IMPROVEMENTS IN THE CONSTRUCTION AND METHODS OF MANUFACTURE OF ELECTRICAL AND OTHER MEASURING INSTRUMENTS
Year: 1883

page_text: LETTERS PATENT to William Edward Ayrton Professor of Technical Physics,
Fellow of the Royal Society, and John Perry Professor of Mechanical
Engineering and Applied Mathematics in the City and Guilds of London
Technical College, Finsbury, in the County of Middlesex, for an Invention of
IMPROVEMENTS IN THE CONSTRUCTION AND METHODS OF MANUFACTURE OF
ELECTRICAL AND OTHER MEASURING INSTRUMENTS,
PROVISIONAL SPECIFICATION left by the said William Edward Ayrton
and John Perry at the Office of the Commissioners of Patents on the
28th April 1883.
WILLIAM EDWARD AYRTON professor of Technical physics Fellow of the Royal
Society, and JOHN PERRY, Professor of Mechanical Engineering & applied
Mathematics in the City & Guilds of London Technical College Finsbury in the
County of Middlesex IMPROVEMENTS IN THE CONSTRUCTION AND METHODS OF
MANUFACTURE OF ELECTRICAL AND OTHER MEASURING INSTRUMENTS
In the construction of our well known Ammeters Voltmeters, Power Meters,
Ohmmeters and other Electrical Measuring Instruments, we have found that when
considerable numbers are manufactured it is very difficult to get workmen to wind
the parts which require to be wound with insulated wire with perfect certainty as
to the effects which will be produced when currents are made to flow through
these Coils. The difficulty is the greater as the wires used are of larger size.
In our pormanent magnet Ammeters and Voltmeters one important defect
has hitherto been the uncertainty as to the magnetic lines of force due to the coil
near' our needle being perpendicular to the position of the needle when no current
flows through the coil. In these instruments as hitherto constructed the bobbin
on which the coil is wound has been permanently fixed to the steel magnet or to
the soft iron pole pieces of the magnet. The first part of our invention consists in
making this attachment in such a way that after the instrument has been coiled

page_text: and fitted, the position of the magnetic axis of the coil may be adjusted; that is,
the bobbin may be adjusted in position without removing the dial of the instru-
ment or the pointer. Several methods may be taken to effect this object, but the
one we have found inost satisfactory, and prefer, cousists in one end of the bobbin
being fastened by one large screw, and the other end by the existing screws, all the
screws passing, however, through slots or holes larger than the scrows themselves.
In this way a small turning adjustment may be made of the bobbin sufficient to
enable us rapidly to remedy the evil which by the present method of construction
cannot be remedied without taking the instrument to picces and rewinding.
It is very necessary in our existing instruments, that when electrical quantities
are measured the angular deflections of our needles or moveable coils should be
proportional to the quantities being measured. This we call "the proportional
law" of our instruments. Our former method of construction was such thut unless
workmen followed the instructions very carefully they frequently had to wind
and unwind the coils of wire of these instruments several times before "the
proportional law" became true. If the coiling has been performed with a
reasonable amount of exactness we find that we can cause "the proportional law"
to hold without alteration in the winding and without taking the instrument to
pieces and we effect this object by adjusting the positions of pieces of soft iron in
the magnetic field produced by the coil. Thus in our Ammeter we use an adjust-
able piece of soft iron in the bobbin, inside the coils of wire. In the common form
of our instrument we find that a soft iron screw lying parallel to the axis of the
bobbin and screwed nearer to or farther from the needle effects the purpose, and
it is easy to perform the adjustment when the instrument is in use. In other
forms of our Ammeter described at a meeting of the Society of Telegraph Engineers
and of Electricians held on May 11th 1882 we now use one or more such screws
or a flat piece of soft iron in the centre of the bobbin whose position may be
adjusted by a screw or screws for the purpose of effecting our object. This the
second part of our Invention is applicable to the fixed coils of all our Instruments,
but we prefer to use it only for such coils as are of small resistance.
We are aware that iron has been introduced into coils of wire used as measuring
Instruments in fact that electro-magnets instead of solenoids have been employed
but this has always been for a totally different object viz to increase the sensibility
of the galvanometer whereas our object is to make the proportional law true by
introducing an adjustable piece of iron into the coil.
