Improvements in Alternating Current Measuring Instruments.
Year: 1898

page_text: PROVISIONAL SPECIFICATION.
Improvements in Alternating Current Measuring Instruments.
We, HARRY PHILLIPS DAVIS, of 327, Neville Street, Pittsburg, and FRANK
CONRAD, of 709, Whitney Avenue, Wilkinsburg, both in the State of Penn-
sylvania, United States of America, Electrical Engineers, do hereby declare the
nature of this invention to be as follows; -
Our invention relates to electrical measuring instruments for alternating
currents or electromotive forces, and particularly to instruments of this class
that indicate either the current or the electromotive force traversing the circuits
with which they are connected at any given instant, as distinguished from such
instruments as effect either a registration or a record of the total current or
electromotive force traversing such circuits during the time the instruments are
in service.
The object of our invention is to provide, an instrument of the character
indicated which shall be simple and compact in construction and accurate in
operation and which shall have a wide range of movement and be adapted to
accurately indicate the current or the electromotive force in any alternating
current circuit, irrespective of the frequency of alternations and changes in
temperature.
Our improved measuring instrument is of the class in which a disc armature is
provided projecting into an air gap in a laminated electromagnet, the electro-
magnet being energized by means of a primary coil and a secondary coil so
disposed with relation to each other that a shifting magnetic field is provided
in the air gap causing the armature to rotate. Continuous rotation of the arma-
ture is prevented by some such means as a spiral spring so that said armature
ultimately takes up a position depending on the current passing through the
primary coil, this position being indicated by a suitable pointer traversing a
scale. In order that the instrument shall be direct-reading, that is to say, in
order to secure proportionality between the torque acting on the armature and
the current in the primary coil, we, according to the present invention make the
disc of varying radius so that as it is rotated less and less of said disc projects

page_text: into the air gap. Furthermore, in order to compensate for changes in frequency
of the alternating current and changes in temperature, we provide a non-inductive
resistance which when the instrument is intended for measuring current is con-
nected as a shunt to the primary coil and is made of copper or some other
material, the temperature resistance co-efficient of which is at least as great as
that of the armature of the instrument; where, however, the instrument is to be
used for measuring electromotive forces the non-inductive resistance is con-
nected in series with the primary coil and has a low temperature resistance
co-efficient being made of German silver for example, in this case in order to
compensate for changes in temperature, we provide the actuating magnet with a
closed coil which tends to demagnetize the core and has a high temperature
resistance coefficient.
In the na
drawings:

