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Патент USA US3098199

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July 16, 1963
T, A_ BUCHHQLD
3,098,189
CRYOGENIC D.C. TO A.C. AMPLIFIER
Filed April 11. 1960
2 Sheets-Sheet 1
Inventor.
Theodor’ A.Buchho/c/,
b9 Way/1%”,
H/LsAttorrvey
July 16, 1963
3,098,189
T. A. BUCHHOLD
CRYOGENIC 11c. T0 A.C. AMPLIFIER
Filed April 11. 1960
2 Sheets-Sheet 2
////////
WW,
Fig.8
Inventor-~
Theodor" ABuchho/d,
by %4///@;% '
His Attorney.
United States Patent O??ce
3,098,189
Patented July 16, 1963
1
2
3,098,189
employed with the ampli?ers shown in FIGURES l and
3 to develop a large current signal for application to the
CRYOGENIC D.C. T0 A.C. AMPLE‘IER
Theodor A. Buchhold, Schenectady, N.Y., assignor to
General Electric (Jompany, a corporation of New York
Filed Apr. 11, 1960, Ser. No. 21,515
7 Claims. (Cl. 32i1—8)
‘ampli?ers;
FIGURE 6 is a cross-sectional view of still a third
term of superconductive ampli?er constructed in accord
ance with the invention;
FIGURE 7 is a schematic ‘circuit diagram of a driving
circuit ‘for energizing the driving coils comprising a part
of the ampli?er shown in FIGURE 6; ‘and
The present invention ‘relates to ‘a new and improved
direct current ampli?er.
More particularly, the invention relates to a new and 10
FIGURE '8 is a schematic circuit diagram of the signal
input [and output circuits comprising part of the ampli?er
improved direct current ampli?er employing supercon
shown in FIGURE 6.
The ampli?er shown in ‘FIGURES i1 ‘and 2 of the
drawings comprise an outer large diameter control wind
tric signals such as are produced by a thermocouple or 15 ing 11 formed of many turns of superconducting wire.
The ampli?er of FIGURES 1 and 2 is adapted to be dis
other similar device, it has heretofore been necessary to
ductive parts which has zero drift, and is essentially noise
free.
In order to measure low amplitude direct current elec
posed within a cold space which will serve to maintain
employ extremely sensitive and expensive ampli?ers in
the temperature of the parts of the ampli?er at a value
advance of the measurement equipment .to ?rst amplify
such that its parts become superconductive (e.g., 4.2°
the signal to measurable levels. In doing this it is of course
essential that the ‘ampli?er inject no noise into the signal 20 Kelvin). Since such equipment is already well known
in the art, and has been described in a number of places
such as is created by thermionic action and so forth, other
(see, for example, copending U.S. application Serial No.
wise the .two become indistinguishable. Because of this
791,953, “Insulated Housing,” ?led February 9, 1959,
limitation, there are certain types of signals having ex
T. A. Buchhold and K. F. Schoch, inventors, assigned to
tremely low amplitude levels which existing ampli?ers
are incapable of amplifying without drowning out the 25 the General Electric Company) 'a further description of
this equipment is believed unnecessary. The outer coil
signal with noise generated in the ampli?er itself. Ad
11 surrounds a plurality of individual multiple turn out
ditionally, the environmental conditions under which am
put windings 12 and 13 which are arranged around the
pli?ers of this type are often used are such that it is im
inner periphery of the control winding 11 in the manner
possible to continually or even periodically adjust the
ampli?er to correct the drift. Under such conditions it 30 best shown in FIGURE 2 of the drawings. The output
windings 13‘ and 12 are inductively coupled with each
is of course desirable that the ampli?er used with the
other and with the control winding 11 and are elec
measuring equipment he as free from drift as possible.
trically interconnected through appropriate connect-ions
‘It is therefore a primary object of the present invention
shown ‘by dotted lines 14- in FIGURE 2 of the drawings.
to provide a new and improved direct current ampli?er
for amplifying low ‘level signals without introducing any 35 These connections serve to connect all of the output
windings 12 ‘and 13 in series circuit relationship with a
extraneous background noise into the signal and there
fore is essentially noise free, and which possesses zero
drift characteristics.
