Improvements in or connected with Electric Measuring Instruments.
Year: 1898

page_text: PROVISIONAL SPECIFICATION.
Improvements in or connected with Electric Measuring Instruments.
We, ALEXANDER BEWICKE BLACKBURN, Manager of the Electric Construction
Company Limited, and WILFRID L. SPENCE, Civil Engineer, of Dunluce, Tetten-
hall Road, both of Wolverhampton, in the County of Stafford, do hereby declare
the nature of this invention to be as follows:-
This invention relates to improvements in or connected with electric meters
of the well known d'Arsonval type and has for its principal object to provide a
meter of great sensitiveness and permanence which is convenient of application
and capable of being produced at a reasonably small cost, and the invention is
more particularly directed to the magnetic field and moving coil structure, to
the magnetic shielding and to the terminals of the meter.
Sensitiveness is secured by having a strong field, permanence by making the
circuit as nearly closed as possible, and with these objects in view we have devised
an arrangement of magnetic field, by suitably shaped pole pieces which not
only has a lower reluctance in relation to the magneto-motive force producing
the field than has hitherto been reached in any commercial instruments known
to us, but also gives an almost absolutely uniform density of lines through an
exceptionally wide range.
The controlling field according to this invention may be produced in any known
manner either by a permanent inagnet of suitable shape or by an electro magnet
designed and excited to give a constant or nearly constant field. Generally how-
ever, we have found a permanent magnet rectangular in section and bent to
approximately semicircular shape, but of somewhat shorter length, with or with-
out specially formed polar ends, very convenient. Pole pieces, or extensions of
wrought iron or other material of high permeability, also semi-circular in form
and of rectangular section adequate to carry the lines without being saturated
are fitted to the ends of the permanent or electro magnet. These pole pieces or
extensions art of equal total section but one is solid and the other is divided into
two parts or limbs the former extension starts from, say, the N pole and extends
round to within a short distance of the opposite or the S pole, while the latter
starting from say the S pole passes round with a limb on each side of and closely
enveloping the N pole piece or extension to within a short distance of the N pole.
The pole pieces being fixed end-on to the permanent magnet the combined structure
thus formis a circular (cylindrical) magnetic circuit with an air gap covering an
are of somewhat less than 180 degrees and of breadth equal to the width of the
pole pieces while the length may be anything from say, 004 inch upwards, from
0:05 to 0065 inch b ing a convenient workable length or clearance. This short
air ga of large area greatly conduces to permanency.
The length of the pole pieces, or the extent to which they overlap, which governs
the working deflection of the meter, is limited by considerations of magnetic
leakage direct from the end of one polar extension to the opposite pole face.

page_text: It can be reduced by properly thinning down the pole pieces from one end to the
other to correspond with the actual flux carried at each point, or more simply
by V notching the ends. With a scale deflection of 90 degrees there is no
difficulty in arranging the extent of overlap to exceed this by 30 degrees or more
at each end so that the slight concentration of lines at the extremities of the pole
pieces will have no appreciable effect on the uniformity of field strength in the
useful part of the gap.
In practice we find it convenient to make the pole pieces out of rings turned in
the lathe, each ring being halved by two saw cuts parallel to, but on opposite
sides of, a diameter.
The moving coil is pivoted on an axis at or near the centre of the magnetic field
system and the coil, close.y surrounding the central solid pole piece, swings in
the uniform field between it and the two outer ones. It can readily be removed
for inspection or repair without disturbing the magnetic circuit if the polar
extensions be not too long the space left to minimise leakage, as already set forth,
being sufficient for this purpose. The coil is wound on a metallic bobbin to damp
the oscillations and is carried at one end of two light levers to which at the
other end is attached a balance weight of copper. The balance weight may be
arranged to swing close to but clear of the semi-circular magnet, acting as an
additional damper. The axis of the moving coil system may consist of a turned
spindle or ivory or like insulating material having flanges to which the levers
are attached. Metallic pivots are driven into both ends of the spindle and over
each the metal collett of a non-magnetic spiral spring is forced, the springs serving
both to carry the current and to control the zero. By the use of two opposing
springs one stronger than the other, or one having a different co-efficient of
expansion from the other, they may be made to compensate to a certain extent
for variations of temperature and resistance in the circuits of the meter, a similar
effect being produced by arranging two like springs to act together instead of
in opposition.
The index finger may either be attached direct to the levers passing outside of
the magnetic field or it may be carried from the coil itself.
The bearing jewels for the pivots may be carried in bridge pieces of non-
magnetic metal which, together with the scale plate of the meter, may be held
by the set screws which are used to secure the three pole picces in fixed relation
to each other.
According to our invention we arrange the case of the meter to shield in some
degree the active parts. The base and front of the meter are preferably separate
castings of magnetic material while the outer shell of the body is a copper or
brass spinning suitably ribbed or embossed, inside of which is slipped or sprung
a shell or tube of iron to complete the magnetic circuit between the front and
base of the meter.
The terminals for back connection are constructed as follows:-A short porcelain
collar is slipped into the hole provided in the base for the terminal, over the
collar is slipped, also from the inside, a long metal nut having a turned down
portion roughened or screwed on the outside which enters the hole in the collar
and projects through it so as to be flush with the underside of the meter base.
The two pieces are held in position and permanently secured by an insulating
cement poured in between the sides of the hole and the roughened portion of the
nut afore-mentioned.
nut.
A plain screw run in from the back suffices to make connection with the long
The same method is applicable for insulating the fixing screws by which the
meter is held on a switchboard.
Dated thi. 9043. 1
c T
1000
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.

