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Патент USA US2112746

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March 29, 1938.
Y '
Filed April 16, 1935
mg. I.
' Inventor‘:
Albrecht‘ Wdlfel
Patented Mar. 29, 1938
Albrecht Wolfe], Rudolstadt, Germany, assignor
to General Electric Company, a corporation of
New York
Application April 16, 1935, Serial No. 16,673
In Germany May 15, 1934
1 Claim.
In order to adapt an X-ray tube to more than
one purpose, for example, therapeutic and radio
graphic, which necessitate different load condi
tions, i. e. different sizes of focal spots, etc., one
may provide the tube with two or more ?la
mentary cathodes of di?erent electron-emitting
capacities. Obviously, the cathodes require dif
ferent amounts of heat. Consequently, when
(01. 250-35)
these objects, my invention contemplates in brief,
the use of a resistance or inductance in series
with the smaller ?lament, i. e. the cathode of
lower heating capacity, and mounting the resist
ance or inductance within the con?nes of the
envelope so as to be surrounded by an evacuated
space in case the envelope is highly evacuated.
that it is desired to- energize the smaller cathode
The invention will be clearly understood when
reference is made to the following description
and the accompanying drawing in which Fig. 1
represents a view, partly in section, of an X-ray
tube containing a cathode construction improved
in accordance with the present invention; Fig. 2
from the same source of electromotive force as
is an end view of the focusing member and as
changing the operation of the tube from one use
10 to the other, as for example, when substituting
the cathode of smaller electron-emitting or heat
ing capacity for the larger cathode, and assuming
r‘ the larger cathode, it is desirable to reduce the
heating voltage impressed across the smaller
In order automatically to reduce the energy
supply to the smaller ?lament under these con~
ditions, it has' been proposed to insert a resist
ance in the heating current circuit of the oath
ode and in case the cathode is energized by an
alternating current source, a choke coil or a trans
former may be used. The choke coil, transform
er or resistance is usually connected in the ?la
ment circuit external to the tube and in order to
make the structure of a unitary design, it has
been proposed to enclose the coil or resistance in
the base cap of the tube. However, when the
voltage-reducing device is enclosed within the
base cap, certain di?iculties arise. In the ?rst
place, the cap on the cathode side of the tube
must be enlarged to accommodate the extra ele
ment which may present some difficulty in ?tting
the tube and its cap into existing apparatus and
protective housings. In addition, more especially
when using a resistance, but also in the case of
a choke coil or transformer in lesser degree,
there is the further disadvantage in that the
voltage-changing device may give off considerable
heat and deleteriously affect the material of the
An object of the present invention is to provide
an improved all-purpose X-ray tube containing
sociated cathodes; while Fig. 3 is an enlarged
diagrammatic view of the cathode structure per
se including the voltage-reducing element.
Referring more particularly to Fig. l, numeral
l designates the envelope of an electrical dis
charge device containing an anode 2, which in
the case of an X-ray tube may have an inclined
face and be provided with a target. The anode
is supported Within the envelope in any suitable
manner, for example, by means of a rigid rod 3
secured to a reentrant stem 4 which terminates
one end of the envelope. The other end of the
envelope is also provided with a reentrant stem
5 in which are fused several rigid rods serving
as combined leading-in conductors and supports
for the improved cathode structure. This struc
ture includes ?laments 6, 1, one of the ?laments,‘
e. g. ?lament 6, being of greater heating capacity
and therefore of greater electron-emitting ca
pacity than the other ?lament. There may also
be provided a hemispherical focusing member 8,
supported within the envelope by means of a
rod 9 which is secured to one of the conductors
passing through the stem 5. The conductor 9
may be connected to one or both of the ?lamena
tary cathodes or as is more usually the case, may
be taken out through the envelope as a separate
lead to which any desired potential may be ap
two or more ?lamentary cathodes of different
The cathodes are preferably constituted of the
same material, for example, tungsten wire of the
electron-emitting or heating capacities, and in
same gage, and since one cathode is intended to
which one cathode may be substituted for the
emit greater amounts of electrons than the other
cathode, it follows that the lengths of the respec
other cathode when desired, and is adapted to
be energized from a common source of electro
50 motive force. A more speci?c object is to em.
ploy in a tube of this character, an improved
form of voltage-reducing device or element in
connection with the cathode of smaller electron
emitting or heating capacity and also an im
55 proved position of the device.
