Improvements in Instruments for Measuring Continuous Currents.
Year: 1897

page_text: [Communicated from abroad by OLIVER BLACKBURN SHALLENBERGER, of Rochester,
in the County of Beaver, and State of Pennsylvania, one of the United States
of North America, Electrical Engineer.]
Improvements in Instruments for Measuring Continuous Currents.
1. WILLIAM PHILLIPS THOMPSON, F.C.S., M.I.M.E., of the Agency for foreign
Patent Solicitors, 322, High Holborn, in the County of Middlesex, 6, Lord Street,
Liverpool, and 6, Bank Street, Manchester, both in the County of Lancaster, and
Lower Temple Street, Birmingham, in the County of Warwick, Civil Engineer,
do hereby declare the nature of this invention to be as follows:-
This invention relates to electrical measuring instruments, and has for its
object to provide an improved device for measuring continuous or direct electric
currents.
The improved measuring instrument is a galvanometer of what is known as the
moving coil type in which the current to be measured passes through a coil of
wire suspended in a magnetic field, the movement of said coil under the influence
of the current being restrained by suitable means.
According to this invention the magnetic field is produced between annular pole
pieces, the moving coil being threaded upon one of said pole pieces and mounted
upon a spindle passing through the centre of the field parallel to the lines of force.
A shield or guard ring of magnetic material is also preferably provided in order
to prevent stray lines of force from fields external to the instrument interfering
with the field of the permanent magnet. Said shield ring takes the form of a
plate bent round so as to almost completely surround the pole pieces, and is pre-
ferably mounted so that it can be adjusted by screws for a purpose hereinafter
mentioned.
In one instrument which embodies the invention two permanent magnets are
provided which are bent to a substantially semi-circular shape. Said magnets aro
placed opposite each other in the same plane, the south pole of one magnet being
adjacent to the south pole of the other magnet, so that practically a ring with two
consequent poles is formed. To each pole a block of magnetic material is secured
on which is mounted an annular pole piece. Said pole pieces project one over the
other the annuli being coaxial, and the faces being parallel. By this arrange-
ment a concentrated and uniform distribution of lines of force between the poles
is attained. To permit of the moving coil being threaded upon the pole piece
said pole piece may be formed in two parts each of which parts consist of a straight
portion with a semi-annulus at the end, or the annulus may be provided with a gap
at one portion through which the coil can be introduced, said gap being filled by
a plug of magnetic material if desired. In order to damp the vibrations of the
instrument and make it dead-beat a damping coil consisting of a single turn of
stout copper for example is preferably provided encircling the second pole piece.
Such coil may be mounted on the same spindle as the galvanometer coil, and is

page_text: preferably placed diametrically opposite the galvanometer coil so that the parts
will be balanced.
In some cases two coils may be provided instead of a single coil and damper and
the bobbins or supporting shells of coils may then be made of good conducting
material, and of sufficient size to act as damping coils.
For finally adjusting the distribution of the magnetism between the poles the
magnetic shield ring is preferably employed. In the absence of such a ring the
adjustment may be effected to a very considerable extent by cutting one or other
of the poles on its inner face, or by throwing them slightly out of parallelism.
The restraining force may be constituted by two flat spiral springs secured at
their inner ends to the spindle, and at their outer ends to fixed stops on the frame,
the said springs if insulated from. the spindle afford a ready means for conveying
the current to the moving coil.
Suitable means such as a pointer and scale for example are provided for indi-
cating the movement of the coil.
Til
Lot 1007
1
W. P. THOMPSON & Co.,
Patent Agents for the Applicant, 322, High Holborn, London, W.C.
COMPLETE SPECIFICATION.
Improvements in Instruments for Measuring Continuous Currents.
I WILLIAM PHILLIPS THOMPSON, F.C.S., M.I.M.E., of the Agency for foreign
Patent Solicitors, 322, High Holborn, in the County of Middlesex, 6, Lord Street,
Liverpool, and 6, Bank Street, Manchester, both in the County of Lancaster, and
Lower Temple Street, Birmingham, in the County of Warwick, Civil Engineer,
do hereby declare the nature of this invention and in what manner the same is to
be performed to be particularly described and ascertained in and by the following
statement:-
This invention relates to electrical measuring instruments and has for its object
to provide an improved device for measuring continuous or direct electric currents.
My improved measuring instrument is a galvanometer of what is known as the
moving coil type in which the current to be measured passes through a coil of wire
suspended in a magnetic field, the movement of said coil under the influence of the
current being restrained by suitable means.
According to my invention I provide a permanent magnet having ring shaped
co-axial pole pieces, the moving coil being secured to a spindle passing through
the centre of the poles. The plane of said coil is perpendicular to the plane of the
opposed polar faces of the magnet, the coil surrounding or being threaded upon
one of the ring poles. I further provide what I term a shield ring of magnetic
material which almost completely surrounds the two annular pole pieces, the
object being to prevent stray lines of force from fields external to the instrument
interfering with the field of the permanent magnet. Said shield ring is also
utilized for the purpose of finally adjusting the distribution of the field between
the magnet poles.
In order that my invention may be clearly understood I have illustrated a
practical embodiment of the same in the accompanying drawings in which
Figure 1 is a side elevation partly in section, and
Figure 2 is a plan with portions of the instrument removed.
Figure 3 is a detailed perspective view of one of the pole pieces and Figure 4 a
similar view of a pair of pole pieces of modified construction.