The third part of our invention consists in the use of one or two adjusting
screws of soft iron passing through one or both of the soft iron pole pieces of the
permanent magnet of such of our Instruments as require the use of permanent
magnets. The end of each screw becomes the end of the real pole piece which
affects the needle most, and by turning the screw we are able to vary the strength
of the magnetic field so that without taking an instrument to pieces we can alter
somewhat the sensibility of the instrument. It is very important to effect this
object, and by the old construction of our instrument it could only be effected by
increasing or diminishing the strength of the permanent magnet by a definite
amount found out by a long series of trials which is an extremely difficult operation
to effect, or by adding to, or subtracting from, the number of convolutions by trial
which also is a very troublesome and lengthy experiment. We are now able in
our Ammeters to arrange that one degree of deflection shall correspond with 1, 2, 3
or any exact number of amperes, and generally in our measuring instruments, that
one degree of deflection shall correspond with an exact whole number of units of
the quantity to be measured. Whereas hitherto the difficulty of making one degree
of deflection correspond with an exact whole number of the quantity to be measured
has been regarded as so great that it has been necessary for the users of such
instruments to be contented to use multipliers with two or three significant figures
like 2-76 amperes, or volts, per degree
The above-mentioned adjustments may at once be applied to our instruments as
hitherto constructed and sold. But in a new Ammeter and Voltmeter which we

page_text: have constructed the iron pole pieces of the permanent magnet are themselves
adjustable being held in any desired position by a pinching screw; and this adjust-
ment of the whole pole piece thus takes the place of the third adjustment mentioned
above
The fourth part of our Invention relates to an entirely new method of con-
struction in Measuring Instruments generally, and it is not only applicable to
Instruments for measuring electric currents as Ammeters and Voltmeters and
Electric power Meters, but it is applicable to any Instruments where force has to
be measured, as in finding the weights of bodies and in measuring the attractive
and repulsive forces between electrified bodies.
In the use of an ordinary spiral spring as in the Spring Balance, the various
coils are nearly at right angles to the axis of the spring, that is the angle of the
spiral, which is the angle between the osculating plane of the central curve of the
wire or. bar forming the spring and the plane at right angles to the axis of the
spring is usually very small, and hence, when the weight is hung from such a
spring or when any force is employed to lengthen it, there is only a very small
tendency for one end of the spring to rotate relatively to the other. Now it has
long been known to mathematicians that when the angle of a spiral becomes great,
and especially when that angle is 45°, the spring not only gets longer or shorter
when axial forces are applied, but there tends to be a relative rotation between the
ends of the spring. This principle has hitherto been unused in the construction of
Machines and Instruments because, in the first place it has not been generally
known, and in the second place it is only under certain circumstances, which we
have discovered by means of calculation and experiment, that the rotation becomes
considerable in comparison with the lengthening or shortening of the spring and
with the force causing it. The phenomenon is not very striking when the wire
which forms the spring is circular in section. The section should be preferably
elliptical or rectangular so that the dimensions in one direction are very much
greater than the dimensions at right angles to the first. We find that when a
strip of elastic material, such as brass, is wrapped round a cylindric or conical coro
and allowed to take a permanent set so that when it is removed the angle of the
spiral is about 45°, lengthening or shortening by axial force is accompanied by a
considerable relative rotation of the ends of the spring, and if one of these ends is
fixed, the other being provided with a pointer capable of moving alongside a
cylindric scale, or in the neighbourhood of a spiral scale, the motion of the end of
the pointer may be made to represent, with any amount of magnification which
may be thought necessary, the lengthening or shortening of the spring by axial
forces
In this form our Invention is immediately available for use in Spring Balances.
The weight hung at the end of the spring causes rotational motion of the pointer
over its scale
Our Invention is applied to Ammeters and Voltmeters with the use of a cylindric
bobbin of wire, preferably of considerable length in comparison with its diameter.
The magnetic action of a current flowing in this wire is allowed to act in sucking
into the inner space a cylindric or conical piece of soft iron or soft iron tube to
whose upper end a pointer is fastened. It is not necessary to describo our method
of guiding this piece of soft iron so that it shall not move laterally but only in the
direction of, and round about the axis of the coil. It is carried by one or more
spiral springs, such as we have described, made of strips of brass or palladium or
non magnetic matter and in some cases of soft iron or other magnetic material.
Sometimes the soft iron is wholly inside the springs, and in other cases it is hollow
and surrounds the springs. The upper end of the spring arrangement is fixed
relatively to the coil so that when the soft iron is sucked into the coil the lower
end of the spring and the soft iron, rotate relatively to the coil, and the pointer
shows on a cylindrical or spiral scale the amount of the attractive force.
