Improvements in Electrical Measuring Instruments.
Year: 1892

page_text: Date of Application, 30th May, 1892
Complete Specification Left, 28th Feb., 1893-Accepted, 6th May, 1893
PROVISIONAL SPECIFICATION.
Improvements in Electrical Measuring Instruments.
We, WILLIAM EDWARD AYRTON, Fellow of the Royal Society, Central
Institution, Exhibition Road, London, S.W., Professor of Applied Physics, and
THOMAS MATHER, Central Institution Exhibition Road, London, S. W., Laboratory
Assistant, do hereby declare the nature of this invention to be as follows:-
Our invention relates to electromagnetic measuring instruments with movable
coils. The improvements consist:-1st, in arranging that the instruments are
astatic, and therefore uninfluenced by external fields; 2nd, in placing the coil or
coils in such positions relative to the direction of the magnetic field or fields by
which the motions are influenced, so as to obtain great sensitiveness about a
particular part of the scale; 3rd in the application of magnetic control to instru-
ments having narrow coils, thereby making their sensitiveness independent of
changes in the strength of the magnetic field, and obtaining a control suitable
for instruments requiring to be very sensitive at one part of their scales; 4th in
combining a switch with " "devices in instruments intended to be very
sensitive about a particular part of the scale; 5th in constructing narrow coil
instruments as portable ballistic galvanometers, which are sometimes arranged as
direct reading coulombmeters, inductometers, gaussmeters, or magnetic field
measurers, of long range, or as ammeters or voltmeters; 6th in arranging the
parts of instruments so that very long ranges may be obtained; and 7th in
combining inovable coil instruments with recording devices.
set up
In describing the improvements we will suppose the magnetic fields in which
the coils are placed to be produced by permanent magnets, but we would have it.
understood that fields produced by other well known means may be used without
departing from the essential nature of our invention. For example, instead of
employing permanent magnetic fields, in which case the instruments (except the
ballistic ones) will measure the mean value of the current passing through the
coil or coils, we may arrange that coils carrying currents are used to produce
the fields. In this case the mean product of the currents in the fixed and movable
coils respectively will be measured, or the same current may pass through the fixed
and movable coils when the product that is measured becomes the square.
The first part of our invention may be carried out by using two oppositely
directed fields each having a coil in it, the two coils forming part of the moving
system, and the currents circulating in the coils in directions to suit the magnetic
fields. Instead of the fields being oppositely directed they may be inclined at an
angle. Three or more fields and coils are sometimes employed and with this device
astaticism may be obtained not merely for a uniform disturbing magnetic field but
even for a non-uniform disturbing field.
The second part of our invention may be carried out in several ways. One is
to set the plane of the coil or coils nearly perpendicular to the magnetic field or
fields when the coils are in the zero position. A second way applicable when two
or more coils are used is to set the coils at an angle to each other and so placed
relative to their magnetic field that about the zero position one coil opposes the
other whilst after a certain deflection has taken place, the coils assist each other.
A similar result may be obtained by setting the coils parallel and the fields at an
angle, or by a combination of the methods above described.
The third part relates to improved methods of applying magnetic control to
narrow coil instruments so as to obtain invariable and great sensibility. Ways
of applying magnetic control to suspended coils are described in Specification
No. 8795 of 1887. In practice these have not been found satisfactory owing to
the pieces of steel being very near the polar faces of the magnet and core, any

page_text: small displacement of the coil altering the force. In instruments with narrow coils
and no core this defect is remedied by putting very small magnets near the axis about
which the coil turns; they are thus at a considerable distance from the poles of
the fixed magnet and in fairly uniform fields. This magnetic control also enables
us to have a controlling moment varying approximately as the sine of the deflection
which is very convenient for instruments which require to be very sensitive about
a particular part of the scale. Instruments arranged in this way are very suitable
for ampere guages, volt guages &c. which require to measure one particular quantity
very exactly. The temperature errors of such instruments used as voltmeters also
tend to compensate each other.
The fourth part of our invention consists in applying a "set up" device in
combination with a switch to instruments having movable coils so that the act of
completing the circuit mechanically moves the pointer to a certain position; the
pointer does not move further until the current through the coil exceeds a certain
value. Breaking the circuit takes off the "set up" and enables the zero of the
instrument to be tested or adjusted without disconnecting the wires from the
terminals. By using this device we are enabled to have the coils in such a position
when the pointer is at zero, that if a current were passed through the coil in
the ordinary direction the coil would move in the wrong direction; but the act of
closing the switch causes the set up device to bring the coil into a position such
that the deflection will be in the right direction when the current passes. The
angle through which the coil turns from zero is therefore increased and hence a
longer range, over which the sensitive part of the scale can be spread, is obtainable.
