Improvements in Electric Measuring Instruments.
Year: 1893

page_text: Date of Application, 10th Jan., 1893
Complete Specification Left, 10th Nov., 1893—Accepted, 23rd Dec., 1893
PROVISIONAL SPECIFICATION.
Improvements in Electric Measuring Instruments.
We, WALTER THOMAS GOOLDEN, and SYDNEY EVERSHED, both of Woodfield
Works, Harrow Road, in the County of Middlesex, Electrical Engineers, do hereby
declare the nature of this invention to be as follows:-
This invention relates to instruments which serve to measure currents and
pressures, by means of electro magnetic forces, and its object is to eliminate, as far
as possible, the errors in the indications of such instruments caused by extraneous
magnetic fields.
We are aware that the movable system in telegraphic and electric instruments
have been made astatic, with the above object in view, but this arrangement
when applied to instruments like ammeters and volt meters is subject to many
objections.
We are also aware that in the case of galvanometers, used on board iron ships,
some protection is afforded to the instruments by a thick casing of. soft iron, which
is evidently a bulky and unsatisfactory arrangement as applied to ammeters and
volt meters and instruments of kindred types, with the object of protecting them
from extraneous magnetic fields, due to neighbouring dynamos or mains carrying
large currents.
But in carrying out our invention we form a shield or shields of soft iron,
external to the instrument to be protected, and in a position normal to the lines of
magnetic force, which it is desired to intercept, the connection between the parts
of the said shields, their section, material, and arrangement, being designed
especially to suit the direction, maximum strength, and conditions of the magnetic
field to be intercepted or diverted, and the part of the instrument to be protected,
as hereinafter more fully described.
Assuming the space to be protected lies within the working coil of the instru-
ment, as it does in nearly all kinds of ammeters and volt meters, we propose to
cover each of the external surfaces of the checks of the working coil bobbin with
a sheet or shield of soft iron, and connect these two sheets by one or more soft.
iron rods or bars, having their ends very carefully fitted into the sheets, in order
that the magnetic resistance of the joints in the shield may be negligible. The
whole of the magnetic induction due to an extraneous magnetic field, impinging on
one cheek of the bobbin, will then be carried to the other cheek, through the path
of iron so provided, by a far smaller magnetic potential-difference than that required
to carry the same induction through the air from cheek to cheek, and consequently
only a small fraction of the extraneous induction will traverse the air between the
cheeks, and the errors in the indications of the instrument will be reduced in like
proportion.
The sectional area of the shield must be sufficient to carry the extraneous
induction together with a large portion of the total induction, produced by the
working coil, (which naturally traverses the shield) and still leave the permeability
of the iron at as near its maximum value as possible.
In constructing such a shield as is here described, the greatest difficulty (and
uncertainty of sufficient protection) is in making the joints in the shield so
magnetically perfect, that their presence does not appreciably affect the function
of the shield; and we therefore propose, as a modification, to make shields without
any joints, as follows:-A piece of soft iron sheet is taken, whose width is equal
to the diameter of the working coil bobbin cheeks, and whose length is equal to
the length of the bobbin over all, plus twice the diameter of the bobbin cheeks.
This strip of iron is then bent at a right angle, at a distance from each end equal
.

page_text: to the diameter of the bobbin cheek, and slipped over the bobbin so that the bent
portions completely cover the cheeks, and the straight portion serves to carry the
induction from cheek to cheek. Several strips of iron, made in this manner may
be used on one bobbin if necessary or convenient.
When the variations in the strength of the extraneous magnetic fields are large,
the coercive force of the shield leads to a small permanent increase or decrease in
the indications of the instrument; and in order to eliminate this effect as far as
possible, we propose to separate the shield into two portions; one as described
closely fitting the bobbin, and the other made in a similar manner to the first, and
occupying a symmetrical position with respect to the bobbin, but so much larger as
to leave a clear air-space of about one tenth of an inch between the two shields.
The first shield will then serve to carry nearly all the induction produced by the.
working coil, leaving the second or outside shield to carry the extraneous induction
only. The effect mentioned above will then be confined practically to the outside
shield, and will not affect the indications of the instrument appreciably. This
principle of applying two separate shields may be extended, and in some cases,
where the extraneous fields are very strong, may be necessary to use more than
two shields.
Shields made according to our invention are equally applicable to instruments
used for direct currents and alternate currents, provided that in the latter case
proper precautions are taken to prevent eddy currents in the shields.
Dated this 10th day of January 1893.
FELL & WILDING,
1, Queen Victoria Street, London, E.C., Agents for the Applicants.
•
COMPLETE SPECIFICATION.
Improvements in Electric Measuring Instruments.
We, WALTER THOMAS GOOLDEN, and SYDNEY EVERSHED, both of Woodfield
Works, Harrow Road, in the County of Middlesex, Electrical Engineers, 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 instruments which serve to measure currents and
pressures, by means of electro magnetic forces, and its object is to eliminate as far
as possible the errors in the indications of such instruments, caused by extraneous
magnetic fields.
We are aware that the movable system in telegraphy and electric instruments
have been made astatic with the above object in view, but the arrangement when
applied to instruments like ammeters and volt-meters is subject to many objections.
We
are also aware that in the case of galvanometers used on board iron ships
some protection is afforded to the instruments by a thick casing of soft iron, which
is evidently a bulky and unsatisfactory arrangement as applied to ammeters and
volt-meters and instruments of kindred types, with the object of protecting them
from extraneous magnetic fields due to neighbouring dynamos or mains carrying
large currents.
But in carrying out our invention we form a shield or shields of soft iron,
external to the instrument to be protected, and in a position normal to the lines of
magnetic force, which it is desired to intercept, the connection between the parts of
the said shields, their section, materials, and arrangement being designed especially
to suit the direction, maximum strength, and conditions of the magnetic field to be
intercepted or diverted, and the part of the instrument to be protected, as hereinafter
more fully described.
Assuming the space to be protected lies within the working coil of the
instrument, as it does in nearly all kinds of ammeters and volt meters, we propose