When our Invention is applied to an Electric-Power-meter the soft iron piece is
replaced by a coil of fine wire of considerable resistance. When currents flow in

page_text: the fixed and moveable coils, the attraction which causes the fine coil to be sucked
into the other may be made proportional to the product of the currents in the two
coils, so that the deflection of the pointer may be made to represent the amount of
this product. If the fine wire coil has its ends attached to the terminals of a
machine or lamp or other recipient of electrical power, and, if the main current
flows through the fixed coil, then the power supplied may be measured by the
motion of the pointer over the scale. The same construction is adopted when our
Invention is applied to an Electro-Dynamometer.
We have already in many other ways employed our Invention, which essentially
consists in a simple method of measuring the amount of motion in a certain
direction as a rotation about that direction.
Instead of using the above-mentioned spiral spring method, we sometimes employ
for the same purpose a combination of an ordinary spiral spring and bifilar suspen-
sion, that is, a body is suspended by an ordinary spiral spring of small angle and
by two threads which in their normal position are not in the same plane, but are
such that a vertical force tending to elongate the spring tends to bring the threads
into the same plane with one another. When force is applied to elongate the spring
it is found that the clongation is accompanied by a rotation, and this effect wo
employ like the similar effect already explained for our spiral springs in the
construction of the above mentioned and other Mensuring Instruments
The fifth part of our Invention relates to a method of using our multireflex.
principle published in La Lumière Electrique in September 1881. in any Instrument
in which it is necessary to measure the small angular deflection of a needle or a
coil. In that publication a focussed beam of light after multi-reflexion falls upon
a white screen where the white spot represents the motion with great magnifica-
tion. Our present method of employing the principle enables us to dispense with
a focussed beam of light. We place upon the object whose angular motion is to be
measured, a sinall mirror. If the object is metallic we make of its own surface a
mirror. In front of this is placed a a fixed mirror at a convenient angle, and a
white object, or a mark on a white object, is seen by an observer's eye after two or
more reflections from the two mirrors. Beyond the edge of the fixed mirror is
fixed a scale at such a distance & of such a curvature that the virtual image of the
white object is always at the same part of the scale; the observer can at once
detect any inaccuracy in position of the scale by noticing if the image inoves
relatively to the scale when he moves his eye. If the moveable mirror moves, the
observer sees the image move along the scale as if it were the point of a needle
quite close to the scale, and, wherever his oye may be, if he sees the image at all,
he sees it in its proper position relatively to the scale.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said William Edward Ayrton and John Perry in the Great Seal Patent
Office on the 27th October 1883.
WILLIAM EDWARD AYRTON, Professor of Technical Physics, Fellow of the Royal
Society, and JOHN PERRY Professor of Mechanical Engineering and Applied
Mathematics in the City & Guilds of London Technical College Finsbury, in the
County of Middlesex. IMPROVEMENTS IN THE CONSTRUCTION, AND METHODS OF
MANUFACTURE OF ELECTRICAL MEASURING INSTRUMENTS.
In the construction of our well-known Ammeters, Voltmeters Power Meters,
Ohmmeters, and other Electrical Measuring Instruments we have found that when
considerable numbers are manufactured it is very difficult to get workmen to
wind the parts which require to be wound with insulated wire, with perfect
certainty as to the effects which will be produced when currents are made to pass
through these coils. The difficulty is the greater as the wires used are of larger
size. In our permanent magnet Ammeters and Voltmeters one important defect lias
hitherto been the uncertainty as to the magnetic lines of force due to the coil near
our needle being perpendicular to the position of the needle when no current flows
through the coil. In these Instruments as hitherto constructed the bobbin A™ A
figures 1 and 2, 4 and 5, on which the coil is wound has been permanently fixed
to the steel magnet or to the soft iron pole pieces of the magnet. The first part of
our Invention consists in making this attachment in such a way that after the
Instrument has been coiled and fitted, the position of the magnetic axis of the coil
may be adjusted, that is the bobbin may be adjusted in position without removing
the dial and pointer of the Instrument. Several methods may be taken to effect
this object. Thus in figures 1 and 2 bolts are shown passing through slotted holes
in the bobbin and fastened underneath, to the case of the Instrument with nuta.