In practicing the invention a noise ‘free zero drift direct
current ampli?er is provided which converts direct cur
capacitor 15 which is of a value to tune the circuit to
series resonance at the operating frequency of the am
rent signals to alternating current signals. The ampli?er
of input signals to be ‘ampli?ed. is shown at 16 together
is comprised by ?rst and second sets of coils which ‘are
with its internal resistance 17 and may comprise a thermo
couple .or other source of low amplitude direct current
pli?er. The ‘ampli?ed alternating current output signal
may then be obtained across the capacitor 15. A source
inductively coupled and which ‘may be formed from
superconductive wire. The ampli?er further includes a
movable member fabricated, at least in part, of super
conductive material which is in position adjacent the
?rst and second coils, and which is moved continuously
to vary the inductive relation between the coils. A direct
current input signal to be ampli?ed is applied to the
input of one of the coils, while the ampli?ed alternating
current output signal is derived ‘from the remaining coil.
‘Other objects, features and many of the attendant ad
vantages of this invention will be appreciated more read
ily as the same becomes better understood ‘by reference
45
potential which changes in magnitude at very slow rates.
This ‘essentially direct current input signal is supplied
to the control winding 11, ‘and the magnetic ?ux pro
duced by the winding 11 as a result of the input signal
will normally link with the multiple turn output windings
12 and 13 where such intercoup-ling is not interrupted by
a superconductive disc 18 supported between the two
sets of windings 12 and 13. The disc 18 constitutes a
rotatable superconductive member which is rotated by a
drive motor (not shown) or other suitable drive means
connected through a shaft 19 to a superconductive hub
to the following detailed description, when considered 55 portion. The hub portion of disc 18 terminates in outer
fan-like superconductive portions 22 which are inter
in connection with the accompanying drawings wherein
like parts in each of the several ?gures are identi?ed by
the same reference character, and wherein;
FIGURE 1 is a sectional side view of a new and im
proved superconductive ampli?er constructed in accord
spaced with alternate open areas indicated at 26.
The
superconductive rotatable disc 18 is supported within a
suitable housing for the ampli?er (not shown) by means
of a plurality of magnetic bearing coils 24 which pref
erably are formed from 1a plurality of turns of super
conductive wire, and ‘are energized from a suitable direct
current source, not shown. A ?ange portion 26 is pro
FIGURE 1;
vided on the superconductive disc 18 against which the
FIGURE 3 is a partial sectional side view of an alter
native form of the ampli?er shown in FIGURE '1;
65 magnetic ‘bearing coils act to support disc 18 in the di
mension transverse to the axis of shaft 19. The ?ange
‘FIGURE 4 is a partial plan view of a disc having alter
portion 26 together with the superconductive hub po-r—
nate magnetically permeable ‘areas and superconductive
tion of the vane serves also to con?ne the magnetic
areas, and which comprises a part of the ampli?er shown
bearing ?ux produced by the bearing coils to the interior
ance with the invention;
FIGURE 2 is a plan view of the ampli?er shown in
in FIGURE 3;
FIGURE 5 is a schematic functional diagram of a
superconductive direct current transformer that can be
hub portion of the rotatable disc so that the bearing ?ux
does not interfere with signal coils of the ampli?er.
‘In operation, the superconductive ampli?er is ?rst re,
. 3,098,189
‘
duced in temperature to its operating range in the neigh
4
purpose it may be possible to employ a D.C. transformer
such as is illustrated schematically in FIGURE 5 of the
borhood of 4.2 Kelvin, and the rotatable disc 18 is caused
to be rotated by the drive motor coupled to the shaft 19
at a constant speed. The direct current signal to be
drawings. This trans-former includes a multiple turn
primary winding 32 which is fabricated ‘from a large
number of turns ‘of superconductive wire, and is excited
ampli?ed is then applied to the input terminal 16 and
induces a flux in ‘the control winding 11 which links with
from a low voltage direct current signal source 33 having
the multiple turn coils 12 and 13 of ‘output windings
an internal resistance 34. The multiple turnyprimary
winding 32'is inductively coupled to a single turn sec
only through those open areas indicated at 23 that occur
Where superconductive areas indicated
ondary winding 35 that in turn is connected to a single
at 2 exist, they will block the lines of flux 'so that the 10 turn superconductive control winding 11 that replaces
in the vane 18.
the multiple turn control windings 11 of the ampli?ers
magnetic lines of ?ux are interrupted, and no linkage
shown in FIGURES l and 3.
, takes place between the control winding 11 and the 'out—
put windings 12 and 13.
