Improvements in Electric Measuring Instruments.
Year: 1893

page_text: Date of Application, 2nd' Nov.,' 1893
Complete Specification Left, 2nd Aug., 1894-Accepted, 8th Sept., 1894
PROVISIONAL SPECIFICATION.
Improvements in Electric Measuring Instruments.
We, WALTER THOMAS GOOLDEN, and SYDNEY EVERSHED both of Woodfield.
Works, Harrow Road, London, Electrical Engineers, do hereby declare the nature
of this invention to be as follows:-
This invention relates to voltmeters and ammeters of the electro magnetic 'class'
intended for the measurement of electric pressures and currents; and its objecte
are to enable any desired' scale and range of readings to be obtained by a few
simple adjustments of the working mechanism; and to make such instruments as
nearly as possible astatic so that their indications are independent of extraneous
magnetic forces.
The first object we propose to effect by an improved working mechanism,
consisting of two concentric semi-cylinders of soft iron sheet, (herein called' cores)
placed within the working coil of the instrument and with their common axis along
that of the coil. One of the cores/is.fixed, while the other, which is of slightly
smaller diameter is attached to an iarbor. lying along, the axis of the working coil
and is therefore free to rotate round the said axis, within the..fixed.core. When a.
current flows in the working coil the two cores become magnetised and tend; to
place themselves so that they form, a nearly complete cylinder; in other words,
the moveable core is repelled by the fixed one and will rotate until the repulsion is
balanced by a weight or spring attached to the arbor in the usual manner. The
scale obtained will depend on the zero position of the moveable core with respect
to the fixed one. If they are exactly symmetrically placed on the same side of the
axis, the repulsive force will have no component tending to cause rotation; the
moveable core must therefore be displaced so that at zero its leading edge may be
outside the corresponding edge, of the fixed core. A very small displacement at
zero will give a very open scale over a short range of current values, while a large
displacement at zero will tend to produce a scale covering a long range.
The ends of the cores may either..be square with the arbor or one or both may
be cut slanting so as to produce an open or close scale as desired. With any.given
pair of cores the only adjustments. required in order to obtain the desired range and
scale are first to place the fixed core so that at zero the moveable core has sufficient
lead to produce the requisite relation between current and deflection, and second,
to alter the weight or the strength of the spring or other controlling force until the
maximum deflection corresponds with the maximum current the instrument is, to
indicate. The framework carrying the working mechanism may be a thin brass
tube, having a flange at one end adapted to be screwed to the dial or coil cheek.of
the instrument; the arbor may then be pivotted at each end of and concentrically
with the tube so that the moveable core rotates inside the tube with a very small
clearance. The fixed core can be arranged to slide. round on the outside of the
tube and thus affords an easy means of making the first adjustment described above.
After adjustment the fixed core must be securely fastened-preferably by soldering
to the tube. The tube with the fixed core on it should just slide easily. within the
bobbin of the instrument.
In order to effect the second object of this invention we propose to magnetise the
two cores of the mechanism described above so that the two ends of each core are.
of like polarity, the other pole being midway between them. To effect this the
working bobbin of the instrument must be wound in two halves in opposite
directions, and the two halves may be separated by a check or disc of soft iron
which will serve to carry the induction which leaves the central pole of the cores.

page_text: In order to reduce the magnetic resistance between the pole on the cores and the
disc, the latter may have a short piece of soft iron tube fixed in its central hole,
and each of the cores may be made into a complete cylinder for a short distance on
each side of their centre of length, so that the air path for the induction is made of
sufficient area. When magnetised in the manner just described, the working
mechanism will be so far astatic that extraneous magnetic forces of one tenth value
of the magnetic force within and due to the coil, will only cause an error of about
one per cent. in the indications of the instrument, while an extraneousr field of half
the above strength only affects the instrument to the extent of about one-quarter
Fer cent. Such small outstanding errors as these may be completely eliminated by
covering the working coil with a magnetic shield made in accordance with the
Specification of Patent No. 562, 1893.
Dated this 1st day of November 1893....
Improvements in, Electric Measuring Instruments.
WALTER THOMAS: GOOLDEN, and SYDNEY EVERSHED, both of Woodfield
Works Harrow Road, London W, 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 :-
The invention, relates to voltmeters and 'ammeters; of the electromagnètic class
intended for the measurement of electric pressures and currents, and its objects are
firstly, to enable any desired scale and. range of readings to be obtained by a few
simple adjustments. of the working mechanism, and secondly, to make such
instruments as nearly as possible astatic, so that their indications are unaffected by:
extraneous magnetic forces..