Further, the exact amount of "set up" does not alter the meaning of the indications
as is usually the case in "set up" instruments, for in our arrangement the zero
position remains unaltered.
The fifth part relates to ballistic galvanometers. Ordinary d'Arsonval galvano-
meters are not very sensitive as ballistic instruments because of the large moment
of inertia of the coils and the possible range is not great being limited by the
support for the iron core. We obviate these defects by employing narrow coils
and at the same time the construction of such instruments is simplified and made
more convenient to use. Our narrow coil ballistic galvanometers are made portable
and to have very long ranges of one or more complete revolutions. Their scales
can also be graduated to read directly in microcoulombs or other convenient units;
in some cases the scales are graduated to read the number of lines of force passing
through magnetic circuits such as those of dynamo machines. In combination with
a test coil arranged to turn suddenly through 180° they can be used to measure
magnetic fields. Arrangements are sometimes provided for stopping the pointer
at its greatest elongation to facilitate reading. In other cases we also combine the
instruments with shunts or series resistances, or both, and put two or more gradua-
tions on the scale thus enabling the same instrument to be used either as a portable
ballistic galvanometer, gaussmeter, ammeter or voltmeter.
The sixth part relates to methods of constructing and arranging the parts of
movable coil instruments so as to obtain very long ranges and is particularly
applicable to ammeters voltmeters and wattmeters.
We arrange the coils like an armature of a dynamo with or without an iron core,
and the field magnet of which is composed of permanent magnet or of a coil or
coils with or without iron. When a current passes round the armature it turns
and its motion is resisted by a spring or other means. A suspended weight whose
distance from the axis is varied by passing the suspension round a cam, snail, or
fusee, is sometimes used as control. By suitably shaping the cam, snail, or fusee,
any desired shape of calibration curve can be obtained and very long ranges
obtained.
Seventhly, by combining our movable coil instruments with recording devices,
we obtain registering instruments having either small rauge and great sensibility,
or long range, according to requirement. For example in cases where it is desired
to know the variation of pressure between the mains of a so-called constant pressure

page_text: system, we employ instruments sensitive about a particular part of the scale,
whereas for registering the output of such a system, (which output usually
varies very considerably), we use instruments of long range in combination with
recording mechanisms.
Dated this 28th day of May 1892.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.
COMPLETE SPECIFICATION.
Improvements in Electrical Measuring Instruments.
We, WILLIAM EDWARD AYRTON Fellow of the Royal Society Central
Institution Exhibition Road London S.W. Professor of Applied Physics, and
THOMAS MATHER Central Institution Exhibition' Road London S.W. Laboratory
Assistant 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 electromagnetic measuring instruments with movable
coils.
The improvements consist :-
-
1st. In arranging the parts so that the instruments are astatic, and, therefore,
uninfluenced by external magnetic fields;
2nd. In placing the coil or coils in such positions relative to the direction
of the magnetic field or fields by which their motions are influenced, so as to obtain
great sensitiveness about a particular part of the scale;
3rd. In the application of magnetic control to instruments having narrow coils
thereby not only making their sensitiveness independent of changes in the strength
of the magnetic field but producing also a control suitable for instruments required
to be very sensitive at one part of their scale.
4th. In combining a switch with "set up" devices in instruments intended to be
sensitive about a particular part of the scale.
5th. In constructing narrow coil instruments as portable ballistic galvano-
meters which are also sometimes arranged as direct reading coulombmeters,
inductometers, gaussmeters, or magnetic field measurers of long range, or as
ammeters, or voltmeters.
6th. In arranging the parts of instruments so that very long ranges may be
obtained.
(2) In our Provisional Specification we described the improvements by reference
to magnetic fields produced by permanent magnets and pointed out that fields
produced by other well known means might be used. In this the Complete
Specification we would have it understood that our improvements relate chiefly to
instruments having fixed permanent magnets and movable coils; cases in which
the magnetic field or fields are produced by means other than permanent
magnets being separately mentioned in the parts of the Specification to which they
relate.
(3) We would also have it understood that although the second, and fourth
parts of our invention are described with reference to instruments in which the
controlling force is gravity, we do not confine ourselves to gravity control, for
spring or other well known controls may be employed without departing from
the essential nature of our invention. To avoid unnecessary repetition we would
here mention that in the drawings hereto attached the fact that the cross sections.
of the coils have section lines in different directions is intended to indicate that.
the currents in these parts are in different directions. The methods of leading
the currents to and from the movable coils which we usually prefer is to use flat.
strips of very thin metal which exert very little control. In the case of ordinary
gravity instruments these strips are generally formed into spirals.