Or there may be screws passing through slotted holes in the bobbin, and screwed
into the wood from the top with an ordinary wood screw. In both these cases tho
dial must be removed to make the adjustment. Or the bobbin may be screwed to
a bridge B figure 5 in the centre of which is a hole in which fits a boss cast on a
piece of brass C figures 4 and 5 screwed to the magnet or soft iron pole pieces, and
which works on the boss as a centre, thus allowing the bridge and bobbin to be
adjusted and clamped in any position by a screw passing through a slottod hole in
the bridge and screwed into the brass. This can be done without removing tho
dial and pointer. In any of these ways a small turning adjustment may be made
of the bobbin sufficient to enable us rapidly to remedy the evil which, by
the previous method of construction, cannot be remedied without taking the
Instrument to pieces and rewinding. Figure 4 is a plan of the Instrument from
the back, with the back removed. Figure 5 is a section through the centre of the
Instrument. In these figures A A is the bobbin on which wire is wound, B B an
iron bridge screwed to the flanges of the bobbin. At the centre g of the bridge is a
hole in which a boss cast on the piece of brass C fits. The bridge and bobbin
revolve on the boss as a centre and are clamped tightly in any position by the
screw e which passes through a slotted hole in the bridge and screws into the brass.
piece C. The piece C is screwed to the soft iron pole pieces of the magnet M.
It is very necessary in our existing Instruments that when electrical quantities are
measured, the angular deflections of our needles or moveable coils should be propor-
tional to the quantities being measured. This we call the 'proportional law of
our Instruments. Our former method of construction was such that unless work-
men followed the instructions very carefully, they frequently had to, wind and

page_text: '
unwind the coils of wire of these Instruments several times before the 'propor-
tional law became truc. If the coiling has been performed with a reasonable
amount of exactness we find that we can cause the proportional law to hold
without alteration in the winding and without taking the Instrument to pieces,
and we effect this object by adjusting the positions of pieces of soft iron in the
magnetic field produced by the coil. Thus in our Ammeter and Voltmeter we use
an adjustable piece of soft iron in the bobbin, inside the coils of wire. In the
permanent-magnet form of our Instruments, specimens of which are shown in
figures 1-6 we find that a soft iron screw lying parallel to the axis of the bobbin
and screwed nearer or further from the needle effects the purpose, and it is easy
to perform the adjustinent when the Instrument is in use. In other forms of our
Ammeter and Voltmeter described at a meeting of "The Society of Telegraph
Engineers and Electricians" held on May 11th 1882 we now use one or more such
screws or a flat piece of soft iron in the centre of the bobbin, whose position may
be adjusted by a scrow or screws for the purpose of effecting our object. Sections
of the Instruments are shown in figures 9 and 10 and it will be seen that we have
effected a further improvement in the employment of a ring of soft iron B which
cncloses a bobbin. The adjusting screws already described to give proportionality
are shown at F in the drawing; there are four on each side. It will be seen that
this ring very much increases the magnetic field produced by the wire wound on
the bobbin. In figure 11 the four screws on each side are shown replaced by flat
pieces of iron G G which are adjustable in position. The Instrument shown in
figures 9. 10. and 11. has the needle controlled by a spring, instead of by a
permanent magnet. And the bobbins of all our Instruments aro generally of the
shapes shown in in figures 1 to 6, or in figures 9 to 11. This the second part of
our Invention is applicable to the fixed coils of all our Instruments but we prefer
to use it only for such coils as are of small resistance. We are aware that iron has
been introduced into coils of wire used as Measuring Instruments, in fact that
electromagnets instead of solenoids have been employed, but this has always been
for a totally different object, namely to increase the sensibility of the galvanometer,
whereas our object is to make the proportional law' true by introducing an
adjustable piece of iron into the coils. The third part of our Invention consists in
the use of one or two adjusting screws of soft iron passing through one or both of
the soft iron pole pieces of the permanent magnets of such of our Instruments as
require the use of permanent magnets as shown at H H figure 7. The end of
ench screw becomes the end of the real pole piece which effects the needle most and
by turning the screw we are able to vary the strength of the magnetic field, so that
without taking the Instrument to pieces we can alter considerably the sensibility
of the Instrument. It is very important to effect this object, and by the old
construction of our Instruments it could only be effected by increasing or
diminishing the strength of the permanent magnet by a definite amount, found out
by a long series of trials which is an extremely difficult operation to effect, or by
adding or subtracting from the number of convolutions by trial which also is a
very troublesome and lengthy experiment. We are now able to arrange in our
Ammeters that one division of the deflection shall correspond with 1, 2, 3 or any
exact number of Ampères, and in our Voltmeters that one division of deflection
shall correspond with 1, 2, 3 or any exact number of volts; and generally in our
Measuring Instruments, that one division of deflection shall correspond with an
exact whole number of units of the quantity to be measured. Whereas hitherto
the difficulty of making one division of the deflection correspond with an exact
whole number of the quantity to be measured has been regarded as so great that
it has been necessary for the users of these Instruments to be contented to use
multipliers with two or three significant figures like 2-76 ampères or volts per
division. The above-mentioned adjustments may be at once applied to our
Instruments as hitherto constructed and sold. Another way in which we effect the
increase or diminution of the strength of the magnetic field is by having the pole
pieces of the magnets themselves adjustable in position. Thus in figure 1 the pole
'

page_text: pieces P P of soft iron have slots in them so that although they are tightly
fastened to the ends of the magnet M M, on slightly loosening the nuts K K the
pole pieces may be brought nearer together or taken further apart thus producing
the necessary change of strength. Again in figure 12 the magnet and pole pieces
are moveable with regard to one another, thus a relative motion of the magnet in the
direction of the arrow will make the field stronger by bringing the magnet nearer
the pole pieces, whereas a motion in the opposite direction will weaken the field.