page_text: Improvements in or connected with Electric Measuring Instruments
We, ALEXANDER BEWICKE BLACKBURN, Manager of the Electric Construction
Company Limited, and WILFRID L. SPENCE, Civil Engineer, of Dunluce, Tetten-
hall Road, both of Wolverhampton, in the County of Stafford, 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 improvements in or connected with electric measuring
instruments, or meters, of the well known d'Arsonval type; and has for its
principal object to provide a meter of great sensitiveness and permanence, which
is convenient of application and which is capable of being produced at a reason-
ably small cost. The improvements are more particularly directed to the magnetic
field and moving coil structure.
Sensitiveness is obtained by having a strong field, and permanence is obtained
by making the circuit as nearly closed as possible, and with these objects in
view we have devised an arrangement of magnetic field with suitably shaped pole
pieces, which not only has a lower reluctance, in relation to the magneto motive
force producing the field, than has been reached in any hitherto known instrument,
but which also gives a uniform, or practically uniform, density of magnetic lines
of force through an exceptionally wide range.
Semi-
The controlling field in the arrangement according to this invention may be
produced in any known manner, either by a permanent magnet of suitable shape,
or by an electro-magnet, designed and excited to give a constant or nearly constant
field. Generally, however we have found a permanent magnet, rectangular in
cross section, and of approximately semi-circular shape, but of somewhat shorter
length, with, or without specially formed polar ends, very convenient.
circular pole pieces, or extensions of a material of high magnetic permeability
such as wrought iron and preferably of rectangular section adequate to carry the
magnetic lines of force without being saturated are secured end-on to the ends
of the said permanent magnet or electro magnet. These pole pieces, or extensions,
are of equal total section, but one is single, and the other consists of two parts,
or limbs; the single polar extension starts from one of the polar ends of the
magnet, and extends round to within a short distance of the opposite polar end,
whilst the two limbs, constituting the other polar extension, start from the said
opposite polar end and pass round to within a short distance of the first named
polar end, one such limb being situated on each side of the single polar extension
leaving an air gap between adjacent sides of the polar extensions. The said
polar extensions and the magnet, thus form an annular magnetic circuit with a
double air gap covering an arc of somewhat less than 180 degrees, and of a
breadth equal to the width of the polar extensions; while the length (distance
across) may be anything from say 004 inch upwards, from 0.05 to 0.065 inch
being a convenient workable length, or clearance. This short air gap of large
area greatly conduces to permanency.
The length of the polar extensions, or the extent to which they overlap, which
governs the working deflection of the meter, is limited by considerations of
magnetic leakage direct from the end of one polar extension to the opposite pole
face. It can be reduced by properly thinning down the polar extensions from one
end to the other, to correspond with the actual flux carried at each point, or
more simply by V notching the ends. With a scale deflection of 90 degrees
there is no difficulty in arranging the extent of overlap to exceed this by
30 degrees or more, at each end, so that the slight concentration of lines, at

page_text: the extremities of the polar extensions, will have no appreciable effect on the
uniformity of field strength in the useful part of the gap.
In practice we find it convenient to make the double pole pieces, or polar
extensions, out of rings turned in the lathe, each ring being halved by two
saw cuts made parellel to, but on opposite sides of a diameter.
The moving coil is pivoted on an axis at, or near the centre of the magnetic
field system, and the coil closely surrounding the central solid polar extension
swings in the uniform field between it and the two outer polar extensions. It
can readily be removed for inspection, or repair, without disturbing the magnetic
circuit, if the polar extensions be not too long, the space left to minimise leakage,
as already set forth, being sufficient for this purpose. The coil is wound on a
metallic bobbin to damp the oscillations, and is carried at one end of two light
levers to which at the other end, is attached a balance weight, preferably of
copper, which may be arranged to swing close to, but clear of the semi-circular
magnet acting as an additional damper. The axis of the moving coil system
preferably consists of a turned spindle, or bobbin, of ivory or other insulating
material, having flanges to which the aforesaid levers are attached. Metallic
pivots are driven into both ends of the axis of this spindle or bobbin; and over
each pivot the metal collett of a non-magnetic spiral spring is forced, the springs
serving both to carry the current, and to control the zero position of the coil.
By using two opposing springs one stronger than the other, or one having a
different co-efficient of expansion from the other, they may be made to com-
pensate, to a certain extent, for variations of temperature and resistance in the
circuits of the meter, a similar effect being produced by arranging two like springs
to act together instead of in opposition.
The index finger may either be attached directly to one of the levers or it
may be carried from the coil itself.
The bearing jewels for the pivots may be carried by bridge pieces of non-
magnetic metal which together with the scale plate of the meter, may be secured
to the magnet by set screws, which may also serve to secure the three pole pieces
in fixed relation to each other.
We will now describe an arrangement according to this invention with reference
to the accompanying drawings premising that we do not limit ourselves to
the precise details illustrated. Figure 1 shews in elevation with the
cover removed, and Figure 2 in transverse section, the general arrangement of
magnetic circuit and moving coil according to this invention. Figure 3 is a
perspective view shewing separately the arrangement of the magnetic field and
Figure 4 shews the way of making the polar extensions. The permanent magnet A
is conveniently, as shewn approximately semi-circular with extended pole
ends a a to facilitate the screwing thereto of the soft iron polar extensions N
and S. The N polar extension is divided into two equal portions lying on
cither side of, and at a short distance from, the single S polar extension, thus
forming with it a double air gap in which the moving coil works as shewn in
Figure 2. The parts of the N polar extension are formed out of a turned ring
divided into two by saw cuts parallel to, but on opposite sides of, a diameter
as shown in Figure 4. Each of the pieces so made is applied to the same pole
end of the permanent magnet A.
from the half of a turned ring and is applied to the pole end of the magnet
The S polar extension is similarly formed
opposite to that to which the N polar extensions are applied. The combined
structure, of the magnet and the three polar extensions, forms an annular magnetic
field system with a very short duplex air gap of large area, thus ensuring
conditions which tend to permanence.
The field existing between the polar
extensions is found, with the proportions shewn, to be almost absolutely uniform
throughout a working range of 90°. Leakage direct between either pole and
the opposite polar extension or extensions, is not considerable, and it is reduced
by V notching the unattached ends of the polar extensions as shewn in Figures 2
and 3.