page_text: (4) Referring to the first part of our invention we would point out that it has
hitherto been considered that the reading of instruments which have moveable coils
deflected by strong stationary permanent magnets are not affected by magnetic
disturbances, but in instruments intended to be sensitive to small changes of current
we find this is not the case. To overcome the changes of reading produced by
such disturbing fields we make our instrument astatic. One way of doing this is
by using oppositely directed fields each having a coil in it, the two coils forming
part of the moving system, and the currents circulating in the coils in directions to
suit the magnetic fields. Instead of the fields being oppositely directed they may
be inclined at an angle. Three or more magnetic fields are sometimes employed
and with this device astaticism may be obtained, not merely for a uniform
disturbing magnetic field, but even for a non-uniform disturbing field. Figs. 1
and 2. (Sheet 1.) show elevation and plan of two magnets N S and N' S', and two
coils a and b, arranged astatically, the directions of the current being indicated by
arrow heads. In Figs. 3 and 4 (Sheet 1.) an instrument having three coils a, b
and three magnets N S, N' S', and N" S" is shown. Instruments of this
latter type give astaticism for uniform, and also for non-uniform disturbing fields.
Both the above arrangements show the coils narrow, and without the fixed iron
core which is commonly used in instruments having movable coils, but this
part of our invention is equally applicable to instruments in which such cores are
employed.
and
Cy
One
(5) The second part of our invention may be carried out in various ways.
is to set the plane of the coil or coils nearly perpendicular to the magnetic field
or fields where the movable system is in the zero position, so that it tends to turn
through an angle of nearly 180° when the current flows through it. An instrument
arranged in this way is shown in elevation and plan in Figs. 3 and 4 (Sheet 1).
When the coils are in the position shown, the deflecting moment when a current
passes is small, for the planes of the coil are nearly perpendicular to the direction
of the magnetic fields. As the moving system turns in a counter-clockwise
direction the plane of the coils approaches to parallelism with the direction of the
field, and the deflecting moment per unit current increases. On the other hand
the gravity control alters slowly as the deflection approaches 90°; hence such
instruments are very sensitive about this part of the scale, a small change in the
current producing a considerable change in the position of the points.
A second way applicable when two or more coils are used, is to set the planes of
the coils at an angle to each other and so placed relative to their fields that about
the zero position one coil opposes the other, whilst after a certain deflection has
taken place the coils assist each other. An arrangement for carrying out this plan
when two coils are used is shown in Figs. 6 and 7 (Sheet 1). A coil a Fig. 6. is
indicated by dotted lines, whilst another coil is shown in section in full lines.
The two coils are fixed together. Fig. 7. gives a diagrammatic view of the same
arrangements supposing the coils a and b to be replaced by equivalent magnets n¹s¹
and n s.
From this diagram it will be readily seen that if the coils be equal, the
system will tend to turn counter-clockwise or clockwise according as the angle
between ns, and N. S is greater or less than the angle between n¹s and NS.
Coil a Fig. 6, therefore, opposes coil b when in the position shown, and the resultant
deflecting moment is the difference of the two moments. After a has passed the
position in which its plane is parallel to Z Z, its deflecting moment changes sign,
and the two coils assist each other. We thereby make an instrument very
unsensitive to changes of current when the moving system is near the zero position,
and very sensitive when a considerable distance therefrom. Similar results may
be obtained by setting the coils parallel and the fields at an angle, and this method
we prefer as giving better astaticism. Combination of the above described methods
may be used."
A modification of the above methods for producing great sensitiveness about a
particular part of the scale, which we sometimes employ, is shown in Fig. 5:
Here a wide coil a pivotted at d embraces the pole N of a magnet the poles of

page_text: which are shaped so as to give a small deflecting action when the coil is near the
zero position shown. When the deflection reaches about 45° the part of the coil
remote from comes into the strong and properly directed field between the
poles N S which are here very close together, and as the gravity control increases.
somewhat slowly from 45° to 90°, great sensitiveness is obtained. It will be
understood that by using two or more magnets & coils instruments of the type
indicated in Fig. 5 may be made astatic.