Description of the Instruments-As is well known our Ammeters, Voltmeters and
other Instruments are sometimes provided with a commutator, a new arrangement
of which is shown in figures 4, 5 and 6. In other particulars except in the use of
three (S T and U) instead of two binding screws, and in the resistance coil V
these figures may be taken as representing the Non-commutator form of
Instrument. Figure 4 is a plan of the Instrument from the back with bottom
piece of wood removed. Figure 5 is a section through the centre longitudinally,
and figure 6 is a plan, from the top, of the Instrument fitted together. M M is a
permanent horse-shoe magnet and consists of two plates of the same size and kept
apart by the pole pieces P P fitted between the ends and held in place by
screws passing through both plates and the pole pieces. The brass bobbin A. A
on which insulated wire is wound is screwed to the metal bridge B B and is bored
from the ends to the centre for soft iron cores to be introduced. The needle N fits
in the centre of the bobbin, and the spindle, to which the needle is firmly fixed,
works in jewel holes one at the bottom and the other above the bobbin.
It will be seen from figure 13 which shows a cross section of the bobbin
through the centre of the needle that the bridge pieces I I both of which go at right
angles to the axis of the bobbin hold the top and bottom jewels. The supports J J
are cast on the bobbin and two thin sheets of metal are soldered on to the ends
of J J to guide the wire. This enables us to have sufficient space to withdraw the
needle from the bobbin at any time without unwinding--a very great improvement
on our previous construction of this part of the Instrument. The pointer O is fixed
to the spindle of the needle at right angles to it, and deflects over the circular
dial d on which is engraved a suitable scale, and which rests on the frame of the
Instrument. Above the dial is the circular brass bezil bin which is fitted a
circular piece of plate glass to protect the dial. The iron bridge B B which is
screwed on to the flanges of the bobbin A A has a hole at its centre into which fits
a boss cast on the piece of brass C C which is screwed to the pole pieces of the
magnet, and also to the frame of the Instrument. A screw passes through a slotted
hole in the bridge and screws into CC so as to clamp the hobbin and bridge in
any position. There are holes bored and screwed in the bridge in which screw the
iron cores F F which are introduced into the bobbin. An armature a a is fitted
on to the ends of the pole pieces and passes through the top of the Instrument.
It is well known to Electrical Engineers, that in the case of our Non-Commutator
Instruments, the wires are passed straight from the bobbin to the two terminals
T S, and that in the case of our Commutator Instruments, the wires are passed
separately from the bobbin through the wood at W to the Commutator. The
Commutator shown in our new Instrument figures 4, 5, and 6 does not in principle
differ from, and is for the same object, as the Commutators hitherto constructed
and invented by us, but it will be observed that the strips of brass through which
the contacts are mado at the two sides of the cylindrical part R R now go up from
the under side of F F which is of wood or ebonite or other such material, and
forms the strong part of the Instrument, to which nearly everything is fastened.
It will be observed by the drawing that the position and construction of the
Commutator enable the connections to be made much more cheaply and easily
than in our old Instruments. The construction of Instruments like that shown in
figures 1, 2 and 3 will be easily understood. In these the pole pieces P P are
moveable as already described, the adjustable cores and method of fastening to the
hase have already been described, and the construction will readily be understood
from the drawings. It will be noticed that in the case of Instruments such as that
•

page_text: shown in figure 1 and also as shown in figure 8 we sometimes used double-pointed
pole pieces to the magnet. This enables us to dispenso with the winding of tho
wire near the centre of the bobbin and so allows, not merely tho needle to bo
removed with great facility but also the bottom jewel to be taken off and
examined without the necessity of unwinding the bobbin. The ellipse shown at
Z Z figure 13 is the elliptic section of the brass bobbin, of double the size shown
in figure 4, and N is a soft iron needle and pivotted to it top and bottom, by
means of the spindle X which passes through the needle and is screwed to it.
