вход по аккаунту


Патент USA US2411997

код для вставки
Dec. 3, 1946.A
Filed D80. 17, 1942
Patented Dec. 3, 1946
imi-so 'STATES 1
p Frederick G. Kelly, West Orange, N. 3'., assigner
to Thomas A. Edison, Incorporated, West
Orange, N. J., a corporation ot'New Jersey
Application December 17, 1942, Serial No. 489,281
10 Claims. (Cl. 171-95)
This invention relates to electrical measuring
instruments and more particularly to ameter
for measuring direct current. ‘
long angular range.
A yet further object is to provide a meter ful-s
filling the above-stated objects which is simple in
design and construction and economical to manu~
It is another object to provide an'improved
permanent magnet rotor for meters of the char»
acter mentioned, having angularly long pole sur
faces which are substantially uniform in pole ,
It is a further object to employ such a perma
nent magnet rotor in conjunction with field poles
to obtain new and improved results.
Other objects are to ‘provide an improved iield
core structure for meters and especially one hav
ing a reduced reluctance, andto employ such im
proved iield core structure in conjunction with a
permanent-magnet rotor to obtain an improved
sensitivity in meter operation.
n further object is 'to effectively damp the rotor
electrically in meters of the character mentioned
by simple means applied to the ñeld structure of
the meter.
magnet rotor 3.
In my invention I employ a permanent magnet
The invention is concerned with D.-C.- meters
oi the moving magnet type and has for its gen
eral objects to provide such D.-C. meters hav
ing a high sensitivity and a uniform scale over a
center 3 there is plvotally held a permanent
Other and allied objects of my invention will
more fully appear from the following description
and the appended claims.
In vthe description of my invention reference
is had to the accompanying drawing, of which:
Figure 1 is a perspective fractional view of. a
meter according to my invention;
rotor having arcuate pole surfaces which sub
tend a wide angle at the center of the rotor and
which are substantially equipotential-i. e., uni
form in pole strength. In a preferred construc
tion of such rotor there is employed a block
shaped permanent magnet il of one of the highly
eiîicient Alnicc magnetic materials having a
thickness dimension (axially of the rotor) which
is substantially equal to the over-all thickness of
the stack oi laminations forming the core E, a
length sufficient largely to span the distancebetween the pole faces, and a width which is small
relative to its length. The magnet has its mag
netic axis parallel to its length dimension just
noted, and has its polar end surfaces arcuately
shaped. Mounted on these end surfaces are arcu
ate shoes it made of a high-permeability and
low-hysteresis material such as of soft iron.
These shoes are held to themagnet by non-mag
netic-caps iI having rim flanges il' which em
brace the shoes and clamp them tightly to the
magnet, the caps being suitably rigidly held to
the shoes as by spot welding. Secured to these
caps at their centers are spindles I2. These
spindles have conical ends or pivots which are
to engage suitable jewels, not shown, at the center
line 3 to hold the rotor 8 in a position wherein
the peripheral surfaces of the shoes I0 confront
the pole faces 4 and 4" and are spaced equally
and uniformly therefrom.
On one of the spindles I2 there is prövided a
pointer I3 having a tailpiece I4 on which a weight
I5 ‘s adjustably mounted- for counterbalancing
the weight of the pointer. This pointer is spaced
Figure 2 is a sectional view of the rotor of the
from the adjacent cap II by a washer I6 and is
meter taken along the magnetic axis of the rotor;
clamped to the spindle against the washer i6 by
Figure 3 is a diagrammatical view showing the 40 a split locking washer I1. The pointer I3 moves
magnet ñux distribution in the ileld core when
across a calibrated scale I8 to indicate the values
the rotor is at the left extremity of its scale Y
range; and
Figure 4 is a view similar to Figure 3 but show- ,
of the quantity being measured. A restoring
force is applied to the rotor by a small helical
spring I9. This spring may be anchored at its
ing the magnet ñux distribution in the core when 45 outer end to any suitably stationary part of the
the rotor is at a mid-scale position.
meter, not shown, and is anchored at its inner end
The meter illustrative of my invention and
to a lug 20 that is clamped between the hub of
shown in the accompanying figures comprises a
>the pointer and the _locking washer I l.