In carrying out
tive cathodes are different.
When a tube of this character is operated, it
is desirable to be able to switch from one cathode
to the other in case the same tube is alternatively
serving two Widely different purposes, as for ex
ample, when used in therapy and radiography.
In case a common source of electromotive force 55
is employed to energize both cathodes, obviously
In the event that the element I0 is required to
the voltage impressed across the cathode of
smaller heating or electron-emitting capacity
must be less than the voltage applied to the
dissipate or consume considerable energy and
thereby is rendered su?iciently incandescent to
emit electrons, it may be desirable to provide a
cathode of larger heating or electron-emitting
capacity. The necessary reduction of voltage in
case it is desired to substitute the smaller cathode
metal shield 13 interposed between the element
for the larger cathode, is conveniently obtained
order to provide a mechanical barrier for electrons
which tend to leave the end of the element Ill
near the envelope. The shield may be supported 10
by means of a resistance or choke coil l0 con
nected in series with the smaller ?lament 1. As
shown, the adjacent ends of the ?laments 6, ‘I are
connected together by a common leading-in con
ductor and the opposite end of the larger ?lament
is connected to another leading-in conductor,
while the free end of the element I0 is connected
to the remaining leading-in conductor. The ?la
m‘ents are conveniently arranged side by side
within the opening at the bottom of the spherical
focusing member 8 as is shown more clearly in
Fig. 2. When the ?laments 6, l are energized
from a common source I I of alternating or direct
current, and when a switch I2 is provided in the
and the anode. This shield may be given any
suitable shape, e. g. an angular con?guration in
within the envelope in any suitable manner.
In addition to the shield I3 and as shown in
Fig. 3, there may be provided other angular mem
bers I4 positioned respectively between the free
ends of the ?laments 6, l and the envelope. All 15
of these shields or screens serve to prevent any
electrons from reaching the anode from unde
sired parts of either cathode or associated ele
While I have described my invention more espe 20
cially in connection with an X-ray tube for the
reason that cathodes of different electron-emit
circuit for selectively connecting the ?lament 6
ting capacity are frequently employed ‘in. this
or ‘I. to the source, the same voltage may now be
connection, it will be understood that the present
invention is not limited to tubes of this type but
is su?iciently broad to embrace all forms of elec
trical discharge apparatus, highly evacuated or
gas-?lled, in which a plurality of ?lamentary
cathodes are employed having different electrical
applied, without danger to the smaller ?lament
1, as isapplied to the larger ?lament 6. Obvi
ously, the voltage actually impressed across the
?lament 1 represents a reduced voltage obtained
by the effect of the resistance or inductance l0
30 which serves to dissipate or consume the energy
of the undesired voltage. It is desirable when the
element i0 is constituted of resistance, that the
latter should be made of the same wire as the
?laments. 6 and ‘I. It is also apparent that
35 the combined resistance of the ?lament 'l and the
resistance II] should be substantially equal to the
resistance 'of the ?lament 6. By placing the
voltage-reducing element I0 within the envelope
instead of being positioned external to the en
velope and contained within a base cap at the
cathode end of the tube, the element is in such a
position as not to cause any deleterious heating
e?'ects, nor does it necessitate a large cathode cap
within which to be accommodated. It will be
‘advantageous to mount theresistance close to the
cathode, since in this case the heat generated in
it will heat only such internal constructional parts
of the tube as are heated by the cathode itself and
are therefore designed to withstand the heat
50 produced.
Moreover, the improved position of
the element I0 o?ers a unitary and compact con
characteristics and which are selectively ener
gized from a common source of electrom'otive
What I claim as new and desire to secure by
Letters Patent of the United States, is,—
An electrical discharge device comprising an 35
envelope containing an anode, a main cathode
and a secondary cathode, said cathodes being ar
ranged adjacent to one another, a voltage-reduc
ingelement within said envelope and connected
to said secondary cathode, the combined resis
tance of the secondary cathode and said element
being substantially equal to the resistance of said
main cathode whereby equal voltages may be ap
plied to the main and secondary cathode circuits
to produce equal heating currents, and a shield
mounted adjacent to said voltage-reducing ele
ment for preventing radiation of electrons by said
element but permitting radiation by the secondary
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