Because the superconductive
By this arrangement, a
low value input current will produce a relatively large ~
disc 18 is rotated it can be appreciated that the linkage
?ux in the primary winding 32 which in turn will result
of the lines of ?ux is alternately interrupted and re 15 in developing a large signal current in the single turn
established thereby inducing an A.C. voltage in the out
secondary Winding 35. This transformed signal current
put windings 12 and 16 which is proportional in value
can then be used to excite the single turn primary wind
ing 11 of the ampli?ers of FIGURES l and 2.
to the direct current signal supplied to the input terminal
A second form of superconductive ampli?er constructed
16. Because the output windings 12 and 13 are tuned
' to series resonance by capacitor 15, maximum output
in accordance with the invention is shown in FIGURE 6
of the drawings. The ampli?er shown in FIGURE 6
power will be obtained from the ampli?er. This alter
nating current output potential may then be further am
comprises an outer housing 41 fabricated from insulating
pli?ed by conventional alternating current ampli?ers.
material or other suitable material which has no, or very
small, magnetic losses, and having a number of receptacles
Also, because disc 18, the control winding 11, and the
output windings 12 and 13 are always superconductive, 25 formed therein for receiving magnetic ?eld producing
it can be appreciated that there is substantially little or
no noise generated in these windings due to thermal
action.
,
driving coils 42 and 43‘ to be described hereinafter. The
magnetic driving coils 42 and 43 are disposed within
housing 41, and are generally toroidal in shape so that
7
Further, if the input signal 'to the ampli?er drops to
they extend completely around the periphery of the hous
ing 41. The driving coils 42 ‘and 43 have superconduc
zero the magnetic ?ux produced by control winding 11
tive plates 44 and 45 which are disposed over their op
' becomes zero so that the output potential from the am—
pli?er drops to zero.
Therefore it can be appreciated
posing faces for ?eld guiding purposes, and which have
that the ampli?er has no zero drift.
A second embodiment of a superconductive ampli?er
a small slit or gap in their cross section so as to prevent
the development of short circuit currents around their
periphery. The superconductive plates 44‘ and 45 de?ne
a space intermediate the magnetic driving coils 4-2 and
43 in which a vibratable superconductive disc 46 is dis
posed. ’I‘he superconductive disc 46 extends over almost
‘employing the principles of the invention is shown in
FIGURES 3 and 4 of the drawings. The superconductive
ampli?er of FIGURES 3 and 4 is constructed and op
crates in a similar manner to that shown in FIGURES 1
and 2, and for that reason includes an outercontrol
the entire cross sectional area ‘of the interior of the am
Winding 11 which is inductively coupled with a set of
inner multiple turns output windings 12 and 13 that are
connected in series circuit relationship to a suitable out
put circuit, not shown. The inductive relation between
the ‘outer coil 11 and the inner ‘coils 12 and 13 is inter
pli?er housing 41, and includes a pair of upright shielding
shown) acting against the hub of the disc and isolated
from signal circuit by ?ange portion 26. The rotatable
plifying circuits of the ampli?er. Disposed Within the
disc has alternate fan-like portions 29' which are ren
48 and 51, respectively, so that in effect each set com
portions 47 which are acted upon by the driving coil mag
netic flux to provide lateral stability, and which further
serve to shield the interior of the housing from the ?elds
of the driving coils 42 and 43. In this manner the mag
rupted by a magnetically permeable steel disc 28. Disc 45 netic ?elds produced by the driving coils 42 land 43‘ are
28 is rotatably supported between the multiple turn sec
isolated from the interior of the ampli?er housing, and
ondary‘windings 12 and 13 by suitable bearing coils (not
therefore prevented from interfering with the signal am
interior of the housing 41 is a second set of magnetic
disc 28 is best illustrated in FIGURE 4- of the drawings 50 signal amplifying coils 48, 49‘ and 51, 5-2. There are
wherein it can be seen that the outer periphery of the
fewer turns in coils 49 and v52 than in their related coils
dered superconductive by the adherence of a suitable
superconductive material to these portions with the re
prises a step‘up transformer. One set of signal ampli- '
fying 'coils 48‘ and 49‘ are disposed over the 'vibratable
maining alternate fan-like portions 31 being magnetically 55 superconductive disc '46 and a second set of coils ‘51 and
permeable. ‘By constructing the superconductive vane
752‘ are positioned under disc 46‘. The opposing faces.