set of laminations I assembled in stacked rela
Merely to simplify the description, the pointer
tion to form' a magnetic core 2. This is a sub
is here shown as being in line with the magnetic
stantially rectangular core terminating in a pair
axis of the magnet 9. Upon energization of the
of adjacent ileld poles 2' and 2". The field poles
coil l there will be set up a flux field between
have arcuate recesses in their adjacent sides
the poles of the magnetic core 2. This flux field
which are curved about a common center 3 there
between to form pole faces 4 and 4'. The poles
~ are thus separated by a central cylindrical air
will flow across from one pole to the other and
will have one or another of two possible direc
tions depending upon the direction of the cur
space 5 and by narrow diametrically opposite
rent in the field coil 1. In response to the flux
slits or gaps .5 at their adjacent peripheral edges.
ñeld set up by a current ilow in the coil l, the
A magnetic field flux is set up in this space by a
rotor will seek a position wherein its magnetic
coil 1 provided on a leg 2a of the core. At the 60 axis will be in line and additive with the field
ñux, with the north and south poles of the rotor
being respectively directly adjacent the south and
north poles of the core. In the present meter
the rotor is biased out of such seeking position
plus and minus by a given angle from the medial
plane M-M between the ñeld poles as is shown in
Figure 4. In the present meter the air-gap
reluctance is kept constant through such range
into a neutral position >by the restoring spring
i9 abovementioned, and is propelled toward that
position according to the value of current fed to
the coil l', the amount of deflection of the rotor
the rotor
the rotor
with shoes
that are `arcuate
long to ' f
be one wherein the pointer registers with any suit
the width of the air gap between each shoe lll of
span the slits 6 at all positions of the rotor within
that range. That this expedient will so hold the
air-gap reluctance constant is readily understood
from neutral position being a measure of the
value of that current. This neutral position may 10 for so long as the air slits t are so spanned
the rotor and the field poles-which is the dimen
sion of the gap along the periphery of the rotor
is chosen at an intermediate point of the scale,
will have the fixed value equal to the peripheral
the deflections in opposite directions from that
point will be in response to plus and minus values 15 length of the shoe less that of one of the slits â, `
lt being understood that the other dimensions
of current in the coil. In the present instance,
of each air gap are of course iixed in the present
however. the neutral position is considered as
structure. A prerequisite for a long linear scale
being at the left extremity of the scale, and all
for the meter is therefore that the rotor not only
readings of the meter are accordingly of the same
sign. While the present meter is considered as 20 have substantially equipotential surfaces subtend
ing a large angle at the pivot axis, but that the
- measuring the current in the ñeld coil ï, it will
spacing between the iieid poles at' the sides of
be understood that that coil current may be
the pole faces-_which is the peripheral length of
supplied as a function of another quantity and
the slits E--shall subtend relatively small angles
that the scale may then be calibrated directly
in terms of that quantity.
25 at that pivot axis. In practice, the separations
between the ends of the shoes i@ should not be
The desirable characteristics for a meter are
substantially less than the clearance between the
a long scale in angular degrees, a uniform scale
able point on the scale. When the neutral position Y
sometimes referred to as a linear scale, and a
shoes and pole faces, but the peripheral length
of the slits â may be very small, as small as will
meter, these desirable characteristics are attained 30 .there yet produce an air-gap reluctance which
is large relative to the reluctance of the ?leld core.
