaving the slit II,
is electrostatically deflected by the electrodes l2,
of direct current voltage which may comprise a
70 battery, and various taps taken from the source
to the electrodes. The negative terminal of the
battery is connected by a conductor 2| to the
focusing member ‘I and an intermediate terminal
is taken to the cathode 5 by a conductor 22. The
75 battery 20 may also provide energy for the heater
.
i. e. given' a transverse or sweeping movement 70
across the tube, depending upon the magnitude of
the control voltage applied to the conductors 26.
As the beam moves across the tube in response
to the electrostatic ?eld exercised by the elec
trodes l2, variable portions of the beam strike the 75
3
2,074,829
electrodes I 3 and I5. Thus, in the arrangement
shown in Fig, 2, when the beam is de?ected to
ward the left-hand electrode l2, practically all of
the beam strikes the left-hand auxiliary anode I3
and little, if any, of the beam will pass through
the slit l4 and reach the anode I5, and practically
none of the beam will strike the right-hand
anode l3. When the beam is de?ected the other
way, i. e toward the right-hand electrode l2, it
traverses the slit H, at which time all of the
beam temporarily passes into the anode l5 and,
upon further de?ection, the beam strikes the
stant magnitude which acts symmetrically along
the length of the cathode and on all of the elec
trons in the beam because the cathode is of an
equipotential character and has no potential drop
therein which might interfere with the focusing
?eld.
The manner in which this improved connec
tion operates to introduce an added focusing
effect will be at once clear when the equipoten
tial lines, suggested by the dotted lines, are con
10
sidered. The positively-charged electron-accel
erating electrode 9 produces a ?eld within the
right~hand auxiliary anode I3. It is obvious space con?ned by the electrodes I2 which may
that, as the beam impinges on either the left- ‘ take the form of a series of loops 3|, symmetri
cally arranged about the slit l I andextending for
; hand anode I3, the main anode [5 or on right
hand anode I3, depending upon the amount and a substantial distance within the said space.
direction of de?ection given the beam by the elec
This distance is determined by the difference in
trodes l2; the respective electrodes I 3 become potential between the electrodes l2 and 9. There
variably charged, both in magnitude and polarity. are similar equipotential lines which take the
These variations of electrical charge can be de
form of inverted loops 32 and also extend down 20
tected or measured by suitable devices and instru
ments connected to the conductors 30 or to the
wardly from the auxiliary anodes l3 for a con
siderable distance into the space between the de
conductor 29.
?ecting electrodes I2.
As in the case of the loops
It is apparent that, due to the manner in which - 3|, the distance that the loops 32. extend into the
thev electrodes are disposed with respect to one
another and, consequently, the manner in which
the tube operates, the variations of the output
current not only represent a satisfactory repro
duction of the voltage variations applied to the
3 O de?ecting electrodes l2 but also amplify these
voltage variations to an extremely large degree.
Thus, the device has application as a radio fre
quency and audio frequency amplifier in radio
and other intelligence communication circuits
and, in addition, can be designed to detect radio
signals efficiently.
Electron beam tubes employing intermediately
positioned de?ecting electrodes are, of course,
well known to the prior art. My speci?c im
40 provement in a tube of this sort consists in em
ploying the de?ecting electrodes not only for the
usual de?ecting function, but also for assisting
the focusing member 1 in maintaining the orig
inal shape of the electron beam as it passes
45 through the tube on its way to the anodes l3, l5.
It will be understood that, in order for the tube
space and their con?guration depend to some ex
trodes l2. The electrical charges on the de?ect
lng electrodes, due to the current ?owing from the
battery through the conductor 25, produce equi
potential loops 33 which extend toward one
another in the manner roughly suggested by the
dotted lines.
-
Now supposing an electron beam is moving to
ward theanode and is temporarily in the space
between the de?ecting electrodes. The individ
ual electrons will tend to cross the respective
equipotential lines at right angles thereto and
in a direction from the lower potential line to
the higher potential line.