28 in this manner, due to the iron, the magnetic resistance
of the signal coils 48 and 49, and 51, 52 respectively, are
between the output coils is. reduced and a higher flux
covered by annular superconductive shields 53 and 54
density is obtained with a consequent improvement in
which have a slit or gap across their entire cross section
ampli?cation. Additional increase in the flux linking the 60 to prevent the development of undesired circulating cur
‘output windings Y12 and 13v is obtained by the provision
rents and which are provided for ?eld forming purposes.
of a toroidal-shaped iron core 30 which extends between
By this arrangement, the superconductive disc 46 may
the control winding 11 and the output windings 12 and
be vibrated up and down in the plane ‘of the drawings by‘
13 around the entire periphery of the windings. In op
the driving coils 4'2 and 43' in the manner. to be described
eration, the embodiment of the invention shown in FIG 65
hereinafter. This results in modifying the coupling be
URES 3 and ,4 functions in precisely the same manner
tween the two setsv of control and output windings 48
as that described with relation to FIGURES 1 and 2, to
and 4-9‘, and 51 and ‘52, respectively, in a manner such as
develop an ampli?ed alternating cln'rent output signal
across its output terminals.
to decrease the magnetic ?ux threading a set of windings
When used in amplifying
signals of. very small currents, it may 'be desirable or 70 when the ‘disc 46 approaches the respective set, and to
increase the magnetic ?ux threading the respective set of
necessary that the ‘signal current be transformed to a
larger value in advance of being supplied to the control
winding of an ampli?er such as shown in FIGURES '1
land 3, ‘modi?ed to the extent that the control winding 11
istormed of a single turn superconductive coil. ~ For this
75
windings when the disc departs from the set. Accord
ingly, vibration (of the superconductive disc 46 functions
to generate an alternating current component in the out
put windings 49 and '52,, respectively, which will have a
3,098,189
5
frequency similar to that of the frequency of vibration
of superconductive disc 46.
The driving and signal circuits for the superconductive
ampli?er of FIGURE 6 are shown in FIGURES 7 and
3 of the drawings. FIGURE 7 illustrates the driving cir
cuit for the magnetic driving coils 42 and 43 which are
shown connected in series circuit relationship across a
6
opposite direction will be induced in these windings which
will cancel out. With regard to the output windings 49
and '52, however, these windings are connected in reverse
relations so that the two induced voltages become addi- -
tive and the induced voltages will be added across the
load resistor to produce an output signal voltage propor
tional to the DC. control current supplied from source
63 and having an alternating current carrier frequency of
the vibrating superconductive disc v46‘. It can be appre
secondary winding ‘56 of a supply transformer. The pri
mary winding 57 of the transformer is connected across
a source 58 of vibrating signals ‘having a frequency the 10 ciated therefore, that the device operates as a direct cur
rent to alternating current ampli?er for low level signals.
same as that at which it is desired to vibrate the super
From- a consideration of the above description, it can
conductive disc 46. ‘The center tap of the secondary
winding 56 and the juncture of the magnetic bearing
be appreciated that because the signal coils 48, 49, and
coi-ls 42 and 43 are connected together through a direct
51, 52, and the disc 46 are fabricated from» supercon
current biasing circuit formed by a battery 59‘ and variable 15 ductive material maintained at an operating temperature
resistor 61. This direct current biasing source func
in the neighborhood of 4.2 Kelvin, extraneous noise will
tions to establish a reference magnetizing current through
not be introduced into the signal due ‘to thermal action
the magnetic ‘driving coils 42 and 43, each of which coils
and the like, and accordingly the ampli?er will be sub
has an appropriate number ‘of turns for positioning the
stantially noise free. Also since there will be no signal
superconductive disc 46 approximately midway in the 20 current induced in the primary windings 48 and 51 of the
space between the two driving coils 42 and 43. There
ampli?er except when a signal is present from the source
after, the alternating current vibrating signal supplied
63 when this signal drops to Zero, similarly the current
from source '58 which is superimposed on the ‘direct bias
induced in the secondary windings 49 and ‘52 will drop
ing current from source 59 will vary the composite sig
to zero, and there will be no ‘drift of the ampli?er from
nal level through the driving coils 42 and 43 in a manner 25 its preadjusted zero reference level.
such as to cause the superconductive disc 46 to be vi
brated up and down at a frequency determined by the
frequency of the source 58.