to an unusually high degree. This is accom
high sensitivity. In the present moving magnet
plis‘ned by providing the moving magnet with
pole surfaces which have substantially uniform
pole strength and which subtend large angles at
the rotor pivot axis, as is aforedescribed, and by
providing a field core having very small reluc
The criteria for a uniform scale distribution
within a given range are (1) that when the field
coil is not energized the rotor shall not be at
tracted magnetically to any one angular posi
tion in preference to another but shall, when not
mechanically biased, float freely' within that
range, and (2) the linkage of the magnet ilux
with the ileld coil shall vary linearly with angular
deflection of the rotor within that range. The
first criterion mentioned establishes that the
total flux of the rotor magnet shall not vary with
angular positioning oi’ the rotor; while, in prac
tice, some variation in total ñux is permitted, the 50
permissible variation becomes stringently small
for highly sensitive meters. Since the present
rotor construction has substantially equipoten
tial pole surfaces, the total magnet ñux will re
main fixed so long as the eiîective reluctance of
_ When a continuous field core made of a high
permeability material is used, these slits need be
only a few thousandths inch long. As typical
physical dimensions, however, the shoes i8 may
each have an angular length of 165°, with 15°
separations at the ends; and the air slits t may
each have a peripheral length suñlcient to sub
tend 15° at the pivot axis. For these values, the
shoes ill will span the air slits over a 150° range
of deiiection of the rotor, which is a range of
traversal of the rotor magnetic axis over 75° plus
and minus from the aforementioned medial plane
between the field poles. This, it will be under
stood, is the maximum range of the rotor, for the-
particular physical dimensions noted, in which
the air-gap reluctance -will remain substantially
When the rotoi~ is at the left extremity of the
scale just mentioned, substantially all of the mag
net ñux will take a return path through the en
tire length of the field core as is indicated in
Figure 3. As the rotor is moved clockwise from
the left extremity of the scale, more and more
of the magnet flux will become localized in the
ileld pole portions, all of the ñux becoming there
the return paths taken by the magnet flux in
going from one magnet pole to the other remains
substantially constant. For purposes of analysis,
‘ localized and none of it passing through the
`this 'effective reluctance may be lconsidered as
' mid-position relative to the scale, as is shown
length of the ñeld core when the rotor reaches a
comprising two components: a component repre 60 in Figure 4. For further clockwise rotation of
the rotor, more and more of the- magnet flux will
senting the reluctance of the air gap between the
magnet shoes I0 and the iield poles, and a com
ponent representing the reluctance which the
magnet flux encounters in the field core itself,
these two components being hereinafter referred
again ñow through the ñeld core so that sub- ’
stantially all 0f it will take a return path through
the length of the core when the rotor reaches the
right extremity of the scale, but the direction of y
flow of the tiux in the field core will be now re
to as the air-gap reluctance and the core> reluc
versed from that which it had when the rotor was
at the left extremity of the scale.
Before considering these reluctances it should
lIt is only the iiux iiowing through the length
be clearly borne in mind that the operative angu
lar range of the rotor is from a position wherein 70 of the ñeld core that links or threads the wind- i
ing of the ñeld coil l. Since in theipresent meter
"the magnet iiux is at least partially aligned with
the rotor has substantially equipotential po-le sur
and opposed to the ñeld iiux to a position wherein
faces and- spans the air slits 6 continuously within
the magnet ñux is again at least partially aligned
the 150° operative range above noted, the linkage
with b_ut aiding the ñeld flux. these being posi
tions wherein the rotor magnet axis is displaced 75 0f the magnet ñux with the field coil Within this
2,41 1,997
range will vary substantially linearly with angu
lar deflection of the rotor. The second criterion
above noted for uniform scale distribution is
therefore fulfilled. That such linear variation of
magnet-flux linkage with the field coil will pro
and tending thus to `damp the movement of the
duce a linear scale distribution will be under
metal may be understood by considering a typical .
stood by considering the torque developed by the
magnet flux in the core opposing the flux change
The improved sensitivity realized in using a
continuous ñeld core construction made of Mu
set of characteristics for a preferred construction
of my meter, and then considering the effect on
equation expressing this developed torque is _
the operating characteristics of the meter when
10 a field core is used having a joint therein form
ing a small air gap.
A typical set of physical and electric charac
Where T. is- the developed torque, K is a. constant,
teristics for the present meter may be approxi
N is the number of turns in the field coil, I is the
mately as follows: the Alnico magnet. S-may be
current in the field coil being measured, ¢ is the 15 .16" long x .187" thick x .032" Wide; the rotor
magnet flux threading the field-coll and 0 is the
shoes l0 may be .01" thick (in the direction radial
angular deflection of the rotor. When the flux `
of lthe rotor) by .187” Wide (in the directions axial
¢ varies linearly with angular deflection of the
of the rotor) ; the clearance between the shoes I0
rotor, as is above noted, the derivative of that
and the pole faces may be .03"; the cross-sec
flux with respect to the anglar deflection of the 20 tional area of the field core may be .5 sq. cm.