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We effect the first object by an improved working mechanism (hereinafter called
a movement) consisting of two hollow concentric semi cylinders of soft iron sheet
(hereinafter called the cores) placed within the working coil of the instrument, and
with their common axis, along the magnetic axis of the coil. One of the cores is-
fixed, while the other which is, of smaller diameter is attached to an arbor lying
along the axis of the coil, and is therefore free to rotate round the said axis within
the fixed core.
This arrangement of coil and movement is shown in Fig. 1. and
Fig. 2. which are respectively a front elevation and 'axial" section. of an ammeter
made according to our present invention. The fixed core NS (Fig. 2) is made by
bending a piece of sheet iron shaped as shown in Fig. 3. on a round mandrel until
the two ears E. meet and form a short length of complete tube at the middle of
the core, with a semi-cylindrical portion projecting on each 'side! The core thus
nade is slipped over the brass tube P (Fig. 2) the ears E' serving to hold it firmly
on the tube. The movable core ns (Fig. 2.) is formed from a rectangular piece of
sheet iron without ears bent into a semi-cylinder of smaller radius than the inside
of the tube P. The movable core is soldered to two brass discs or collets D
(Fig. 2) which are themselves soldered on the arbor A which is pivotted at J J
and carries an adjustable weight W and an index H ranging over a scale on a dial
in the usual manner. Thus the movable core is free to rotate with a very small
clearance between its outer surface and the inside of the tube P. When a current
flows in the working coil C the cores become maguetised and tend to place
themselves so that they form a nearly complete hollow cylinder. In other words.
the inovable core as is repelled by the fixed cote NS and will rotate on its axis
until the tangential component of the repulsion is balanced by the weight W.
The relative positions of the cores when the instrument is at zero are indicated in
Fig. 1 by the dotted semi-circles n, N, and more clearly in Fig. 4 and Fig. 5 which
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page_text: are radial sections through the tube P in the plane X Y Fig. 2. The arrow in
Fig. 4 & Fig. 5 indicates the direction of rotation of the arbor and movable, core,
and the longitudinal edges of the two cores marked I, I, and f, f, are herein referred
to as the leading edges and following edges respectively. For the sake of clearness
Fig. 4 and Fig. 5 are drawn to twice the scale of the remaining figures. Fig. 4
shows a position of the fixed core with reference to the zero position of the
movable one when a moderate range of readings is desired; and if the angle
indicated by the dotted lines and marked L is increased to about 90°, so that when
at zero the leading edge of the movable core is about 90° in advance of the
corresponding edge of the fixed core, the tangential component of the repulsion
between the cores will be a maximum and the beginning of the scale will be
expanded to the utmost extent. In other words the longest possible range of
readings will thus be obtained, and the scale will be well adapted for an ordinary
ammeter. Fig. 5 on the other hand shows the cores set with a very small angular
lead L at zero, so that the tangential component of the repulsion is very small.
With this small angular lead the portion of the scale near zero will be greatly
compressed and a very open scale will be obtained over a small range of current
values, in other words a suitable scale for an ordinary voltmeter. Thus any
desired openness of scale can be obtained by simply altering the angular lead of
the inovable core, and this is effected by turning the fixed core round on the
tube Puntil the desired result is obtained; it is then soldered to the tube, .and
the weight W having been adjusted until the scale is of the desired angular
extent, is soldered to the rod on which it is screwed. No further adjustments aro
required.
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In order to obtain the greatest force from cores of any given size their length
must be equal and their adjacent ends in the same radial plane.
purposes a better scale is obtained both for voltmeters and ammeters by cutting off
the corner at each end of the following edge of the fixed core. The slanting edges
so formed inay with advantage be continued on to the corners of the ear which
projects from the following edge. These modifications are indicated in Fig. 3 by
dotted lines.
The approximate sizes of the various parts described may be ascertained by
reference to the scale of inches given on the drawings attached hereto jubut
although these sizes represent what we find it convenient to use in practice, the
size and proportions of the parts in the movement may be varied between very
wide limits without affecting its action. We prefer to make the cores extend
round an arc of about 180 degrees, but greater or less ares for one or both cores
may be used:
The movable core may conveniently be made of such thickness as to act as a
balance weight to the index H. When the instrument -is, tube. used. for alternate
currents it is desirable to use iron of not more than 15 thousandths of an inch in
thickness, using two or more thicknesses if more force is required; in this case also
the tube P should have a wide longitudinal slot cut in it from end to end to eliminate
eddy currents. The movement should be made to slide easily, but with. as little
clearance as possible within the working coil. In Fig. 2 the clearance is purposely
enlarged to avoid confusion.
