Improvements in Electrical Measuring Instruments.
Year: 1893

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Date of Application, 27th Mar., 1893
Complete Specification Left, 23rd Dec., 1893-Accepted, 10th Feb., 1891
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PROVISIONAL SPECIFICATION.
Improvements in Electrical Measuring Instruments.
I, BENJAMIN JOSEPH BARNARD MILLS, of 23 Southampton Buildings in the
County of Middlesex, Patent Agent, do hereby declare the nature of this inven-
tion, a communication to me from Arthur Edwin Kennelly, of Orange, in the County
of Essex, and State of New Jersey, in the United States of America, Electrician,
to be as follows:-
This invention relates to electrical measuring instruments of that class employing
"a magnet of constant strength, between the poles of which is pivotted an armature
controlling a pointer moveable over a graduated scale. The main object of the
invention is to improve, the construction of instruments of the character indicated,
whereby increased simplicity and more perfect operation are secured.
The permanent magnet preferably employed
having its ends nearly meeting.
of the horse-shoe or curved form,
To the ends of the magnet are suitably secured wrought-iron pole-faces of
such form as to have their ends overlapping for a considerable length. The ends
of these pole-faces are preferably rounded, and are considerably thinner than the
main portion.
Each pole-face is preferably cut away at one side to make room for the spindle
and hub of the armature, which will be arranged to rotate between the over-
lapping pole-faces. The armature is a thin disc and preferably made of aluminium,
and to the periphery of this armature is secured a rubber ring having an outer
groove and on one side of the ring a number of notches at intervals to receive'the
wires of the armature winding, as hereafter described. One of these notches will
be somewhat wider than the others. The centre of the armature will be stamped
out into cup form for the purpose hereinafter described. .
The preferred mode of winding the armature is to run the wires radially across
the upper face of the disc and into the cup formed at the centre of the armature
so as to be bent readily around the spindle of the armature, and then to the notch
opposite the one where the wire is led onto the armature; then around the
armature in the groove formed in the rubber ring, to the first notch; then again
radially across the armature to the second notch, and then around the armature in
the groove of the rubber ring in the opposite direction, back. to the first notch,
then radially across the armature to a third notch, then around the groove of the
rubber ring to the first notch, then again radially across the armature to the third
notch, and around the groove of the rubber ring in the opposite direction back to
the first notch and so on until the winding is completed. The armature will thus
be given a differential winding.
Secured to the spindle are two restoring springs for returning the pointer or
indicating device to the zero point. One or both of these restoring springs may
be included in the electrical circuit of the instrument.
The circuit through the meter will be from the plus terminal to the restoring
spring; thence to the spindle of the armature, thence through the armature coils;
thence to a second restoring spring; thence to the minus terminal. The spindle
of the armature, is suitably supported between plates carried by a non-magnetic
bar secured to the magnet, said spindle being properly insulated from the plates."
It will readily be understood that the armature winding described will have a
considerable number of wires extending from the wide notch to the centre of the
disc, and in order that the armature may be perfectly balanced, a counter-balance
: .

page_text: weight is secured to an arm extending from the spindle of the armature and placed
opposite to said notch. This collection of wires on the armature surface are so
located as to be outside of the magnetic field between the facing and overlapping
pole-faces of the magnet, and preferably normally stand directly in line with the
pointer. By bending the wires down into the cup or hub of the armature as
described, the same are held in place without special fastenings and without
forming a thick portion, which might interfere with the movement of the armature
in its thin field between the pole-faces of the magnet.
When the circuit of the instrument is closed, the armature will give a steady,
uniform pull, depending on the current on the line, for all positions, so that the
scale can be divided into divisions of regular length and calibrated by a single
trial. Owing to the form of the armature, the poles of the magnet can be quite
large and can be brought very close together, thereby largely reducing the
magnetic resistance of the air gap, and making the operation of the instrument
much more efficient and regular than with any other form of armature, and the
armature pointer can be mounted so as to be capable of movement through a longer
arc than heretofore practicable.
In practice the scale can be made 120° or more, thereby making it possible to
use large divisions on the scale. When the circuit through the instrument is
broken, the pointer will gradually be returned to zero by means of the restoring
springs. The instrument will be dead-beat, since currents will be generated in the
metal disc, forming the body of the armature, tending to oppose sudden movements
thereof. The armature described gives a continuous rotary effect with no other
appreciable component, such as a side strain on its bearing, and hence the
instrument gives very perfect results.
While the instrument has been above described in relation to its use as an
ammeter, it can obviously be used as a volt meter. This would generally require
the use of a resistance coil in the main circuit, and preferably the omission of the
shunt.
The winding of the armature can also be varied to some extent. There need
not necessarily be two wires in each armature notch, although there will be two
or more when the winding has many turns. The differential condition will not be
materially affected if one wire only be led to each notch, provided that the returns
to the entrance notch are successively carried in opposite directions. It may not
always be desirable to enter the first radial wire on the disc exactly opposite to
the large notch, as the differential arrangement will generally be more effectively
carried out by commencing at some other radius. This is for the reason that the
differential balance cannot be perfect everywhere around the periphery, and it is
desirable to make it strictly correct for full load.
Dated March 27th 1002
COMPLETE SPECIFICATION.
Improvements in Electrical Measuring Instruments.
I, BENJAMIN JOSEPH BARNARD MILLS, of 23 Southampton Buildings in the
County of Middlesex, Patent Agent, do hereby declare the nature of this invention,
a communication to me from Arthur Edwin Kennelly, of Orange, in the County of
Essex, and State of New Jersey, in the United States of America, Electrician,
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 of that class employing