(.312" x .25") and the core may have a mean
magnetic path approximately 5 cm. long; and, as
for the electrical characteristics, the coil 'l may
have 1,000 turns, the magnet may have a total
is a constant, The torque thus becomes directly
of 180 lines, producing a maximum flux den
proportionate to the current I in the coil. Since
» sity in the core of 360 gauss, and the permeability
the restoring torque of the spring I9 is propor
of the Mumetal at this density may be 30,000.
tional to the angle 0, the rotor deflections become
In the meter above described, there is a torque
directly proportionate to the coil current to give
rotor in response to the field coil current.~ The
a uniform scale distribution.
of .54 g. mm, exerted on the rotor in response to
It is noted that the magnet flux threading the 30 only 2.9 ma. current in the ñeld coil l, this cur
rent producing 2.9 amp. turns for the present coil.
field coil reaches maximum values in opposite di
the present design, the power input into the
rections when the rotor is at the scale extremi
for 2.9 ma. current is approximately 50 mu w.
ties, and passes through a zero value when the
By proper choice of restoring spring I9 the torque
rotor is at the middle of the scale. rI'he greater
of .5 g. mm. is utilized to produce a full-scale
these maximum values become, the greater will
deflection of 150°. I find that the scale calibra
be the factor
tion over this range is well-nigh perfectly uni
> form.
_ The high sensitivity of the present meter
and the greater therefore will be the sensitivity 40 follows as a result of the very low loss which is
obtained in the field core. It has been noted
of the meter-_i. e., the torque developed by a
given field coil current. I have found that while
the physical dimensions of the roto-r and of the
air gap between the rotor and field poles are im
that a rotor deflection from one scale extremity
to mid-scale position is accompanied by a change
in magnet flux in the field coil to the extent of `
portant factors in determining the sensitivity of `
180 lines, which is the total magnet flux. Each
the meter, it is the absolute value of the reluc
change in magnet ñux through the field core '
tance of the field core which is a prime deter
mining factor. In the present-J meter I have em
with deflection represents a magnetomotive
force loss in the meter system equal to the mag
netomotive force drop which an amount of flux
small reluctance. This low value of reluctance 50 of the value of that change would have in the
field c_ore. This magnetomotive force loss 'must
is attained by the use of laminations l which are
be supplied by the field coil, and will for any
made of a material having a high permeability of
given meter be a fixed precentage of the amp. '
the order of 15,000 gauss per oersted or more.
turns provided by the field coil since the magnet
such as that known commercially as Mumetal,
flux change in the field core is in direct propor
and extended continuously through the full
tion with the deflection of the rotor. In the
length of the field core. Preferably in this field
present meter 1.45 amp.- turns in the field coil
core construction, I provide one field vpole as a
produces a half-scale deflection. The magneto
straight extension of the leg 2a of the core. This
motive force loss at half-scale deflection, which
permits the coil to be prewound by machine meth
ods and the laminations to be thereafter inserted 60 is that equal to the drop of 180 lines in the field
core, is found however to be only approximately
individually into the coil by first springing the
ployed a ñeld core construction having a very
leg thereof corresponding to the leg 2a of the core
to the side as each lamination is passed through
the coil.
For damping the movement of the rotor, a short
is provided around the core 2. This short may
.05 amp. turns for the present Mumetal core 2.l
The loss in amp. turns is therefore in the ratio
comprise a copper sleeve 22 surrounding the leg
2a aforementioned, and may conveniently form
or only about 3.5%.
a, base onto which the coil l may be wound, The
damping action of this sleeve follows as a result
of the change in value of magnet flux through the
core with change in angular positioning of the
magnet, for each such change in flux will induce
a current into the copper sleeve about the leg
2a ofthe core, and this current will produce a
Were the field core not continuous along its
length but provided with an air gap, the mini
mum effective length of that gap would be in
practice about .0025 cm. even were the lamina
tions interleavedv at that gap. The presence of
this gap would not materially change the value
of magnet flux in the field core since the length
of this gap is small relative to that of the air
gaps between the field poles and the rotor shoes
- I0, and we may therefore consider the total mag
net fiux in the core to be again 180 lines. For a
core having .5 sq. cm. cross-sectional area, this
ing capable of being fiexed to the side to per- .
mit the laminations to be inserted individually
into said coil in the assembling of said meter.
gap would have .005 c. g. s. unit of reluctance and
3. A sensitive moving magnet type of direct
current measuring instrument adapted to have
would for 180 lines of flux produce a magneto
motive force drop of .72 amp. turns
a uniform scale comprising a field core struc
ture having pole faces arcuate about a common
(.005 X 180
This ‘loss would have to be wholly overcome at
mid-scale deflection-_since there is a total mag
net fiux change in the core at that defiection
and thus to obtain a mid-scale defiection there
would be required approximately a 50% increase
in amp. turns. This means that each deflection
would require a 50% increase in amp. turns a's
a result of the .0025 air gap above presumed.