page_text: Referring now to Figures 1 and 2, NS and N¹ S₁ are permanent magnets having
their like poles rigidly secured to each other through interposed blocks of magnetic
material K R. Said blocks are secured to a suitable bed plate W of non-magnetic
material. The block R is provided with a polar extension which as will be
seen by reference to Figure 3 consists of two straight bars each terminating in
a semi-annulus so that when said bars are placed together they form in effect a
single bar terminating in a complete ring P. The block R is provided with a
similar polar extension constructed of two pieces q¹ q2 forming a complete ring Q.
The parts being arranged as shewn in Figure 1 it will be seen that a concentrated
magnetic field will be formed in the space between the rings P and Q, the lines
of force being perpendicular to the opposing faces of the rings, and distributed
in a substantially uniform manner throughout the field. M is what I call a shield
ring inounted as shewn so as to almost completely surround the pole pieces P' and Q.
Said ring is cut away as shewn in Figure 2, and is sufficiently separated from the mag-
net at other points so as not to short-circuit the magnetic field to an injurious extent.
It is supported in place by the three adjusting screws t¹ 2 and 3 by means of which
its position with reference to the pole pieces may be varied in order to correct any
lack of uniformity in the distribution of the field between the said pole pieces.
A is a coil of wire, surrounding or threaded upon the ring pole Q as shown in
Figure 1, and free to move circumferentially upon it. It is mounted upon a
light shell of aluminium or other suitable material which is attached by lugs to a
metallic support H carried by a shaft J. This shaft is delicately mounted in jewel
bearings as is usual in the construction of such instruments. B is a damper
consisting preferably of a solid ring of metal of high conductivity, surrounding or
threaded upon the ring pole P as shewn in Figure 1. It is attached to the support-
ing piece H, which is prolonged at right angles to the shaft for the purpose, and
is placed diametrically opposite the coil A.
D is a flat spiral spring, made preferably of phosphor bronze attached at its
inner end to the shaft J through an insulated collet d and at its outer end to the
post F, which is insulated from the base W. C is a similar spring attached to the
shaft J by means of the collet e, which may be insulated or not as desired, and to
the circumferentially-adjustable arm E which may also be insulated if desired.
Said springs restrain the movement of the shaft J and the coil and cause the
pointer to assume an approximately fixed zero position when no current is flowing.
One terminal of the coil A is electrically connected through the conductor 3 and
the spring C to the conductor I which leads to one of the binding posts (not shewn)
of the instrument, and the other terminal of the coil is connected through the con-
ductor 4 and the spring D to the conductor 2 which leads to the other binding
post, (not shewn).
I is a pointer, preferably made of aluminium, which is attached to the support-
ing piece H, which is carried up and bent at right angles for the purpose. A
suitable scale, not shewn in the drawings, is provided for indicating the deflections
of the pointer, as is usual in the construction of such instruments.
Owing to the uniform distribution of field between the pole pieces in my
improved instrument, the proportionality between the current passing through the
coil and the angular deflection is maintained through a very considerable range;
I have succeeded in obtaining in practice a range of over 270°. It is not neces-
sary that the points of attachment of the annular pole pieces to the magnet poles
proper should be diametrically opposite if a uniform field be otherwise obtained;
the arrangement shewn is, however preferable because the damping coil can be
mounted upon the shaft J diametrically to the coil A, and the two coils may be
made to balance. The construction of the annular pole pieces in two parts one
of which may be detached is for the purpose of permitting the coil and damper to.
be threaded upon them in setting up the instrument; the construction may, how-
ever, be varied, for example as shewn in Figure 4, the pole pieces consist of single
straight parts p and q having ring extensions P and Q each of which is made
with a gap as shewn at O to permit the coil or damper to be placed in position.

page_text: Properly shaped pieces of magnetic material may be inserted in the gaps after
the coil and damper have been put in place.
.
The magnetic shield M is not absolutely essential although I prefer to use it
owing to the facility it affords for adjusting the instrument. When the shield is
dispensed with any lack of uniformity in the field may be corrected to a very
considerable extent by cutting one or other of the annular poles on its end face,
or by throwing them slightly out of parallelism.
I sometimes employ a single magnet instead of two as shewn in the drawings.
For some uses two coils are provided one of which occupies the position of the
damper B in which case the supports or bobbins on which the coils are wound
are made of highly conducting material, and of sufficient size to serve as a damper.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, as communicated to me by
my foreign correspondent, I declare that what I claim is :-
1. For measuring continuous currents a galvanometer having a movable coil and
a magnetic field formed by a permanent magnet with substantially co-axial annular
pole pieces on one of which the coil is threaded, and with or without a shielding
sleeve of magnetic material surrounding the magnet poles substantially as
described.
2. A current meter constructed substantially as described with reference to
Figures 1 and 2 of the accompanying drawings either with or without the separate
damping coil B, substantially as described.
3. The modification of the invention wherein the magnet poles are made with a
gap as described with reference to Figure 4 of the drawings.
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C., Patent Agents for the Applicant.