By noting the posi
tion of the arrows 344, which are drawn at right
angles to the dotted lines, it will be seen that
there is a constant force acting on the electrons
which continually urges them toward the center
of the space and away from the de?ecting elec
. trodes themselves. This continual urge tends to
to provide a satisfactory reproduction of the volt
age variations applied to the conductors 26 in
terms of current variations at the conductors 30,
keep the electrons in the beam packed together
50 it is necessary that the electrons maintain the
quently, that portion of the beam which strikes
same position within vthe electron beam as it
moves through the tube until it ?nally reaches
either one of the electrodes l3 or the main anode
l5 represents an accurate proportion of the en
the electron-receiving surfaces.
Whereas heretofore, the de?ecting electrodes
tirebeam determinable by the magnitude and
polarity of the voltages applied to the de?ecting
had a tendency to diverge or spread the beam
away from its original shape due to the electro
static ?elds'introduced between de?ecting mem
bers, in accordance with my invention, I have
found that it is entirely feasible to so modify the
60 de?ecting members and their electrical circuits
as to prevent this divergence or spreading out,
and actually to provide an added focusing action
at the position of the de?ecting electrodes. This
desirable effect is brought about by the use of a
potential which may be either positive or negative
with respect to the cathode, depending upon the
distance between the plates, and which is applied
in the same polarity and magnitude to each of
the de?ecting electrodes. For this purpose, the
70 de?ecting electrodes are electrically connected by
a high resistance 24 and a conductor taken from
the mid-tap of the resistance back to a suitable
positive or negative terminal on the battery 20.
As in the case of the focusing electrode 1', the de
75 ?ecting electrodes also introduce a ?eld of con
25
tent upon the relative potentials existing between
the auxiliary anodes i3 and the de?ecting elec
and’ to maintain the original shape of the beam
as it moves through the de?ection zone.
electrodes.
Conse
Therefore, the tube as a whole tends
to amplify without distortion, and the current
available at the output circuit conductors 3!] rep
resents a faithful reproduction of the voltage
variations applied to the conductors 26. It will
be understood that the above-stated theory, 60
based upon the position of equipotential lines, is
offered purely for explanatory purposes and .I
do not desire to be limited thereto.
It is a fact,
confirmed by many tests and regardless of theory,
that, when the de?ecting members are main
tained at a'?xed average potential with respect
to the equipotential cathode and different from
the potential of the anode in accordance with
my invention, the output current variations fol
low with much more faithfulness the changes 70
in potential applied to the de?ecting members
than when the latter are not so connected.
While I have shown the combined de?ecting
and focusing electrodes ll being used in con
nection with a focusing member _1, it is obvious 75
2,074,829
that the latter may be dispensed with, if desired,
as su?icient focusing effect for ordinary pur
poses may ordinarily be obtained from the de
?ecting electrodes. However, it is considered
preferable to utilize the focusing member ‘I in
addition to the improved de?ecting electrodes.
Fig. 3 shows the application -of the invention
to a cathode beam tube which utilizes a ?uo
rescent screen 36. In this figure, the anodes I 3
10 and i5 have, of course, been omitted since the
electrons now impinge upon the screen to form
a light image, for visual or photographic pur
poses, of the voltage'variations applied to the
conductors 26.
In this ?gure, elements, which
15 correspond to the elements shown in Fig., 2, have
been given the same reference'characters. Thus,
the de?ecting electrodes II are bridged by a re
sistance 24, and a conductor 25 taken preferably
from the mid-tap of the resistance to the battery
20. As in the case of Fig. 2, there is present in
the space between the electrodes l2 equipotential
lines, indicated roughly by the loops 3| and 33,
which tend to converge the individual electrons
of the beam passing through the tube into a
25 packed condition, similar to the condition pro
duced by the focusing action of the member ‘I.
The arrows 34 represent the direction in which
the electron-constraining forces act and it will
be noted that this force is always exercised to
ward the center axis of the tube. Unless the de
?ecting electrodes II are provided with a ?xed
average potential negative or positive with re
spect to the equipotential cathode and different
from the anode, which potential is applied to
both electrodes, of the same polarity and mag
nitude, the electron beam tends to spread and
to be attracted toward the electrodes while the
beam is being de?ected so that the light image,
as shown on the ?uorescent screen, does not rep
40 resent a faithful reproduction of the voltage
variations applied to the conductors 26.
