IMPROVEMENTS IN APPARATUS FOR MEASURING ELECTRIC CURRENTS
Year: 1884

page_text: I, Sir WILLIAM THOMSON Knight, of Glasgow College, Doctor of Laws and
Professor of Natural Philosophy in the University and College of Glasgow, in the
County of Lanark, North Britain do hereby declare the nature of the said invention
for "IMPROVEMENTS IN APPARATUS FOR MEASURING ELECTRIC CURRENTS" to be
as follows:-
Having by mathematical reasoning been led to expect that a mass of soft iron,
if spherical or of an elongated shape, provided the length bears not too great a
proportion to the breadth or to the shortest diameter, experiences forces in a
magnetic field which are but little affected by residual magnetism, and which are
very nearly proportional to the rate per unit of distance at which the square of the
intensity of the magnetic force of the field varies from place to place in any
direction; I have devised the following means of employing this mathematical
conclusion for the measurements of electric currents.-I make a conductor of
electricity of such shape and such form, as to give a magnetic field with suitable
variation of the intensity of its magnetic force, in a space through which a small
piece of soft iron can travel while properly supported for measuring the force
required to balance it in any position. -The most suitable distribution of the
electro-magnetic force for most purposes is when the square of its magnitude varies
directly as the distance from a point in the prolongation of the line travelled by
the piece of soft iron; because this law makes the force experienced by the piece
of soft iron independent of its position in the field. This condition may be fulfilled
by trial with any desired accuracy by taking a single flat conductor, or a flat
conductor doubled back upon itself, and cutting it to different thicknesses in
different parts in the neighbourhood of the space through which the soft iron is to
move, the line of motion being approximately parallel to the line of the current.—But
I prefer for most purposes to make the line of motion approximately perpendicular
to the current. When the current to be measured is so small that a moderately
fine wire or flexible insulated conductor of considerable length can be used, the
easiest way of producing a field to fulfil the desired condition is by winding the
conductor into the space between a cylinder and a truncated conoid of slightly
concave meridional section towards the thick end.
For more powerful currents I use a ribbon of copper cut to a taper form,
narrower at one end than the other, and wind this up either with the narrow end
or the broad end inwards, and thus obtain a coil with a cylindrical and a tapering
boundary fulfilling the required conditions. For very powerful currents, for
example of more than 100 ampères I use a thick broad ribband, or flat bar, of
copper bent round upon itself so that the current is in opposite directions on the
two sides of a space of sufficient width between the two parts of the bar to allow
a piece of soft iron of convenient magnitude to travel. And to give the proper
variation of the electro-magnetic force throughout the space traversed by the soft
iron, I make the ribband, or bar, of copper thicker toward one edge and thinner

page_text: toward the other.-The instrument is suitable for measuring either continuous or
alternate currents, but for alternate currents the bobbin must be of insulating
material, and for this purpose I prefer to make, the tube several rounds of paper
hardened by shellac varnish or other suitable cement, with ends of vulcanite or other
convenient insulating material.-When the instrument is designed for measuring
continuous currents I enclose the bobbin within a soft iron cylinder so as to nearly
enough annul the influence on the interior magnetic field of terrestrial magnetic
force, or magnetic forces, due to external magnets or electric machines in the
neighbourhood.
When the instrument is to be used for measuring alternate currents, instead of a
continuous ball or cylinder of soft iron, I use a number of thin plane plates of soft
iron, or a bundle of thin soft iron square or round wires put together in the proper
shape and form and insulated from each other by very thin paper or other
insulating material; or thin ferrotype iron rolled with an intervening layer of very
thin insulating material, into the form of a short thick cylinder or other convenient
form.
When the instrument is designed for use on land the method which I prefer for
supporting the soft iron is to hang it on one end of a balance arm properly counter-
poised so that the virtual centre of gravity of the whole is as nearly as possible
in the line of the bearing knife edges, and the arrangement constitutes a balance
which will rest in any position through a range of five or ten degrees or more on
either side of its middle position.-To counterbalance the action of the electro-
magnetic force I hang a weight on a sharp-edged hook attached to the arm or on a
hook attached to the bottom of the piece of soft iron, (this last arrangement being
convenient when the electro-magnetic force is upwards). The magnitude of this
weight is such that the balance rests in its middle position with any desired degree
of current. The weight is varied according to the square of the strength of the
medium current in the use that is made of the instrument.-For standardising
tests the bearing point of the hook on which the weight is hung should be exactly
or very nearly in the plane of the two bearing points of the main knife edges of
the balance and the point from which the soft iron is hung.-For inspectioual use
in ordinary practice the bearing point of the weight should be sufficiently far below
this plane to give whatever degree of stability is desired: and thus with one and
the same weight, currents deviating by 10 or 20 per cent or more above or below
the magnitude of that which balances the instrument in its middle position are
measured with sufficient approximation for most practical purposes by means of a
pointer attached to the balance and a scale of divisions graduated to correspond to
stated fractions: for example each division to correspond to one per cent, of the
current which gives the instrument its middle position. This convenient condition
is fulfilled by the same scale, whatever be the weight which is hung on the balance.