page_text: The moving coil C is supported on an axis formed of a central flanged bobbin B
of ivory, or other insulating material, which carries two light levers L to one
end of which the coil C is attached, and at the other end of which is attached
a corresponding balance weight W.
Steel pivots P are driven into both ends of an axial hole through the bobbin B,
and over each of the said pivots P the metal collett s of a non-magnetic spiral
metal springs is forced. These springs serve both to carry the current to the
coil and to control its position to the zero point of the scale.
The bearing jewels for the pivots are carried by non-magnetic bridge pieces D
which together with the scale plate of the meter, are fixed by the screws t serving
to hold the polar extensions in fixed relation to each other. The jewels are
mounted so as to insulate the pivots which would otherwise be short-circuited
through the bridge pieces and pole pieces.
We arrange the case of the meter to shield, in some degree, the active parts.
The base E, and front F, of the meter are preferably separate castings of magnetio
material while the side or outer shell G of the body is preferably spun from
copper, or brass, and is suitably ribbed, or embossed as at g, a shell, or tube
of iron g being slipped, or sprung, inside of it to complete the magnetic circuit
between the front and base of the meter.
The terminals for back connection are constructed as follows: A short
porcelain collar is slipped into the hole provided in the base for the terminal,
over the collar is slipped, also from the inside, a long metal nut having a turned
down portion roughened, or screwed, on the outside, which enters the hole in the
collar and projects through it so as to be flush with the underside of the meter
base. The two pieces are held in position and permanently secured by an
insulating cement poured in between the sides of the hole and the roughened
portion of the nut aforementioned.
A plain screw run in from the back suffices to make connection with the long
nut.
The same method is applicable for insulating the fixing screws by which the
meter is held on a switchboard.
We do not claim broadly a moving coil threaded over a curved pole, as we
are aware that this is not broadly new, and we are also aware of the Specification
of Letters Patent No. 17577 of 1896, and of the Specification of the American
Patent No. 579344 dated March 23rd 1897, and we make no claim, under this
application, to what is described therein.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed, we declare that what we
claim is:--
1. In electric measuring instruments, an annular magnetic field system com-
prising a semi-circular, or approximately semi-circular, magnet carrying semi-
circular, or approximately semi-circular, overlapping polar extensions disposed
so as to form a twin air-gap of low reluctance, substantially as, and for the
purposes, hereinbefore described and illustrated by the accompanying drawings.
2. In, or for, electric measuring instruments, polar extension pieces formed
from a turned ring by saw cuts parallel to, but on opposite sides of, a diameter;
substantially as hereinbefore described and illustrated in Figure 3 of the accom-
panying drawings.
3. In electric measuring instruments of the moving coil type herein referred
o, triple polar extensions extending through 180° or more, side by side so as to
give a strictly uniform field throughout a working range of 90° or thereabout:
all substantially as hereinbefore described and illustrated in the accompanying
Irawings.
4. In electric measuring instruments of the type hereinbefore referred to,
he construction of the moving coil axis with a central insulating double flanged
obbin, having inserted in its axis, two metallic pivots, insulated from each

page_text: other, and each carrying the metal collet of a spiral spring forming part of the
circuit of the meter, and insulated by the bearing jewels arrangement; all
substantially as hereinbefore described and illustrated in the accompanying
drawings.
5. An electric measuring instrument constructed substantially as hereinbefore 5
described and illustrated in the accompanying drawings.
Dated this 11th day of February 1899.
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.