However, in view of the focusing eifect pro
vided by the de?ecting electrodes 12 in accord
ance with my invention, the electron beam main
tains its original focused condition as‘ it moves
through the tube and, hence, the light image on
the ?uorescent screen varies exactly in accord- -
ance with the variations of voltage applied to the
conductors 26. As was explained in connection
50 with Fig. 2, the focusing member ‘I may be en
tirely‘ dispensed with and the focusing e?ect ob
tained 'solely from the de?ecting members, but
it isv considered preferable to utilize the electrode
7 in addition to the focusing effect offered by the
electrodes i 2.
While I have described and illustrated my in
vention in connection with an electron beam tube
in which the cathode rays are obtained initially
from an equipotential source of electrons and the
60 beam takes on a more or less elongatecon?gura
tidn, it is to be understood that the invention is
not limited to this form of cathode or shape of
beam. Obviously, the combined de?ecting and
ditions as are provided in the case of the equi
potential cathode.
What I claim as-new and desire to secure by
Letters Patent of the United States, is:1. An electron beam tube comprising an en
velope containing an electron gun including an
equipotential cathode for producing electrons in
a fast-moving focused condition, and a common
means for de?ecting the beam across the tube
and for simultaneously maintaining the beam in 10
its focused condition, and means for receiving the
electrons.
2. In combination, an electron beam tube com
prising an envelope containing an equipotential
source of electrons, an anode, a plurality of com
15
bined electron-de?ecting and electron-focusing
electrodes positioned on opposite sides of the en
velope between said source and said anode, a
source of electromotive force, the negative termi
nal of which is connected to said source of elec 20
trons, and connections from a positive terminal
on said source of electromotive force to the elec
tron-de?ecting electrodes, and a connection from
a different terminal on said source to said anode.
3. An electron beam tube comprising an en
velope containing an equipotential source of elec
trons, an electron-receiving member, an electron
accelerating electrode positioned between said
source and said member, and a combined elec
tron~de?ecting and electron-focusing electrode 30
interposed between said electron-accelerating
electrode and said electron-receiving member.
4. An electron beam tube comprising an en
velope containing a source of electrons includ
ing an indirectly heated cathode, an electron-re
ceiving member, an electron-accelerating elec
trode positioned between said source and said
member, combined means for sweeping the elec
tron beam across the face of said member and
for focusing said electrons, said means including 40
a pair of electrodes disposed on opposite sides
of the envelope, a resistance connected between
said electrodes, and a conductor between said
resistance and said electron source, said com
bined sweeping and focusing electrodes being 45
charged to a different potential from the poten
tial of said electron-accelerating electrode and
?xed with respect to the average potential of
said source of electrons.
5. In combination, an electron beam tube com
prising an envelope containing an indirectly
50
heated cathode, an anode, a plurality of com
b‘ned electron-de?ecting and electron-focusing
electrodes respectively positioned on opposite
sides of said envelope between said cathode and 55
anode, a resistance between said electron-de
?eeting electrodes, a source of electromotive
force, said cathode, anode, and resistance being
connected to different potential terminals on
said source of electromotive force.
'
6. An electron beam tube comprising an en
velope containing an electron gun including an
60
equipotential cathode for producing electrons in
focusing function exercised by the .electrodes I2
a fast-moving focused condition and a com
is equally applicable to beams of any and all
shapes, depending upon the shape of the open
mon means for de?ecting the beam across the
tube and for simultaneously maintaining the
.beam in its focused condition, and means for
receiving the electrons, said common means in
cluding a plurality of metal members maintained
at a ‘fixed average potential with respect to said 70
ings 8, ll, etc. Furthermore, it is also evident
that instead of an indirectly heated cathode as
shown and described, any suitable and well
known form of directly heated cathode, for ex
ample, a flat spiral ?lament may be employed,
in which case the connection 22 is brought back
to a point on the ?lament circuit which repre
sents the average potential of the ?lament so
75 as to retain the same symmetrical electrical con
electron gun but independently operable by a
control deflecting voltage.