When the instrument is to be available for use at sea I make it with two soft iron
masses, moving each in a suitable magnetic field, the two magnetic fields being so
situated relatively to each other that the soft iron masses may be supported one on
each end of a balanced lever such as is hereinbefore described for supporting one
mass of soft iron in the form of the instrument adapted for use on land. The
balancing force is given by a spring of flat or cylindrical spiral form, while the
pointer scale and other attachments are similar to those herein before described in
connection with the form which the instrument takes when used on land.
J
:

page_text: COMPLETE SPECIFICATION.
I, Sir WILLIAM THOMSON, Knight, of Glasgow College, Doctor of Laws, and
Professor of Natural Philosophy in the University and College of Glasgow in the
County of Lanark, North Britain do hereby declare the nature of my said
invention for IMPROVEMENTS IN APPARATUS FOR MEASURING ELECTRIC CURRENTS
and in what manner the saine is to be performed, to be particularly described
and ascertained in and by the following statement :-
Having by mathematical reasoning been led to expect that a mass of soft iron,
if spherical, or of an elongated shape provided the length bears not too great a
proportion to the breadth or to the shortest diameter, experiences forces in a
magnetic field which are but little affected by residual magnetism, and which are
very nearly proportional to the rate per unit of distance, at which the square of
the intensity of the magnetic force of the field, varies from place to place in any
direction; I have devised the means hereinafter set forth for applying this mathe-
matical conclusion for the measurement of electric currents. I make a conductor
of electricity of such shape and such form, as to give a magnetic field with suitable
variation of the intensity of its magnetic force, in a space through which a small
piece of soft iron can travel while properly supported for measuring the force
required to balance it in any position. A suitable distribution of the electro-
magnetic force for some purposes, especially for a very accurate standardiser, is
when the square of its magnitude varies through the used part of the field, directly
as the distance from a point in the prolongation of the line in which the piece of
soft iron moves, because this law makes the force experienced by the piece of soft
iron independent of its position in the field.-This condition, or any modified
condition to give convenient lengths of scale divisions, may be fulfilled by trial,
with any desired accuracy by taking a single flat conductor, or a flat conductor
doubled back upon itself, and cutting it to different breadths in different parts in
the neighbourhood of the space through which the soft iron is to move, the line of
motion being approximately parallel to the line of the current.
In this case, and when the instrument is designed for use on land, the method
which I prefer for supporting the soft iron is to secure it on one end of a balance
arm properly counter poised, so that the virtual centre of gravity of the whole is as
nearly as possible in the line of the bearing knife edges. To counterbalance the
action of the electro-magnetic force, I hang a weight on a knife edge attached to
the arm or on a hook attached to the bottom of the piece of soft iron, (this last
arrangement being convenient when the electro-magnetic force is upwards). The
weight is varied according to the square of the strength of the medium current, in
the use that is made of the instrument.
An instrument for measuring electric currents, constructed according to the
principles herein before set forth, and fulfilling the condition that the motion of the
soft iron mass shall be in a direction parallel to the direction of the current, is
represented at Figures 1 and 2 of the annexed drawings these being respectively a
view partly in front elevation and partly in vertical section, and a side elevation,

page_text: In this instrument the balance arm a, carries two soft iron masses b, situated so that
each is in a magnetic field due to the current flowing through the conductor, which
is hereinafter more particularly described. The balance arm a, is counterpoised by
the arms C, and d, according to the principles herein before set forth, and is supported
by the knife-edges e, resting upon the V-grooves f, carried by the plate g. A knife-
edge support t, is provided at the lower end of the balance arm a, for the suspension
thereon of the weight u, employed in the action of the instrument as hereinbefore
described. A suitable arrangement for a paper or equivalent scalo is carried by the
frame h, secured by the arm i, to the conductor of the instrument so as to prevent
any error due to shifting relatively to the pointer j, secured to the balance arm a.