page_text: properly balance the armature, we may affix to it on the opposite side of the
shaft a suitable weight 13.
The outer end of the shaft is provided with a hand or pointer 14 which
co-operates with a suitable dial plate 15 on the plate 4, as is usual in this class of
instruments. 16 is a spiral spring connecting the shaft 11 with a bearing
bracket 17, and is of such composition and strength as to exert the desired degree
of force in opposition to the rotative effect imparted by the current in the coils 8
and 9. 18 is a permanent magnet between the poles of which the armature 10
projects and which co-operates with such armature, in a manner well under-
stood in the art, to make the instrument dead-beat.
In order to adapt the instrument to different current frequencies, we employ
a non-inductive resistance coil 19 which may be located at any convenient point
and supported in any manner desired. In the drawings we have shewn such
coil as clamped to the inner side of plate 4. In order to prevent self-induction,
one half of coil 19 should be reversely wound with reference to the other half.
In case the instrument is designed and constructed to measure the amount of
current flowing at any given instant in the circuit with which its coil 8 is con-
nected, we employ a resistance coil 19, made of copper or some other material
the temperature resistance coefficient of which is at least as great as that of the
material of which the disc 10 is made. This coil should be wound non-inductively
Without the
and connected in shunt to the coil S, as indicated in Figure 4.
coil 19, the meter would read too high with a high frequency of alternations and
too low with a low frequency of alternations, since the difference of potential
between the terminals of coil 8 increases with an increase in the rate of alterna-
If the non-
tions and decreases with a decrease in the rate of alternations.
inductive coil 19 be connected in shunt to the coil 8, however, the difference of
potential between its terminals will remain substantially constant for all
frequencies from which it follows that as the rate of alternations increases, the
coil 8 will take less and the coil 19 more current. On the other hand, as the
rate of alternations decreases, the coil 8 will take more and the coil 19 less
current. By properly proportioning these coils the torque exerted upon the disc
may, therefore, be made substantially independent of changes in the rate of
alternations. As has already been stated, the shunt should have a temperature
resistance coefficient at least as high as that of the disc, so that, as the heat of
the disc tends to decrease the torque exerted upon it on.account of the increased
resistance; the shunt coil will become heated, and will, therefore, take less
current, thus forcing more current through the coil 8 to compensate for the
effect of heat in the disc.
In the case of a voltmeter, the coil 19 should also be wound non-inductively,
but should be made of metal having a low temperature resistance co-efficient,
such for example, as German silver, and be connected in series with coil 8, as
indicated in Figure 6 of the drawings.. If no resistance 19 be employed, the
tendency of the voltmeter is to read too low with high frequencies, and too high
witli low-frequencies, due to the fact that the self-induction of the coil 8 increases
Since the resistance 19
with an increase in the rate of alternations, and vice versa.
has no self-induction, the difference of potential between its terminals will
remain substantially constant for all frequencies of alternations, and being con-
nected in series with the coil 8, the two may be so proportioned that changes in
the rate of alternations will not affect the reading of the instrument to any
appreciable extent. As the coil 19, when connected in series with coil 8, does
not compensate for changes in temperature, we surround one limb of the core of
the actuating magnet 6 with a closed coil 20 which consists of a few turns of wire
made of metal which has a high temperature resistance coefficient, such, for
example, as copper. The current set up in this coil 20 will tend to demagnetize
the core and thus reduce the torque exerted upon the disc 10.
of this
amount
current
demagnetizing
ance of the coil, sucli current will decrease
Since the
the resist-
depends upon
the coil becomes heated,
as

page_text: To
"
thus correspondingly decreasing the demagnetizing effect, the result being that
the resultant torque exerted upon the disc. will not vary, to any appréciable extent
with changes in temperature.,.
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Dated this 27th day of September 1898:"
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WYP THOMPSON & Co
322, High Holborn, London, WC.;
Patent Agents for the Applicant
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FINAL SPECIFICATION...
Saran's ་དག
Improvements in Alternating Current Measuring Instruments..,
་ ་་་་
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1
We, HARRY PHILLIPS DAVIS, of 327, Neville Street,, Pittsburg, and FRANK
CONRAD; of 709; Whitney Avenue, Wilkinsburg, both in the State of Penn-
sylvania, United States of America, 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:-
Our invention relates to electrical measuring instruments, for alternating
currents or electromotive forces, and particularly to instruments of this class
that indicate either the current or the electromotive force, traversing the circuits
with which, they are connected at any given instant, as distinguished from such
instruments as effect either a registration. or a record of the total current or
electromotive force traversing such circuits during, the time the instruments are
in service.
"
The object of our invention, is..to provide an instrument of the character
indicated which shall be simple and compact in construction and accurate in
operation, and which shall have a wide range of movement and be adapted to
accurately indicate the current or the electromotive force in any alternating
current circuit, irrespective of the frequency. of alternations and changes in
temperature:
.
-Our improved measuring instrument is of the class in which a disc armature is
provided projecting into an air gap in a laminated electromagnét, the “electro-
magnet being energised by means of a primary coil and a secondary coil so
disposed, with, relation, to each other. that a shifting magnetic field is provided
in the air gap.causing the armature to rotate. Continuous rotation of the arma-
ture is prevented by some such means as a spiral spring so that said armature
ultimately takes up a a position depending on the current passing through the
primary coil, this position being indicated by a suitable pointer traversing" a
scale... In order that the instrument shall be direct-reading, that is to say, in
order to secure proportionality between the torque acting on the armature-and
the current, in the primary coil, we, according to the present invention
make the disc of varying radius that is to say with radii of different
length so 'that as it is, rotated less and less of said disc projects
into the air, gap. Furthermore, in order to compensate for changes in frequency
of the alternating current and changes in temperature, we provide a non-inductive
resistance which when the instrument is intended for measuring current is con-
nected as a shunt. to, the primary coil and is made of copper or some other
material, the temperature resistance co-efficient of which is at least as great as
that of the armature of the instrument; where, however, the instrument is to be
used for measuring electromotive forces the non-inductive resistance is con-
nected in series with the primary coil and has a low temperature resistances
co-efficient being made of German silver. for example, in this case in order to
compensate for changes in temperature, we provide the actuating magnet with a