(6) The third part of our invention relates to improved method of applying
magnetic control to narrow coil instruments whereby we obtain not only invariable
sensibility, but also get a control suitable for instruments requiring to be very
sensitive at one part of their scale. Ways of applying magnetic control to
galvanometers are described in Specification No. 879587. In practice these have
not been found satisfactory, for owing to the pieces of steel being very near the
polar faces of the magnet and core, any small displacement of the coil altered the
controlling forces. In instruments with narrow coil and no iron core this defect is.
remedied by putting very small magnets near the axis about which the coil turns ;
they are thus at considerable distance from the fixed magnets and in fairly uniform.
fields. This arrangement of magnetic control enables us to have a controlling
moment varying approximately as the sine of the angle of deflection even over
considerable angles, and by suitably fixing the small magnets relative to the coil
we obtain great sensitiveness about a particular part of the scale. Fig. 8 (Sheet 1.)
illustrates one of the ways in which we carry out this part of our invention. Here
the coil has small magnets such as n s fixed nearly perpendicular to its plane..
When deflected so that the plane of the coil is about parallel to the line N S, a.
small change of current produces a large motion of the coil. Instruments arranged
in this or analogous ways are suitable for ampere gauges, volt gauges, etc. which
require to measure one particular quantity very exactly, and also for voltmeters on
ship board, or other places in which gravity control is not permissible.
"
(7) The fourth part of our invention consists in applying a set up" device in
combination with a switch to instruments intended to be very sensitive over par-
ticular ranges. In completing the circuit the pointer, and the movable system
attached thereto, is "set up" and does not move further until the current through
the coil exceed a certain value. Breaking the circuit takes off the "set up," and
enables the zero of the instrument to be tested or adjusted (without disconnecting
the wires from the terminals). By using this device we are enabled to have the
coils in such a position when the pointer is at zero, that if a current were sent.
through the coil in the ordinary direction the coil would tend to move in the
wrong direction. But with our arrangement the act of completing the circuit
causes the "set up" device to bring the coils into such a position that the deflection
will be in the right direction when the current passes. The angle through which
the coil turns from zero is, therefore, increased, and hence a longer angular
distance over which the sensitive part of the scale can be spread, is obtainable.
Fig. 9 (Sheet 2) shows one of the methods we employ. There is a break in the
circuit between the parts h, and the direction of the current through the coil is.
such as would make it turn in a clockwise direction if the current were passed
when the coil was in the position shown. Completing the circuit by moving the
knob in the direction of the arrow displaces the coil so that its plane passes.
beyond the position perpendicular to NS; and the current then tends to turn the
coil in a counter-clockwise direction. The amount of "set up" does not alter the
meaning of the indications as is usually the case in "set up" instruments, for in
our arrangement the zero position is not altered by the setting up. In cases where
such instruments are to be used as ainmeters in which the whole current to be
measured passes through the coils, we arrange the switch so that, instead of the
act of taking off the "set up" breaking the circuit, it is caused to shortcircuit the
instrument
27
(8) The fifth part of our invention relates to movable coil instruments employed
as ballistic galvanometers. Ordinary d'Arsonval galvanometers with wide coils

page_text: embracing an iron core are not very sensitive as ballistic instruments because of
the large moment of inertia of the coils, and the possible range is not great, this
being limited by the support for the iron core. We obviate these defects by
employing narrow coils. At the same time the construction is simplified and the
instruments made more convenient to use. We make our narrow-coil-ballistic-
galvanometers so as to be portable, and to have very long ranges of one or more
complete revolutions. Their scales are sometimes graduated to read directly in
microcoulombs or other convenient units, and in other cases the scales are
graduated to show the change in the number of magnetic lines of force passing
through magnetic circuits such as those of dynamo machines when the current
passing round the magnet is stopped or changed in strength. In combination with
test coils arranged to turn suddenly through 180° or other convenient angle the
instruments can be used to measure magnetic fields. To facilitate reading we
sometimes provide arrangements for stopping the pointer at its greatest elongation.
In order to make our instruments also suitable for use as ammeters or volt-
meters, or both, we in some cases combine shunt or series resistances, or both,
with the instrument and put two or more graduations on the scale,-such
instruments can then be used either as portable ballistic galvanometers, magnetic
field or magnetic flux measurers and also as ammeters and voltmeters. One form
of instrument we employ as portable ballistic galvanometer and magnetic field and
flux measurer is shown in elevation and plan in Figs. 10 and 11 (Sheet 2). The
narrow coil a is suspended between the pole NS of a C shape permanent magnet
so that the plane of the coil is approximately parallel to the direction of the
magnetic lines of force when the pointer is at zero. It will be readily seen that
the pointer may move through one or more complete revolutions depending on the
impulse given to the coil by the transient current which passes through the coil.
The scale is shown graduated so that 200 parts correspond to one revolution, and
the number of turns in the coil and the strength of the magnet, or of the control,
or the relative position of coil and magnet may be arranged so that a quantity of
200 microcoulombs discharged through the coil produces one complete revolution.