This figure also shows the way in which the pointer is attached to this spindle X
there is a piece of brass attached which is soldered to the spindle X which is
curved on its upper surface as shown. The loose piece W is screwed to V by
ineans of two little screws; and the pointer, which has a corrugation all along its
length, as is well known to Electricians who have used our Instruments, is firmly
caught between W and Y. The fastening of this spindle has hitherto given rise to
very considerable difficulty on account of its liability to change in position
relatively to the needle. This method of fastening wo consider a very great
improvement. The fourth part of our Invention relates to an entirely new method
of construction in Measuring Instruments generally, and it is not only applicable to
Instruments for measuring electric currents as Ammeters, Voltmeters and Electric
Power Meters, but it is applicable to any Instrument where force has to be
measured as in Weighing Machines, Pressure and Vacuum Gauges, Dynamometers,
Electrometers etc. In the use of an ordinary spiral spring as in the Spring Balance
the various coils are nearly at right angles to the axis of the spring; that is the
angle of the spiral which is the angle between' the osculating plane of the central
curve of the wire or bar forming the spring, and the plane at right angles to the
axis of the spring, is usually very small and hence when the weight is hung from
such a spring or when the force is applied to lengthen it, there is only a very small
tendency for one end of the spring to rotate relatively to the other. Now it has
long been known to Mathematicians that when the angle of a spiral becomes great,
and especially when that anglo is 45°, the spring not only gets longer or shorter
when axial forces are applied, but there tends to be a relative rotation between
the ends of the spring. This principle has hitherto been unused in the construction
of Machines and Instruments because, in the first place, it has not been generally
known, and in the second place, it is only under certain circumstances which
wo have discovered by calculation and experiment that the rotation becomes
considerable in comparison with the lengthening or shortening of the spring and
with the force causing it. The phenomenon is not very striking when the wire
which forms the spring is circular in section. The section should be preferably
elliptical or rectangular so that the dimensions in one direction are very much
greater than the dimensions at right angles to the first. We find that when a strip
of elastic material such as brass is wrapped round a cylindric or conical core and
allowed to take a permanent set so that when it is removed the angle of the spiral
is about 45°, lengthoning or shortening by axial forces is accompanied by a
considerable relative rotation of the ends of the spring, and if one of these ends
is fixed, the other being provided with a pointer capable of moving alongside
a cylindric or other scale (or in the neighbourhood of a spiral scale) the motion of
the end of the pointor may be made to represent, with any amount of magnification
which may be thought necessary, the lengthening or shortening of the spring by
axial forces, whether the axis of the wire or strip is made to be in a spiral on a
cylindrical surface or a conical or other surface of revolution, and however the
section of the strip may lie in position relatively to the general axis of the
spiral The spring here described is called by us our "Magnifying Spring." In
this form our Invention is immediately available for use in Spring Balances.
The weight hung at the end of the spring causes rotational motion of the
pointer over the scale. Our Invention is applied to Ammeters and Voltmeters
with the uso of a cylindric bobbin of wire, preferably of considerable length in
comparison with its diameter. The magnetic action of a current flowing in this

page_text: wire is allowed to act in sucking into the inner space a cylindric or conical piece
of soft iron or soft iron tube, to which a pointer is fastened. The piece of
soft iron is carried by one or more spiral springs such as we have described,
made of silver or steel or any suitable metal. Sometimes the soft iron is wholly
inside the spring, and in other cases it is hollow and surrounds the spring. In
some cases the spring is of soft iron and is itself attracted so that the presence of
any other core of iron may be dispensed with. The upper end of the spring
arrangement is fixed relatively to the coil so that when the soft iron is sucked into
the coil the lower end of the spring and the soft iron rotate relatively to the coil,
and the pointer shows on a scale the amount of the attractive force. In figure 14
the electro-magnet consists of a hollow core part of which B B is soft iron and
part D E of brass or other non-magnetic metal. The outside tube CC and the
plates XX top and bottom are also of soft iron. The space F F is filled with
insulated wire in electric connection with the terminals so that when a current is
sent through the Instrument an intense magnetic field is formed between D and E
which are the poles of the electro magnet. To the soft iron tube A A the spiral
spring as above described is attached, the other end being attached to the piece L
which is fixed relatively to the bobbin. The spindle G G which is fixed to the
moving iron core A A moves freely in bearings H H so that the only movements
of which A is capable are, one of rotation, and one parallel to the axis of the
bobbin. As the iron core A projects into the strong magnetic field between D
and E it is strongly attracted towards E when the current is made. This attraction
on A is a measure of the current passing through the coil of wire. The attraction
causes a slight elongation of the spiral spring which is accompanied as hereinbefore.