The conductor of the instrument is of copper and, in the case represented at
Figures 1 and 2 of the annexed drawings, is of considerable cross sectional area as
shewn more particularly at Figure 2. The conductor consists essentially of two
horizontal portions A and B connected by the vertical portions C, but in order to
avoid circumlocution it may be supposed to be thus constituted :-two copper
plates of the requisite thickness are cut according to the shape of the outline which
is at Figure 1 represented as enclosing the three connected parts A, B, and C,
respectively. These plates have their adjacent surfaces parallel and, it may be
supposed, are metallically connected at k, in such a manner that together they form
a continuous metallic circuit between the terininals, which upon the annexed
drawings are represented in the form of stranded copper wire cables l, and m.
These may be provided with any convenient arrangement for putting the instru-
ment into, or removing it from, an electric circuit. The shape given to the
conductors A and B at the parts adjacent to the masses b, is that by which I obtain,
in the field traversed by the said soft iron masses b, by means of a current flowing
through the conductor, the requisite disposition of magnetic force. To prevent
undue oscillation of the pointer j, I provide a light aluminium tube n, suspended
pendulum-wise by two fibres or fine threads o, to the arms p. A crank handle or
lever q, outside the case, enables the tube n, by motion of the arms p, to be brought
close up to, and to touch the pointer j, and thus, by the friction between the tuben,
and the pointer j, stops any such oscillations. A case, having at its upper part a
glass front, protects the instrument from currents of air and from dust. The upper
part r, of this case, is of wood, while the lower part 8, consists of an iron casing
which surrounds, but is insulated from, the conductor of the instrument, in order to
prevent magnetic matter in the neighbourhood from injuriously affecting the
magnetic field or fields of the instrument. The instrument is provided with
levelling screws v, and a spirit level to enable the said instrument to bo properly
adjusted.
In order to annul any residual magnetism that may be supposed to remain in
the soft iron masses b, I provide a reversing key for effecting rapid reversals of the
current. This key I construct in a manner analogous to that hereinafter described
with reference to Figures 3, 4, 5, 5a, and 5b, except that the contacts are made by
suitably connected pins carried by the moveable part u', dipping into mercury cups
formed in the fixed part t¹.
I prefer for some purposes to make the line of motion of the soft iron masses
approximately perpendicular to the current. When the current to be measured is
so small that a moderately fine wire, or flexible insulated conductor, of considerable
length can be used, the easiest way of producing a field to fulfil the desired condition
is by winding the conductor on a cylindrical or conical core, so as to fill up the
space to an outer boundary which may be of cylindrical or conical or conoidal
figure. The instrument so constituted is suitable for measuring cither continuous
or alternating currents, but for alternating currents the bobbin must be of insulating
material, and for this purpose I prefer to form the tube of such bobbin of several
rounds of paper hardened by shellac varnish or other suitable cement, with ends of
vulcanite or other convenient insulating material. When the instrument is designed
for measuring continuous currents I enclose the bobbin within an iron cylinder or

page_text: sheath so as to nearly annul the influence on the interior magnetic field of
terrestrial magnetic force or magnetic forces due to external magnets or electric
machines in the neighbourhood.
An instrument constructed upon these last mentioned principles and fulfilling the
condition that the direction of motion of the soft iron mass shall be practically
perpendicular to the direction of the current, is represented at Figures 3 to 5
(inclusive) of the annexed drawings, Figure 3 being a front elevation, Figure 4 part
end elevation and part section, and Figure 5 a plan. The electro-magnetic coil w,
of the form represented at Figures 3 and 4 is supported by the brass frame-work x,
so that the core of the said coil w, is in a vertical position. The sheath for the
purposes hereinbefore mentioned and which is not shown on the annexed drawings,
and is not necessary except when the instrument is to be used in the neighbour-
hood of magnetic matter, consists of an iron case enclosing the electro-magnetic
coil w, but having an aperture for the balance arm which is hereinafter described.
The brass framework x, shewn in the drawing as screwed to the wooden case y,
enclosing the instrument and protecting it from currents of air and from dust.