'
7. An electron beam tube comprising an elec
tron gun including an indirectly heated cathode
for producing electrons in a fast-moving f0- 76
£074,829
cused condition, a main anode, a plurality of
auxiliary anodes positioned between said main
anode and the electron gun, means including
electrodes charged to a positive potential of ?xed
value with respect to said electron gun for de
?eeting the electron beam across the face of the
auxiliary anodes and for maintaining the focus
of said electrons, said auxiliary anodes being
maintained at a ?xed average potential with re
10 spect to said electron gun and different from
the potential of the main anode.
8. An electron beam tube comprising an elec
tron gun including an indirectly heated cathode
for producing electrons in a fast-moving‘focused
condition, an anode vfor receiving the electron
beam, and intermediately positioned electrodes
for de?ecting the beam across the anode and for
simultaneously maintaining the beam in its fo
cused condition, said anode comprising a plurality
of physically separate members which are charged
in a variable manner depending upon the extent
of the beam portion which strikes the respective
' members.
9. In combination, an electron beam tube com
H) Li prising an envelope containing a source of elec
trons, a main anode, a plurality of auxiliary an
odes, and a plurality of electron-de?ecting elec
trodes positioned on opposite sides of the en
velope between said source and said main anode,
30
resistances respectively between said auxiliary
anodes and between said electron-de?ecting elec
trodes, a source of electromotive force, the nega
tive terminal of which is connected to the source
of electrons, and connections between di?erent
positive terminals on said source of electromotive
force and each of said resistances.
10. An electron beam'tube comprising an en
velope containing an equipotential source of elec
trons, an electron-focusing electrode surrounding
40 said source, an electron-receiving member, an
electron-accelerating electrode positioned between
said source and said member, and a combined
electron-de?ecting and electron-focusing elec
trode interposed between said electron-accelerat
45 ing electrode and said electron-receiving member.
11. An electron beam tube comprising an elec
tron gun including an indirectly heated cathode
5
for producing electrons in a fast-moving focused
condition, an anode for receiving the electron
beam and intermediately positioned electrodes for
de?ecting the beam across said anode and for
simultaneously maintaining the beam in its fo
cused condition, said anode comprising a plurality
of physically separate members which are charged
in a variable manner depending upon the extent
of the beam portion which strikes the respective
members, said intermediate electrodes being 10
maintained at a ?xed average potential with re—
spect to said electron gun.
12. An electron beam tube comprising an en
velope containing a source of electrons including
an indirectly heated cathode, an electron-focus
ing electrode surrounding said source, an electron
15
receiving member, an electron-accelerating elec
trode positioned between said source and said
member, and a common means arranged on the’
opposite side of said electron-accelerating elec 20
trode from said electron-focusing electrode for
simultaneously de?ecting and maintaining the
focus of the electrons, said means including an
electrode for providing an electric ?eld having a
contour similar to the shape of the electron beam 25
as it leaves said electron-focusing electrode.
13. An electron beam tube comprising an en
velope containing a source'of electrons including
an indirectly heated cathode, means for con—
straining said electrons to a beam of rectangular 30
cr0ss~section, an electron-receiving member, an
electron-accelerating electrode positioned between
said source and said member, and a pair of metal
plates arranged on the opposite side of said elec
tron-accelerating electrode from said electron— 35
constraining means, said plates being positioned
in planes corresponding to opposite sides of said
beam, said plates having such positions with re
spect to the remaining elements of the tube and
being adapted to be charged to such a ?xed av 40
erage potential with respect to said electron
source that the electric ?eld set up by said plates
serves to maintain the initial focus given to the
beam by said electron-constraining means and
permit a free movement of the focused beam when
_ a de?ecting voltage is applied to said plates.
JOHN M. CAGE.
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