It will be understood that the present meter
may be readily adapted to higher or lower cur
rent measurements than that hereinbefore
noted. This adaptation is preferably made by
using fewer turns of larger wire or more turns
of finer wire in the field coil. If a multiple range
meter is desired, appropriate taps may be
brought out from the field coil. In all ranges,
however, there may be utilized the same power
of 50 microwatts input for a full-scale deflection.
Since in the meter of this invention I do not
need _to transmit the current under measure
ment through torsion (hair) springs nor be lim
ited to the delicacies of a moving coil, I may
wind the field coil with but one turn having for
center line therebetween and separated by air
gaps at their adjacent peripheral edges, a per
10 manent magnet structure pivoted at said center
line and provided with substantially equipoten
tial pole shoes of non-permanent magnetic ma
terial having faces uniformly spaced from said
field pole ffaces, the pole faces of said field and
magnet structures being wide so that the magnet
pole faces will bridge the peripheral gaps be
tween the field pole faces throughout a sub
stantial range and said field core structure be
ing continuous from pole face to pole face and
20 made of a low loss material having a perme
ability of the order of 15,000 or more, means
for pivotally biasing said magnet structure in
vone direction, and a field coil associated with
said core structure for setting up a magnetic
ñux between said field pole faces to produce a
torque influence on said pivoted magnetI struc
ture in the other direction.
4. In a meter responsive to continuous current:
the combination of a field core having a pair of
field poles with adjacent faces curved arcuately
about a common center line therebetween and
separated by air gaps at their adjacent peripheral
edges; a permanent-magnet rotor pivoted at said
center line and having diametrically opposite
example only a few millionths of one ohm re
sistance, or I may. wind the coil with many turns 35 pole shoes of non-permanent magnetic material
with arcuate peripheral surfaces of substantially
having as many as >15,000’ohms resistance. Of
uniform pole strength, each of said rotor poles
course, for intermediate current ranges, there
subtending a multiple times greater angle at said
may be used any desirable intermediate values
center line than is the angle subtended at said
of turns and resistance for the field coil.
The embodiment of my invention herein shown 40 center line by the peripheral gaps between said
field poles; means pivotally biasing said rotor in
and described is illustrative but not limitative of
one direction; and a coil associated with said field
my invention for the same is subject to changes
core energizable by current to be measured for
and modifications without departure from the
producing a magnetic field between said field
scope of my invention, which I endeavor to ex
press according to the following claims. '
45 poles to deflect said rotor in the other direction.
5.- In a meter responsive to continuous current
I claim:
comprising a field core having a pair of field poles
1. In a meter of the moving magnet type:
with adjacent faces curved arcuatelyabout a
the combination of a core of magnetic material
common center line therebetween and separated
continuous throughout the length thereof and
terminating in field poles having arcuate pole 50 by air gaps at their adjacent peripheral edges:
the combination of a permanent-magnet rotor
faces curved about a common center line there
pivoted at said center line and having diametri
between and separated by air gaps at their ad
cally opposite pole shoes of non-permanent mag
jacent peripheral edges; a coil associated with
netic material with peripheral surfaces of sub
said core for producing a magnetic field between
said ñeld poles; and a magnet' structure pivoted 55 stantially uniform pole strength, each of said
surfaces subtending an angle at said center line
which is the major part of a straight angle, said
rotor having an operative range wherein the pole
magnet structure having arcuate pole shoes ofj
surfaces thereof bridge the peripheral gaps be'
high permeable material confronting said pole
faces, said pole shoes having an angular length 60 tween said field poles;` means pivotally biasing
said rotor into a position within said range; and
suflicient to span the peripheral gaps between
a coil associated with said field core for receiving
said _field poles, throughout a substantial angu
current to be measured and producing a magnetic
lar range.
field between said field poles, said rotor being
2. In a meter including a pivoted permanent
subjected to a torque influence by said magnetic
magnet structure: the combination of lamina
. field which is substantially proportional to said
tions assembled in stacked relation and forming
current within said range.