described by a large rotation which is shown by a pointer P over a scale, which
may be cylindric and carried ou the cylindric surface Y or flat as Z Z with a
mirror W W to avoid errors by parallax. As a means of varying the power of
the Instrument an adjustable iron pieco K is provided which can be screwed
nearer to or farther from the core A. And in addition on account of the great
simplicity of construction it is easy to add or remove some of the convolutions of
wire and so vary at will the sensibility; and in consequence of this facility of
adjustment we are able to make each division of the scale correspond with an
exact number of ampères, volts, or other quantity to be measured, and thus
entirely avoid the use of troublesome multipliers like '1-34 ampères per division.'
Screw M is a stop by which the spindle is stopped when it has moved the
maximum distance desired. This design is applicable equally as an Ammeter and
as a Voltmeter, the only difference being that the former is wound with a thick
wire, and the latter with a very fine wire having a resistance of several hundred.
ohms. This is our Non-commutator Instrument,' On the other hand we wind
our coil with a number of separate strands, and connect to the contact-pieces of
a commutator of some such shape as that shown in figures 4, 5 and 6 when a
Commutator Instrument' is desired. The position of this commutator may be on
the wooden base of the Instrument, or it may be placed vertically with its ends
against the base and underside of the upper plate X. In figure 15 is a variety of
adjusting-piece K where it is hollow, having the boro large enough to admit the
moving core A figure 16. Instead of having a spindle inside for the purpose of
guiding as in figure 14 there is here employed a very thin brass tube g g outside
it so that it will very much resist the bending due to any lateral attraction. In
figure 17 F F is a simple solenoid with brass frame. In this caso the iron core
attracted is a long tube A A projecting into the solenoid a certain distance which
we have determined. The spring is wholly inside the tube and has its lower
end attached to the tube at G which is in rigid connection with the frame through
the glass plate. The method of guiding can easily be seen from the figure. The
tube is made of very soft iron, and is also very thin so that when, the current
is put on it reaches the point of magnetic saturation when only a very small
current is flowing so that during nearly the entire movement the attraction
on the iron, and therefore the deflections, of the pointer are proportional to the

page_text: current flowing; and in the case of the Voltmeter to the difference of potentials
between the terminals which it is required to measure. In figure 18 is shown an
arrangement with the iron core A A of a conical shape and inside the spiral spring.
In figure 19 is a similar arrangement, but here the core is cylindric instead of
conical. When our Invention is applied to an Electric Powermeter the soft iron
When currents
core is replaced by a coil of fine wire of considerable resistance.
flow in the fixed and moveable coils, the attraction which causes the fine wire coil
to be sucked into the other may be made proportional to the product of the
currents in the two coils so that the deflection of the pointer may be made to
represent the amount of this product. If the fine wire coil has its ends attached
to the terminals of a Machine, or Lamp, or other recipient of electrical power, and
if the main current flows through the fixed coil then the power supplied may be
measured by the motion of the pointer over the scale. The same construction is
We have
adopted when our Invention is applied to an Electro-dynamometer.