page_text: a magnet of constant strength, between the poles of which is pivoted an armature
controlling a pointer movable over a graduated scale.
The main object of the invention is to improve the construction of instruments
of the character indicated, whereby increased simplicity and more perfect operation
are secured.
In the accompanying drawing illustrating the invention,
Figure 1 is a plan view of the instrument with its cover removed;
Figure 2 is a section on line X X of Figure 1, looking in the direction of the
arrow;
Figure 3 is a detail view illustrating the construction and winding of the armature,
(which winding is omitted in Figure 2 to avoid confusion of lines);
Figure 4 shows another way of winding the armature; and
Figure 5 is a cross section through the centre of the armature, some of the parts
being omitted for clearness.
The permanent magnet 1, preferably employed, is of horseshoe or curved form,
having ends nearly meeting at one side as shown in Figure 1. To the ends of this
permanent magnet are secured wrought iron pole faces 2, 3, by means of screws 4.
These pole faces preferably terminate in rounded ends 5, which ends are con-
siderably thinner than the main portion of the pole faces, as shown in Figure 2.
Each pole face is preferably cut away at 6 to make room for the spindle and hub of
the armature, as most clearly shown in Figure 2. 7 is a non-magnetic bar secured
to the permanent magnet and forming a support for the plate 8 which supports the
bearing 9 for the spindle 10 and the insulated plate 11, which carries an arm 12, to
which a light coil spring 13 is connected, the opposite end of said coil being
connected to the spindle in such manner as to tend to turn the pointer 14 to zero.
131 is a second spring co-operating with the first and supported, in the manner
indicated in the drawing, so as to be insulated from the spindle. 15 is a small
counterbalance weight.
The armature is a thin disc, and preferably consists of an aluminium disc 16, to
the periphery of which is secured a rubber ring 17, having a groove around it, and
having notches 18 at intervals on one side, and at one point having a wider notch 19
to receive the wire of the armature winding as hereafter described. At the centre
the disc 16 is stamped into cup form, forming a hub 20 which rests on the sleeve 21
on the spindle.
The preferred mode of winding the disc armature is indicated in Figure 3.
Starting from the point 22 an insulated wire is led radially across the upper face
of the aluminium disc, being bent down into, and around the centre of, the hub 20,
then radially along the surface of the plate to the diametrical notch 18, then around
toward the left in the groove of the ring to the notch 19, then again across the face
of the disc to said notch 18, then around in the groove of the ring toward the right,
thus giving a differential winding on the periphery; the winding then extends to
the next notch 18, and so on until there are radial wires extending to each of the
notches around the periphery of the ring. The end 22 is connected to the spindle,
as indicated in Figure 2, while the other end 23 is connected to the spring 131,
insulated from the spindle by the sleeve 24. +, are the line terminals of the
instrument and are connected by means of a wire 25 including a resistance 26, which
is, for convenience, wound directly on the body of the permanent magnet. From
the same terminal a wire 27 extends to the insulated plate 11, thence through the
armature coil and wire 23 to spring 131, thence to 23', to the terminal, thus
completing the circuit of the instrument.
-
-
It will thus be seen that the winding consists of a series of radii in all of which
the current flows either to or from the centre. Supposing the latter case the
arrangement approximates to a plane current sheet spreading uniformly from centre
to edge of the disc, except at the radial portion 19 which is never allowed to enter
the field, and which allows the current turns to pass back from the edge to the
centre of the disc.
Since the two opposite polar faces are semicircular, they can only cover one half