a core of magnetic material continuous through
6. In a meter responsive to continuous current
out the length thereof, said core having leg por
comprising a, field structure having a pair of field
tions terminating in field poles, said poles hav->
ing arcuate pole faces cooperating with said 70 poles with adjacent faces curved arcuately about
a common center line therebetween and sepa
magnet structure, one of said poles being a
rated by air gaps at their adjacent peripheral
straight extension of one leg portion of' said
edges, and a coil associated with said field struc
core; and a field coil on said one leg portion for
producing a magnet field between said pole
ture and energizable by said current for produc
faces, the leg portions of said laminations be 75 ing a magnetic ñeld between said field poles, each
between said pole faces for response to changes
in the magnetic flux produced by said coil, said
of said poles subtending an angle at said center
line which is the major part of a straight angle:
a permanent-magnet rotor pivoted at said center
line and deflected angularly according to the
value of said current within a predetermined
range of movement of the rotor, said rotor com
prising a magnet symmetrically disposed relative
to said center line and having a width which is
narrow in comparison to that of said field poles,
and comprising a permanent magnet having ar
cuate pole shoes of highpermeable material at
the polar ends of the magnet and bridging the
peripheral gaps between the ñeld -poles through
out a range of pivotal movement of the rotor, said
rotor producing a magnetic iiux in said core
through said coil which varies substantially lin- '
early with angular deflection of the rotor through
said range, and means for producing a biasing
and further comprising pole shoes of non-perma
nent magnetic material at the pole ends of said
magnet and curved arcuately about said center
line, each of said shoes subtending an angle at
said center line which is the major part of a
straight angle and the surfaces of said shoes 15
torque on said rotor which varies substantially
linearly with reflection of the rotor through said
8. In a meter adapted to measure continuous
current according to a uniform scale: the com
bination of a field core having a pair of field poles
of non-permanent magnetic material having
faces spaced uniformly from said field pole faces,
9. In a meter of the moving-magnet type: the
combination of a field-core structure terminating
in a pair of field poles having concave pole faces
being substantially equipotential, said range be
at their adjacent sides and separated by air gaps
ing that wherein said pole shoes bridge the pe
at their edges; a coil associated with said core
ripheral gaps between said field poles, and said
structure for producing a magnetic field between
rotor being biased into a position Within said
said pole faces; and a pivoted magnet structure
20 between said pole faces and responsive to changes
7. In a meter responsive to continuous current:
in intensity of the magnetic iield produced by
the combination of a field structure comprising
said coil, said magnet structure comprising a
field poles having adjacent faces curved arcuately
permanent-magnet member magnetized trans
about a common center line therebetween and
versely to its pivot axis and curved non-perma
separated by air gaps at their adjacent periph
nent pole shoes associated with the pole end
eral edges; a permanent-magnet rotor pivoted at
faces of said magnet member and confronting
saidl center line and having opposite pole shoes of
the pole faces of said field core, said pole shoes
non-permanent magnetic material with substan
spanning said air gaps continuously within the
tially equipotential pole surfaces arcuately dis
operative range of said pivoted magnet structure.
posed about said center line, the peripheral spac 30
10. In a meter of the moving-magnet type: the
ing between said rotor poles being of the order of
combination of a core of magnetic material ter
the magnitude of the length of air gap between
minating in field poles, said ñeld' poles having
the rotor poles and said field poles, said rotor
pole faces curved arcuately about a common cen
having an operative range wherein the poles
ter line therebetween and separated by air gaps
thereof bridge the peripheral gaps between said
at their adjacent peripheral edges; a coil associ
field poles; means for biasing said rotor into a
ated with said core for producing a magnetic field
position within said range; and a coil, associated
between said pole faces; and a pivoted permanent
with said field structure and energizable by the ‘ magnet rotor between said pole faces character
current to be measured, for producing a magnetic
ized as being substantially without magnetic bias
field between said field poles to deñect the rotor 40 within a given deiiection range thereof, said rotor
out of its biased position.
comprising substantially equipotential pole shoes
said rotor pole faces bridging the peripheral gaps
with adjacent faces curved arcuately about a 45 between said field pole faces throughout said de
common center line therebetween, and a coil as
flection range.
sociated with said core for receiving current to be
measured; and a rotor pivoted at said center line
Без категории
Размер файла
880 Кб
Пожаловаться на содержимое документа