By using a test coil having a total area of 5000 square centimetres and arranged
to turn through 180° in combination with such an instrument, the total resistance
of the circuit being 100 ohms, we obtain a direct reading magnetic field measurer.
The same instrument used in series with a test coil of one turn round may be
employed to measure changes in the magnetic flux passing through that coil; if
the total resistance of the circuit be 100 ohms the swings of the instrument
multiplied by 10,000 gives the changes of flux measured in C.O.S. units. We
would have it understood that we do not confine ourselves to instruments of the
particular range above described. In some cases we use two C shaped magnets
arranged with similar poles together instead of the one magnet shown in Fig. 11;
circular magnets are sometimes employed. For instruments intended to be used in
close proximity to dynamo, we arrange them so as to be astatic as described in the
first part of our invention.
(9) The sixth part of our invention relates to methods of constructing instru-
ments with very long ranges, and is particularly applicable to ammeters, voltmeters
and wattmeters. In carrying out this part we arrange the movable coils like the
armature of a dynamo or electro motor, with, or without an iron core, and the field
magnet of which is composed of a permanent magnet, or of a coil, or coils, with or
without iron. When a current passes through the armature, it turns and its
motion is resisted by a spring or springs or by other means. In such arrangements
the turning moment exerted by the armature depends on the product of the
strength of the magnetic field and the current through the armature. By using a
long spring as control, the rotations of an armature turning in a permanent
magnetic field will be approximately proportional to the current passing through
the armature; such instruments have nearly uniformly divided scales. Similar
scales are sometimes obtained by using as control a weight suspended by a
thin strip or fibre, wrapped on a cam as shown in Fig. 12, the cam being fixed to

page_text: the armature. By employing cams shaped as shown in Fig. 13 we make the
instruments very sensitive about a particular part of the scale. For example the
part of the cam ef, is nearly circular about the centre of the spindle, and hence a
considerable angle produces only a slight change in the controlling moment exerted
by the suspended weight W. The cams shown in Figs. 12 and 13 are suitable for
instruments whose deflections are limited to one revolution, but it will be readily
understood that by using long springs, or by replacing the cam by fusees deflections
of more than one revolution can be obtained. By suitably shaping the fusees we
make instruments having calibration curves of any desired nature.
In some cases we produce the magnetic field in which the armature turns by
electro magnets or by coils. Instruments arranged in the latter way are suitable
for long range wattmeters.
We would have it understood that we do not confine ourselves to the use of
electro motors having movable armatures and fixed magnets for we may arrange
the armatures to be stationary, and the field magnet to revolve without departing
from the essential nature of our invention.
Having thus particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed and carried into effect we
declare that what we claim is :-
1. The use of two or more movable coils with stationary permanent magnets as
shown in Figures 1, 2, 3, 4, 5, 6, and 7, and described in paragraphs 4, 5, and 8, or
any modification thereof, in order to obtain an astatic instrument.
2. Two or more movable coils with stationary permanent magnets as shown in
Figures 3, 4, 5, 6, and 7, and described in paragraph 5, or any equivalent arrange-
ment for obtaining great sensibility in one part of the scale.
3. The use of the arrangement of one or more movable coils, and one or more
stationary permanent magnets as shown in Figure 5, and described in paragraph 5,
for obtaining great sensibility in one part of the scale.
4. The use of one or more magnetic needles with their axes nearly perpendicular
to the plane of the coil or coils, as shown in Figure 8, and described in paragraph 6,
in the construction of an instrument with a control other than gravity so that
the controlling moment may vary approximately according to a sine law, and thus
producing an instrument which is very sensitive about a particular part of the
scale.
5. The employment of a "set up" device and switch as shown in Figure 9, and
described in paragraph 7, or any equivalent arrangement for enabling the zero of
an instrument to be in an otherwise impossible part of the scale thus increasing
the angular range of the pointer and obtaining great sensibility at a particular part
of the scale.
6. The construction of ballistic galvanometers of very long range in the manner
indicated in Figures 10 and 11 and described in paragraph 8.
7. The employment for a long range ammeter, voltmeter, or wattmeter, of an
electromotor having a movable or fixed armature with or without an iron core,
and a fixed or movable field magnet which may consist either of a permanent, or
of an electromagnet, or be simply a coil or coils, the motion of the movable part
being resisted by a force produced by springs, or by weights, combined with cams
or fusees, as shown in Figures 12 and 13, and described in paragraph 9, so as to
obtain controls varying according to any desired law as the one part of the motor
moves relatively to the other.
Dated thie 28th day of Wahruary 1803
W. E. AYRTON.
T. MATHER.