page_text: of the surface of the disc, and the radial section 19 occupies some position in the
unoccupied, semicircle. When current is supplied to the disc winding, all the radii
between the poles will aid in setting up a rotatory couple tending to move the disc
round its axis either to the right or left. As the disc turns, as many active wires
enter the field at one side as leave it on the other, so that the rotatory couple
remains constant so long as the notch 19 does not approach the field too closely.
Of the half revolution that is ideally permissable to give the disc without bringing
the notch 19 between the poles, it is practically convenient to utilize 120 or
130 degrees, and with this arrangement the distance between the section 19
and the pole faces is never so small as to seriously affect the uniformity of the
rotatory couple.
The active wires carried round the periphery of the disc, as for example from
the notch marked 18 in Figure 4 to that marked 19, are all outside the circle of the
pole faces, and by carrying the wires round in alternately opposite directions, any
stray field passing them exerts no appreciable resultant magnetic influence.
It will be seen that the armature winding described gives a considerable number
of wires extending from the notch 19 to the centre of the disc, and in order that
the armature may be perfectly balanced, the counterbalance 15 is placed opposite
to said notch. This collection of wires on the armature surface is so located
as to be outside of the magnetic field between the facing, overlapping, magnet
poles, and preferably normally stands directly in line with the pointer 14 as indicated
in Figure 1.
'
By bending the wire down into the hub 20 as described it is held in place without
special fastenings, and without forming a thick portion which might interfere with
the movement of the armature in its thin field between the magnet poles.
When the circuit of the instrument is closed, the armature will give a steady,
uniform pull, depending on the current on the line, for all positions, so that the
scale 28 can be divided into divisions of regular length and calibrated by a single
trial. Owing to the form of the armature, the poles of the magnet' can be quite
large and can be brought very close together, thereby largely reducing the
magnetic resistance of the air gap, and making the operation of the instrument much
more efficient and regular than with any other form of armature, and the armature
pointer can be mounted so as to be capable of movement through a longer arc
than heretofore practicable. In practice the scale can be made 120° or more,
thereby making it possible to use large divisions on the scale. When the circuit
through the instrument is broken, the pointer will gradually be returned to zero
by means of the restoring springs 13, 131.
The instrument will be dead-beat, since currents will be generated in the
metal disc, forming the body of the armature, tending to oppose sudden movements
thereof.
The armature described gives a continuous rotary effect with no other appreciable
component, such as a side strain on its bearing, and hence the instrument gives
very perfect results.
In setting up the instrument, the armature is put in position before the pole
face 2 is secured to the magnet. Said pole face is then put in place, and the steel
magnet magnetized by means of a suitable coil or otherwise.
While the instrument has been above described in relation to its use as an
ammeter, it can obviously be used as a volt meter.
This would generally require the use of a resistance coil in the main circuit,
and preferably the omission of the shunt.
The winding of the armature can also be varied to some extent. There need
not necessarily be two wires in each armature notch, although there will be two
or more when the winding has many turns. The differential condition will not be
materially affected if one wire only be led to each notch, as shown in Figure 4,
provided that the returns to the entrance notch 19 are successively carried in
opposite directions.
It may not always be desirable to enter the first radial wire on the disc exactly

page_text: opposite to the large notch 19, as the differential arrangement will generally be
more effectively carried out by commencing at some other radius, as indicated
at ŕ Figure 4. This is for the reason that the differential balance cannot be
perfect everywhere around the periphery, and it is desirable to make it strictly
correct for full load.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is :--
:--
An electrical measuring instrument comprising a disc armature, moving in its
plane between the poles of a permanent magnet, and a winding upon the said
armature extending radially froin centre to periphery, then round the periphery to
a common narrow radial sector over which return is made to the centre, the said
reserved sector of return never entering the magnetic field within the range of the
disc's motion.
Dated December 23rⱭ 1893,
B. J. B. MILLS,
By Harris & Mills,
23, Southampton Buildings, W.C., Agents.