page_text: to cover each of the external surfaces of the cheeks of the working coil bobbin
with a sheet or shield plate of soft iron and connect these two shield-plates
by one or more longitudinal pieces of soft iron in the form of rods or bars, having
their ends very carefully fitted to the sheets, in order, that the magnetic resistance
of the joints in the shield, may be negligible.
The magnetic induction due to an extraneous magnetic field, impinging on one
cheek of the bobbin, will then be carried to the other cheek, through the path of
iron, so provided by a far smaller magnetic potential difference than that required
to carry the same induction through the air from cheek to cheek, and consequently
only a small fraction of the extraneous induction will traverse the air between the
cheeks and the errors in the indications of the instrument will be reduced in like
proportion.
The sectional area of the longitudinal portions of the shield must be sufficient to
carry the extraneous induction together with a large portion of the total induction,
produced by the working coil (which naturally traverses the shield), and still leave
the permeability of the iron at, as near its maximum value as possible.
In constructing such a shield as is here described, the greatest difficulty (and
uncertainty of sufficient protection) is in making the joints between the longitudinal
pieces of iron and the shield plates so magnetically perfect, that their presence does
not appreciably affect the function of the shield; and we therefore propose, as a
modification, to make shields without any joints as follows:-A piece of soft iron
sheet is taken, whose width is equal to the diameter of the working coil bobbin
checks, and whose length is equal to the length of the bobbin over all, plus twice
the diameter of the bobbin cheeks.
This strip of iron is then bent at a right angle, at a distance from each end equal
to the diameter of the bobbin cheek, so that the bent portions completely cover the
cheeks, and the straight or longitudinal portion serves to carry the induction from
cheek to cheek. Several strips of iron, made in this manner, may be used on one
bobbin if necessary or convenient.
Shields made according to our invention are equally applicable to instruments
used for direct currents and alternate currents provided that in the latter proper
precautions are taken to prevent eddy currents in the shields.
Several ways of applying our invention to an ammeter or voltmeter are shown in
the drawings accompanying this Specification.
Figures 1 and 2 are respectively an axial section and end elevation of the working
parts of a measuring instrument.
The working-coil W and internal mechanism P are those described in the
Specification of Patent No. 1628 of 1887, but our present invention is equally
applicable to any other measuring instrument of kindred type.
Two shield plates of soft iron sheet S S serve to receive all extraneous magnetic
induction which would otherwise impinge on the bobbin cheeks of the coil W; and
the greater portion of such induction in passing from one shield plate to the other
traverses the soft iron rods R. R. In order to make the magnetic resistance of the
longitudinal path, so traversed, as small as possible, the rods R R have their ends
cut truly square and flat, so that when screwed to the shield plates S S a perfect
joint is made. The size and number of rods used vary with the amount of error to
be eliminated.
Methods of carrying out our invention by means of a shield of iron without joints,
with the object of completely eliminating the difficulties met with in making
magnetically perfect joints are shown in Figures 3, 4, 5 and 6. In Figures 3 and 4
a piece of soft iron sheet S R. S is shown bent over the ends of the coil W so that
the end portions of the iron sheet serve the purpose of shield plates, and the central
or longitudinal portion R has the same function as the iron rods R R in Figures 1
and 2. A hole is made in one of the end portions of the sheet to admit the internal
mechanism of the instrument P.
In making a shield similar to that shown in Figures 3 and 4 it is sometimes
onvenient to use a sufficiently thick piece of sheet iron to furnish the requisite

page_text: sectional area in the longitudinal piece R. In such cases we form the shield of two
or more separate pieces of sheet iron as illustrated in Figures 5 and 6 which are end
elevations of a coil W protected by a shield formed respectively of two and four
pieces of sheet iron. In this way the sectional area of R is doubled in Figure 5
and quadrupled in Figure 6 while still using the same thickness of sheet iron as in
Figures 3 and 4.
The actual dimensions of the parts of a magnetic shield depend on the strength
of the magnetic field due to the coil W and the magnitude of the error to be
eliminated. A rough rule is that the total sectional area of the longitudinal
pieces R R should not be less than one fifteenth the whole superficial area of the
external face of the shield plates S at one end of the coil. The shield plates should
be from one fiftieth to one twenty fifth of an inch in thickness. In order that the
shielding may be effective the iron parts of the internal mechanism of the instrument
must not occupy more than about one half the whole length between the shield
plates and when as in Figures 3, 4, 5 and 6 there is a hole in one of the end plates
the iron parts of the internal mechanism must be arranged as far from the hole as
possible.
Having now particularly described and ascertained the nature of this inven-
tion and in what manner it is to be performed, we declare that what we claim is
1st. In an electric measuring instrument a magnetic shield having soft iron shield
plates adapted to receive extraneous magnetic induction, and one or more longi-
tudinal pieces of soft iron of sufficient area and adapted to carry the said induction
from one shield plate to the other; for the purpose set forth and substantially as
described.
2nd. In an electric measuring instrument a magnetic shield having soft iron shield-
plates magnetically connected by means of one or more soft iron rods, substantially
as described with reference to Figures 1 and 2.
3rd. In an electric measuring instrument a jointless magnetic shield of bent soft
iron sheet for the purposes set forth and substantially as described with reference
to Figures 3, 4, 5 and 6.
Dated this 9th day of November 1893.
FELL & WILDING,
1, Queen Victoria Street, London, E.C., Agents for the Applicant.