The signal circuits for exciting the control windings
From the foregoing description, it can be appreciated
therefore, that the invention provides a new and im
proved superconductive ampli?er for amplifying low level
signals without introducing any extraneous background
4S and 51 are shown in FIGURE 8 of the drawings. The
noise into the signal. Hence, the ampli?er is essentially
control windings 48 and 51 are connected in series circuit
noise free. Further, from a consideration of the nature
relationship with a source of direct current electric sig
of the operation of the ampli?er it can be appreciated
nals 63‘ having an internal resistance 64, and which might
that the ampli?er possesses no zero drift characteristics
comprise a thermocouple or similar sensing device. The
since no signal output occurs until a signal is applied
direct current signal currents produced by the signal 35 to the input which rises above zero level.
source 63 will How in the control windings 48 and '51 to
Having described several embodiments of the new and
produce a ?ux that links with the output windings 49‘ land
improved ampli?er constructed in accordance with the
52. The signal output windings 49‘ and '52 are likewise
invention, it is believed obvious that other modi?cations
connected in series circuit relationship, however, it should
and variations of the present invention are possible in the
be noted that connections to the input and output ter 40 light of the above teachings. It is therefore to be under
minals of the winding '52 are reversed with respect to the
stood that changes may be made in the particular embodi
output winding 49 so that even though signal currents in
ments of the invention described which are within the
duced in this coil are in the reverse direction from those
full intended scope of the invention as de?ned by the
induced in the signal output winding 49, the interconnec
appended claims.
tion ‘of the coils is such that the currents will be cumula 4:5
What I claim as new and desire to secure rby Letters
tive or additive as explained more fully hereinafter. The
Patent of the United States is:
resultant ampli?ed ‘output signal is supplied to an output
1. A noise-free zero ‘drift ampli?er including in com
load resistor 65 to develop an output potential that is
bination ?rst and second inductively coupled windings
available across the ‘output terminals 66.
fabricated from superconducting materials, a movable
In operation, assuming the parts of the ampli?er to 50 member fabricated at least in part of superconductive ma
terials in juxtaposition to said ?rst and second windings
supplied to the magnetic bearing coils 42, 43 is adjusted
for influencing the inductive relation between the wind
to properly suspend the superconductive disc 46 approxi
ings, means for moving said movable member at a pre
mately midway in the space between the signal coils 48,
determined rate to thereby vary the inductive relation
4-9 and 51, 52. Thereafter, the vibrating or oscillating 55 between the coils, means for impressing an input signal
signals 58 are supplied to the magnetic bearing coils
to be ampli?ed on said ?rst winding, and means for de
42, 43 so that the superconductive disc 46 is caused to
riving an ampli?ed alternating current replica of said
oscillate up and down at a frequency determined by the
input signal from said second winding.
frequency of the source 58. With the ampli?er in this
2. A noise-free zero ‘drift ampli?er including in com
condition, application of an input signal to be ampli?ed 60 bination a ?rst magnetic ?eld producing control winding
to the control windings from source 63 induces a cur
fabricated of superconducting material, a second output
rent flow through these windings and establishes a mag
winding
fabricated from superconducting materials juxta
netic ?eld that links with the output windings 49 and 52.
posed to said control winding in inductive relationship,
The inductive couplings ‘between these respective pairs of
a movable member positioned to intercept the magnetic
windings will be modi?ed by the vibrating superconduc 65 lines of ?ux inductively coupling said ?rst and second
be at superconducting temperature, the direct current bias
tive disc 46 which will cause a decrease in the flux link
ing a respective set of windings as the disc 46 approaches
the windings, and will cause an increase in the flux link
windings and having alternate superconductive and mag
netically permeable areas for interrupting the inductive
relationship between said ?rst and second windings, means
for moving said movable member at a predetermined
set of windings. This will have the effect of an increas 70
rate
to thereby vary the inductive relation between the
ing or collapsing magnetic ?eld on the windings, and
windings, means for impressing an input signal to be
will induce electric voltages in each of them. With re
ampli?ed upon said ?rst winding, and means for deriving
spect to the control windings ‘48 and 51, one winding
an ampli?ed replica of said input signal from said second
will be subjected to an increasing ?ux and the other to
a decreasing ?ux so that voltages of equal magnitude but 75 winding.
ing the windings as the disc departs from the respective
3,098,189
I
3. A noise-free zero drift ampli?er including in com
bination a ?rst magnetic ?eld producing control winding
fabricated of superconducting material, a second output
winding fabricated from superconducting materials juxta
posed to said ?rst coil in inductive relationship, a rotatable
disc positioned to intercept the magnetic lines of ?ux
inductively coupling said ?rst and second windings and
having alternate superconductive and open areas for inter
rupting the inductive relationship between said ?rst and
second coils, means for impressing ‘an input signal to be
ampli?ed upon said control winding, and means for de
riving ‘an ampli?ed replica of said input signal from said
output winding.