When the iron sheath hereinbefore referred to is provided, the framework is
secured thereto, and the whole is enclosed in the wooden case y, which under any
construction is provided with a glass front. The brass framework x, also carries
two V-grooves, upon which rest the knife edges a', of the balance arm b'. The
balance arm b', I make of aluminium, so as to give the requisite stiffness with as
little weight as possible, and on the end next the electro-magnetic coil w, is fitted
a semi-circular or hollow knife edge c', constructed of a hard non-magnetic metal
such as phosphor bronze, and having its plane perpendicular to that of the balance
arm b'. The other end of the balance arin b¹, constitutes the pointer, and for this
purpose it has secured on it by gum or otherwise a piece of white paper on which
is traced a thin black line which forms the mark by which the indications of the
pointer are read on the vertical scale d¹. A piece of wire, of aluminium, or other
convenient metal is shewn attached to the balance arm b', at e¹, to provide a
means for adjusting, so that the centre of gravity of the balance arin and its
directly-connected parts shall be as nearly as possible in the bearing point of the
knife edges.
For the convenience of transport, a wooden platform, a portion of which is
represented at f, is provided, and is furnished with a slot in its upper surface in
which slot the balance arm is secured by a pin. The lower end of this platform is
hinged to the frame of the scalo d', whilst its upper end is supported upon a cam
by means of which this end of the platform may be raised so as to lift the balance
arm 6', off its knife edges.
The plunger consists of a short thick cylinder or sphere g₁, of soft iron as
represented in dotted-lines at Figure 3 carried by a thin screwed aluminium rod /',
the screw upon which is provided for adjusting the position of the soft iron
mass g', within the magnetic field of the electro-magnetic coil w. A hook is
formed on each end of this aluminium rod h¹, the upper of which rests upon the
knife edge c', while the lower hook i¹, serves for supporting the weights which are
used in the action of the instrument as hereinafter described. Within the core of
the coil w, is a small annular projection which serves as a stop for the plunger, and
gives a means of observing any alteration in the position of the soft iron mass g',
on the aluminium rod ', since any such alteration would vitiate the scale readings.
This source of error is guarded against by observing that when the soft iron
mass g', rests upon the said annular support, the pointer b', indicates or should
indicate a definite position marked on the scale d³. The terminals of the
instrument consist of two long brass rods j', having an end of each lirmly secured
to the ebonite or vulcanite block k', fixed to one end of the case y, these ends being
in metallic connection with the two ends of the circuit containing the electro-
magnetic coil w, and the resistances and reversing key hereinafter described. The
brass rods j, pass at their free ends out through the case y, and, on the outside,.

page_text: each rod is bent at right angles to itself, and in such a manner that the two
arms, so formed are in the same plane, but inclined to each other in such manner
that they form parts of the sides of an angle. On each of these arms l', a ring m³,
is formed as represented more particularly at Figure 4. The terminal, for
attachment to the cut or break in the circuit in which the instrument is to be
placed, consists of two pins of brass or of copper, or other suitable metal, fixed in a
block of non-conducting material, at a distance apart which is about equal to the
distance between the parallel portions of the brass rods, and is therefore less
than the distance between the nearest parts of the brass rings m¹. To put the
instrument in circuit the ends of the two arms A, are pressed towards each other
sufficiently to allow the pins of the said terminal of the circuit, to be inserted in
the rings m. The arms ", when released from this position, bold the pins firmly
by virtue of the torsional resilience of the brass rods, acting upon the arms 1,
and their connected rings m'. To prevent the pins being shaken out of the rings
ly reason of vibrations or otherwise, ench pin is slightly curved in the direction of
its length so as to present concave surfaces toward each other. An arrangement
to prevent undue oscillations of the pointer b', is provided by means of the fine
thread n¹ kept stretched by a suitable weight o¹, and having its upper end secured
to any convenient hinged support, such as the brass moveable frame p', as
represented in dotted lines at Figure of the annexed drawings, and which may
be moved about the vertical axis q', by the handle r, so as to bring the hanging
thread n', in contact with the pointer b', and thus produce the necessary
checking effect by the friction of the pointer b', on the thread n'. On the shelf s¹,
forming the lower part of, and projecting out from the wooden case y, I place a
convenient form of reversing-key to permit of rapid reversals of the current
through the coil w, and thus annul any residual magnetism that might be
supposed to remain in the soft iron mass g¹. This reversing-key consists of a fixed
part t, and a rocker or moveable part ul, and for convenience of description I have
represented at Figures 50, and 5b, the adjacent surfaces respectively of the said
parts which carry the conductors. The fixed part t', is secured to the shelf 8', and
bears the crossed conductors v. (Figure 5º). The rocking or moveable part u¹,
which carries the parallel conductors w', (Figure 5) has a conical recess x¹, at the
intersection of the diagonals of its lower surface, which recess receives a round-
ended pin y¹, secured in any convenient manner in a corresponding position on the
upper surface of the fixed part t, of the key. This pin y', constitutes one of the
three bearing points on which the rocking part u¹, rests, the other two bearing
points being one or other end of its pair of conductors w¹, resting upon the
corresponding end of the pair of conductors v'. A spring, pressing on one end of
the rocking or moveable part u', secures good contact being maintained at that end,
and that the current through the coil w, kept flowing in one direction. The
reversal of the direction of the current through the coil w, is effected by pressing
down the end of the rocking part u, opposite to that end upon which the
spring z¹, bears.