page_text: closed coil which tends to demagnetize the 'core' and has a high,temperature.
resistance co-efficient:
*****
In the drawings left with our Provisional. Specification.
Figure 1 is a plan, and...
*་་་་
Figure 2 a similar view with the cover plate and indicating devices removed..
Figure 3 is a view partially in section on the line III-III, Figure 1. and
partially in side elevation:
"
"I")
Figure 4 is a diagram of the circuit of the instrument when intended for
measuring current:
Figure 5 is a 'view partly in rear and partly. sectional.. at right angles to
Figure 3.
.......
"
"
Figure 6 is a diagram of the circuit of the instrument when. used. to measure
electromotive forces.
1is an enclosing case provided with a glass front 2. and a back plate 3...to
which is connected by the posts 5 a front plate 4, said plates forming a frame
work on which" operating parts are mounted. 6 is a laminated core, having, an
air gap 7. Mounted upon each arm of the core and adjacent. to the, air, gap 7 is
located a coil 9 that is closed upon itself. One side of said coil 9 is located in
a slot in the core (as shewn in Figures 2 and 5) so that the magnetic field due to
the current set up in these coils is displaced with reference to..the field set up,
by the current passing' through the coil 8, and inasmuch, as. the currents, in the
coils 9 are produced by induction from, the current in the..coil 8, there, will also..
be a 'displacement of phase and a shifting magnetic field will result. The
armature or secondary member of the motor device consists. of. a disc. 10, the
periphery of which has the form of one turn of a spiral, as is clearly indicated.
in Figure 2 of the drawings. By reason of this construction,. the disc, will
project into the air gap 7 a maximum distance when the disc is at its initial or
zero position, that is, when there is no current flowing through the coil S. As
the.
the current increases through the coils 8, the armature will be rotated, in
direction of the arrow by reason of the action of the shifting magnetic field, and
as each succeeding part in the disc has a radius of less length than the pre-
ceding one the degree to which the armature projects into the air-gap will
decrease. This construction of armature enables us to provide an instrument
having a uniform scale and one which will accurately indicate the current or the
electromotive force, as the case may be, in the circuit.
"
If a circular disc were employed, the torque exerted upon it would be pro-
portional to the square of the current, whereas a torque proportional to the
current is desired in order that the motor may be a direct reading instrument,
i.c., one in which equal increments in the current produce equal angular deflection
of the pointer throughout the scale. We secure this result by making the disc
of such shape that its degree of projection between the poles of the magnet will
decrease as it moves away from the initial or zero, position.
The armature 10 is rigidly mounted upon a shaft 11 which is preferably
mounted in jewel bearings as is usual in instruments of this class, and in order
that the armature may be properly balanced on the shaft, we cut out a segmental
portion 12 on the side having the greater radii, and if this is not sufficient to
properly, balance the armature, we may affix to it on the opposite side of the
shaft a suitable weight 13.
The outer end of the shaft is provided with a hand. or pointer 14" which
co-operates with a suitable dial plate 15 on the plate 4, as is usual in this class of
instruments.. 16 is a spiral spring connecting the shaft 11 with a bearing
bracket 17, and is of such composition and strength as to exert the desired degree
of force in opposition to the rotative effect imparted by the current in the coils 8
and 9. 18 is a permanent magnet between the poles of which, the armature 10
projects and which co-operates with such armature, in a manner well under-
stood in the art, to make, the instrument dead-beat.
In order to adapt the instrument, to different current frequencies, we employ
し
"