already in many other ways employed our Invention, which consists essentially
in a simple method of measuring the amount of motion in a certain direction, as a
rotation about that direction; and hence a small linear motion may be magnified
into a much greater linear motion at the end of an arm, without the use of toothed
wheels or any of the other well-known forms of multiplying gear which necessarily
are accompanied by serious friction. Instead of employing the above spiral spring
method we sometimes employ, for the same purpose, a combination of an ordinary
spiral spring and bifilar suspension. That is, a body is suspended by an ordinary
spiral spring of small angle and by two threads which, in their normal position,
are not in the same plane, but are such that a vertical force tending to elongate
the spring tends to bring the threads into the same plane with one another. When
force is applied to elongato the spring it is found that the elongation is accompanied
by a rotation, and this offect we employ like the similar effect already explained
for our spiral springs in the construction of the above-mentioned and other
Measuring Instruments. The fifth part of our Invention relates to a method of
using our multi-reflex principle published in "La Lumière Electrique" in Sep-
tember 1881, in any Instrument in which it is necessary to measure the small
angular deflection of a needle or a coil. In that publication .a focussed beam of
light after multi-reflection falls upon a white screen where the spot of light repre-
sents the motion with great magnification. Our present method of employing the
principle enables us to dispense with a focussed beam of light. We place upon the
object whose augular motion is to be measured a small mirror. In front of this is
placed a fixed mirror at a convenient angle, and a white object or a mark on a
white object is seen by an observer's eye, after two or more reflections from the
two mirrors. Beyond the edge of the fixed mirror is fixed a scale at such a
distance and of such a curvature that the virtual images of the white object is
always at some part of the scale. The observer can at once detect any inaccuracy
in the position of the scale by noticing if the image moves relatively to
the scale when he moves his eye. If the moveable mirror moves, the
'observer sees the image move along the scale as if it were the point of a
needle quite close to the scale, and wherever his eye may be, if he sees
the image at all, he sees it in its proper position relatively to the scale.
Having thus described the nature of our Invention and the manner of performing
the same we would have it understood that we claim (1) The use of adjustable soft
iron cores in the bobbins of our Ammeter, Voltmeter and other Instruments, as
shown at F in figures 2, 4, 5, 9, 10 and in G figure 11, for the purpose of obtaining
our proportional law' as already described (2) A combination of a soft iron ring B
with the cores F shown in figures 9 and 10 in our well known spring Instruments.
(3) The use of adjustable pole pieces P P with the permanent magnets of our
Amineters and Voltmeters as shown in figure 1, to modify the sensibility of the
Instrument. (4) The use of soft iron adjusting screws H H shown in figure 7 for
the purpose of modifying the sensibility of our Ammeters and Voltmeters. (5) The
use of the arrangequent shown in figure 12 whereby the permanent magnet is

page_text: moveable relatively to the pole pieces P P, substantially as described, for the
purpose of modifying the sensibility of our Ammeters and Voltmeters. (6) The
arrangement of the keeper a a figure 4 for the permanent magnet, the holes for
which are shown in a a figure 6, the keeper being withdrawable by a motion
at right angles to the face of the Instrument (7) The use, in our Ammeters and
Voltmeters, of pole pieces such as those shown in figures 1 and 8, whose ends are
concave to the needle, for the purpose of obtaining the proportional law, without
winding wire on the bobbin near the needle. (8) The use, in our Ammeters and
Voltmeters, of a soft iron bridge piece connecting the two ends of the bobbiu,
substantially as described and shown in B figure 5. (9) The method of attachment
of the pointer to the needle spindle, substantially as described and shown in
figure 13. (10) The arrangement of the brass bobbin near its middle with thin
sheets of metal as guards, which enables us to withdraw our needle without
unwinding the wire on the bobbin, substantially as described and shown in figures 5
and 13. (11) The arrangement of the contact pieces of the commutator shown
at R R figure 5 whereby they come up through the wood or ebonite base, being
fastened to the underside, thus enabling connections to be made without bringing
wires through this base. (12) The general arrangement of the parts of the Instru-
ment shown in figures 1, 2 and 3. (13) The general arrangement of the parts of
the Instrument shown in figures 4, 5 and 6. (14) The use of our 'Magnifying
Spring in converting motion in the direction of the axis of the spiral into a
rotation about that axis in Measuring Instruments substantially as described.
(15) The use of our Magnifying Spring' in a Weighing Machine, substantially as
described. (16) The use of our Magnifying Spring' in such shapes as are shown
in figures 14, 16, 17, 18 and 19 in converting the attraction of a solenoid on a core
of soft iron or on another coil of wire, into the rotatory motion of a pointer such
as P in the above-mentioned figures. (17) The general arrangement and construc-
tion of the Instrument shown in figure 14, substantially as described. (18) The
general arrangement and construction of the Instrument shown in figure 14 with
the modification shown in figure 16, substantially as described. (19) The applica-
tion of our multi-reflex principle to Measuring Instruments, substantially as
described. (20) The general arrangement and construction of the Instrument shown
in figure 14, with the modification shown in figure 15, substantially as described
(21) The general arrangement and construction of the Instrument shown in
figure 17, with the possible modifications shown in figures 18 and 19, substantially
as described. In Witness whereof 1 the said William Edward Ayrton, have
hereunto on my own behalf, and on behalf of John Perry set my hand and seal
this twenty seventh day of October One thousand eight hundred and eighty
three
'
WILLIAM EDWARD AYRTON (L.S.)