I
l
4. A noise-free zero drift ampli?er including in com
bination a ?rst magnetic ?eld producing control wind
ing fabricated of superconducting material, a set of multi
ple turn output windings fabricated from superconducting
materials juxtaposed to said ?rst coil in inductive rela
tionship and connected in series circuit relationship, a
8
impressing an input signal to be ampli?ed upon said
control winding, and means for deriving an ampli?ed
replica of said input signal from said second set of coils.
6. A noise-free zero drift ampli?er including in com
bination a housing, a ?rst set of magnetic ?eld produc
ing coils fabricated from superconducting materials posi
tioned in said housing, a vibratable superconducting mem
ber disposed in said housing and adapted to be vibrated
within the housing by said ?rst set of magnetic ?eld pro—
ducing coils, and a second set‘ of inductively coupled
signal windings having di?erent numbers of turns fabri
cated from superconducting materials and disposed within
said housing in juxtaposition to said vibratable member,
said superconductive vi'bratable member being adapted
to modify the inductive relation between the second set
of windings, means for impressing a signal to be ampli
?ed on the one of said signal windings having the fewer
turns, and means 'for deriving an ampli?ed replica of the
input signal form the remaining one of said signal wind
rotatable disc positioned to intercept the magnetic lines 20 ings having the larger number of turns.
7. A noise-free zero drift ampli?er including in com
of flux inductively coupling said control and output wind
bination a housing, a vibrata-ble superconducting disc dis
ings and having alternate superconductive and open areas
posed in said housing, a ?rst set of superconducting coils
for interrupting the inductive relationship between said
disposed on opposite sides of the disc and acting against
control and output windings, a set of bearing coils for
magnetically supporting'said disc in, said relationship,
25 the ends thereof to cause the disc to be vibrated within
means for isolating said bearing coils from said control
and output windings, means for rotating said rotatable
the housing, a respective second’ set of inductively coupled
signals windings having different numbers of turns fabri
cated from superconductive material disposed in said
housing on respective sides of said disc, said vibratable
disc, means for impressing an input signal to be ampli?ed '
upon said control winding, and means for deriving an
ampli?ed replica of said input signal from said output
superconductive disc being adapted to modify the induc
winding.
tive relation between each of the windings in each set,
' ' 5. A noise~free zero drift ampli?er including in com
means for isolating the magnetic flux produced by said
?rst set of coils from in?uencing said signal windings,
bination a ?rst large diameter magnetic ?eld producing
means for applying an input signal to be ampli?ed to the
control win-ding fabricated of superconducting material,
a set of multipletturn output windings fabricated from 35 one of said signal windings having fewer turns, andrmeans
for deriving an ampli?ed replica of said input signal from
superconducting materials disposed- within said control
windings in inductive relationship and connected in series
the remaining one of said signal windings having the
circuit relationship, a core member having low magnetic
larger number of turns.
reluctance in-tercoupling said control and output windings,
References ‘Cited in the ?le of this patent
a rotatable disc positioned to intercept the magnetic lines
V UNITED STATES PATENTS
of ?ux inductively coupling said control and output wind
ings and having alternate superconductive and magneti
2,689,332
Greene ______________ _._ Sept. 14, 1954
cally permeable areas for interrupting the inductive rela
2,704,431
Steele _______________ __ Mar. 22, 1955
tionshiptbetween said control and output windings, a set 45 2,725,474
Fricsson et al _________ __ Nov. 29, 1955
of bearing coils for‘ magnetically supporting said disc in
2,820,915
Mathews ______ -a ____ __ Ian. 21, 1958
said relationship, means for isolating said bearing coils
FOREIGN PATENTS
from said control and output windings, means for rotat
ing said disc at a substantially constant rate, mean-s for
25,457
France ____ ..___.L ______ __ Mar. 17, 1923
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