We effect the second object of our invention by magnetising the cores of the
movement so that the two ends of each core are of the same polarity, the other
pole being midway between them. This is effected by arranging the working coil
in two halves C and G (Fig. 6) and causing the current in C to How in the opposite
sense to that in G so that all four ends of the cores are of one polarity; for example
they may all be north poles, as shown in Fig. 6. The two halves of the working
coil are separated by a disc of iron d which serves to carry the magnetic induction
which leaves the central pole of the cores. In order to reduce the magnetic
resistance between the pole on the fixed core N N and the disc d a short length
of iron tube T is fixed in the central bole of the disc d. This short tube
preferably of the same length as the tubular part of the fixed core N N. For the

page_text: "
same reason the movable core n n is made with a central tubular portion like the
fixed core, so as to afford a proper path for the induction leaving the central pole
of the movable.core. Thus in an astatic instrument made, according to our
invention both cores are formed in the manner described with reference to: Fig. 3,
but in order to obtain sufficient repulsion between the poles N n N (Fig. 6) it is
necessary to make the tubular part of the cores longer than is required in a
non-astatic movement like that illustrated in Fig. 2, and this increased length renders
it desirable to increase the whole length of the cores in proportion. This alteration
of the dimensions of the cores for an astatic instrument is shown in Fig. 6 which
is drawn to the same, scale as Figs. 1, 2, and 3. In other respects the astatic 1
movement is like the non-astatic; it obeys the same laws as to repulsion and kind
of scale produced, as has been described and illustrated by Fig. 4 and Fig. 5 with
reference to the non-astatic movement.
An instrument made as described with reference to Fig. 6 is so far astatic that
extraneous magnetic forces of one tenth the value of the magnetic force within.
and duo to the working coils will only cause an error of about one per cent. in the
indication of the instrument, while an extraneous magnetic force one twentieth of
that within and due to the working coils only affects the indication of the instrument
by about
Instruments made according to our invention may be wound either for use as volt-
meters or as ammeters, and we find for most purposes a convenient force of repulsion.
is produced when the coil is so wound that the product of maximum current, which
the instrument is intended to measure, into the number of turns of wire on the coil,
is about 600.
The nature of the controlling force applied to the arbor of an instrument made
in accordance with this Specification forms no part of our present invention. We
have described and illustrated herein the use of a gravity control, but any other
known and convenient control may be applied: For example when instruments are
required for use on board ship, or for other purposes where a gravity control is
undesirable, we dispense with the weight W (Figs. 1, 2 & 6) and in its place use a
non-magnetic metal spring of the same shape as an ordinary watch hair-spring.
The inner end of this spring is attached to the arbor in the usual manner, and its
outer end is fixed in a suitable manner to the flauge to which the tube.P (Figs. 2
& 6) is attached. The spring is arranged so that it is in a radial plane.
For the sake of clearness in the drawings attached to this Specification, those
parts of the iron cores, tubes and discs that are in section are shaded black; those
parts of the non-magnetic portions of the movement which are in sectiou are indicated
by section lines in the customary manner.
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In the following claims, and generally throughout this Specification, the word
"iron" is used in the sense "suitable magnetic material," and the word "brass
used in the sense "suitable non-magnetic material.”
Having now particularly described and ascertained the nature of our said inven-
tion, and in what mauner the same is to be performed, we declare that what we
claim is :-
. (1.) In an electric measuring instrument, fixed and movable iron cores in the form
of concentric hollow semi-cylinders adapted to repel each other when magnetised
by the current to be measured, substantially as described and for the purposes set
forth.
(2.) In an electric measuring instrument, a working mechanism or movement
comprising a hollow semi-cylindrical iron core fixed on and concentric with a
brass tube, an arbor within and concentric with the said brass tube and a secoud
bollow semi-cylindrical iron core fixed to and concentric with the said arbor, and
adapted to be repelled by the first mentioned core and cause rotation of the said
arbor when both cores are magnetised by the current to be measured; for the
purposes set forth and substantially as described with reference to Figs. 1, 2, 3, 4,
and 5.
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page_text: (3.) An instrument for the measurement of electric pressure or ourrent made
substantially as described with reference to Figs. 1, 2, 3, 4, and 5.
(4.) Anastatic instrument for the measurement of electric pressure or current
made substantially as described with reference to Figs. 4, 5, and 6.
Deted this 30th of July 1894
WALTER T. GOOLDEN.
SYDNEY EVERSHED.