page_text: a
non-inductive resistance coil 19 which may be located at any convenient point"
and supported in any manner desired. In the drawings we: have shewn such
coil as clamped to the inner side of plate 4. In order to prevent self-induction,
one half of coil 19 should be reversely wound with reference to the other half"
In case the instrument is designed and constructed to measure the amount of
current flowing at any given instant in the circuit, with which its coil 8 is con-
nected, we employ a resistance coil 19, made of copper or some other material
the temperature resistance coefficient of which is at least as great as that of the
material of which the disc 10 is made. This coil should be wound non-inductively
and connected in shunt to the coil S, as indicated in Figure 4.. Without the
coil 19, the meter would read too high with a high frequency of alternations; and
too low with a low frequency of alternations, since the difference of potential
between the terminals of coil 8 increases with an increase in the rate of alterna
tions and decreases with a decrease in the rate of alternations. If the non-
inductive coil 19 be connected in shunt to the coil 8, however, the difference' of:
potential between its terminals will remain substantially constant: for all,
frequencies from, which it follows that as the rate of alternations increases, the
coil S will take less and the coil 19 more current. On the other hand, as the
rate of alternations decreases, the coil 8 will take more and the coil 19 less
current. By properly proportioning these coils the torque exerted upon the disc
may, therefore, be made substantially independent of changes in the rate of
alternations. As has already been stated, the shunt should have a temperature
resistance coefficient at least as high as that of the disc, so that, as the heat of
the disc tends to decrease the torque exerted upon it on account of the increased
resistance, the shunt coil will become heated, and will, therefore, take less.
current, thus forcing more current through the coil 8 to compensate for the
effect of lieat in the disc.
.
In the case of a voltmeter, the coil 19 should also be wound non-inductively,
but should be made of metal having a low temperature resistance co efficient,
such for example, as German silver, and be connected in series with coil 8, as.
indicated in Figure 6 of the drawings. If no resistance 19 be employed, the
tendency of the voltmeter is to read too low with high frequencies, and too higli
with low frequencies, due to the fact that the self-induction of the coil 8 increases
Since the resistance 19
with an increase in the rate of alternations, and vice versa.
has no self-induction, the difference of potential between its terminals will
remain substantially constant for all frequencies of alternations, and being con-
nected in series with the coil 8, the two may be so proportioned that changes in
the rate of alternation's will not effect the reading of the instrument to any
appreciable extent. As the coil 19, when connected in series with coil 8, does
not compensate for changes in temperature, we surround one limb of the core of
the actuating magnet 6 with a closed coil 20 which consists of a few turns of wire
made of metal which has a high temperature resistance coefficient, such, for
example, as copper. The current set up in this coil 20 will tend to demagnetize
the core 6 and thus reduce the torque exerted upon the disc 10.
amount of this demagnetising current
upon
will decrease as the coil
of the coil, such current
thus correspondingly decreasing the demagnetising effect, the result being that
the resultant torque exerted upon the disc will not vary to any appreciable extent
with changes in temperature.
depends
the
Since the
resistance
becomes heated,
Having now particularly described and ascertained the nature of our said
invention and in what manner the same is to be performed we declare that what
we claim is :-
1. For an electrical measuring instrument an armature or secondary member
consisting of a disc having radii of different length, for the purpose specified.
2. The combination with an instrument for measuring alternating currents
of a non-inductive resistance connected as a shunt to the primary coil the

page_text: temperature resistance coefficient of said resistance being at least as great as
that of the armature of the instrument for the purpose specified.
3. The combination with an instrument for measuring alternating electro-
motive forces, of a non-inductive resistance connected in series with the primary
coil and having a low temperature resistance coefficient, for the purpose specified.
4. In an alternating current measuring instrument a closed demagnetizing
coil on the actuating magnet, said coil having a high temperature resistance
coefficient so as to render the instrument substantially independent of changes
of temperature.
5. Measuring instruments constructed substantially as described with reference
to the accompanying drawings.
Tate the ofth dow of Tune 1900
W. P. THOMPSON & Co.,
Of London, Liverpool, Manchester and Birmingham.