To extend the range of the instrument, and to permit of its use as a potential
indicator, I sometimes employ a set of resistance coils which permits of any
desirable amount of resistance being put in series with the electro-magnetic coil w.
These resistance coils and their connections may be arranged in the manner set
forth in the specification filed in pursuance of Letters Patent granted to ine the
said Sir William Thomson number 2028 A.D. 1883, or they may be supported and
arranged as to their connections in the manner shewn at Figures 3, 4, and 5,
wherein a brass cylinder a³, open at each end, and of suitable length is supported
on the underside of the shelf 8', and constitutes the bobbin on which the resistance
coils are wound. These resistances, the contiguous coils of which are separated by
vulcanite washers, consist of 20 coils in 2 series of 10, each coil of the one series
of 10 having a resistance of 50 ohms, and each coil of the other series a resistance
of 500 ohms. On the upper surface of each of the two sets of contact studs 6³,

page_text: which are arranged in line, a V-groove is formed and the bar c³, below each such
set of studs b', has a similar V-groove formed on its under side. A spring contact
piece d*, for each set of studs makes contact between any of the studs of such
set and the corresponding bar c¹, below, by means of the round ended pin e³, and
the two screws f*, which respectively slide in the V-grooves thereof.
For more powerful currents than are measureable by the instrument hereinlast
described, I may use a ribbon of copper cut to a taper form, and wind this up
either with the narrow end or the broad end inwards, and thus obtain a coil with
a cylindrical and a tapering boundary fulfilling the conditions required for motion
of the soft iron mass in a direction perpendicular to the line of the current. For
very powerful currents, for example of more than 100 amperes, I use a thick
broad ribbon or flat bar of copper bent round upon itself so that the current is in
opposite directions on the two sides of a space of sufficient width, between the two
parts of the bar, to allow a piece of soft iron of convenient magnitude to travel to
and fro within it. And to give the proper variation of the electro-magnetic force
throughout the space traversed by the soft iron, which, in this case, is approximately
perpendicular to the line of current, I make the ribbon or bar of copper thicker
toward one edge and thinner, toward the other.
When the instrument is to be available for-use at sea I make it with two soft
iron masses, moving each in (a suitable magnetic field, the two magnetic fields
being so situated relatively to each other that the soft iron masses may be
supported one on each end of a balanced lever such as is herein before described,
for supporting one mass of soft iron in any of the forms of the instrument adapted
for use on land. The balancing force is given by a spring of flat or cylindrical
spiral form, while the pointer, scale and other attachments are similar to those
hereinbefore described in connection with the form which the instrument takes
when used on land.
When the instrument is to be used for measuring alternate currents instead of a
continuous sphere or cylinder of soft iron I use a number of thin plane plates of
soft iron, or a bundle of thin soft iron square or round wires, put together in the
proper shape and form, and insulated from each other by very thin paper, or other
insulating material; or thin ferrotype iron rolled with an intervening layer of very
thin insulating material, into the form of a short thick cylinder or other
convenient form.
I sometimes construct the instrument represented at Figures 3, 4 and 5 of the
annexed drawings, with the coil w, inverted, in which case, I secure a knife edge
on the pointer b¹, for the support of the weights which are otherwise suspended
from the plunger h¹. These knife edges are so situated on the pointer b', that the
weight supported by it counterpoises the electro-magnetic pull of the coil w.
For standardising tests, the bearing point of the hook on which the weight is
hung, should be exactly, or very nearly, in the plane of the two bearing points of
the main knife edges of the balance, and the point from which the soft iron is
hung. For inspectional use in ordinary practice, the bearing point of the weight
should be below this plane, sufficiently far to give whatever degree of stability is
desired; or, the desired stability may be given by selecting, as the place moved
through by the soft iron mass, a part of the magnetic field in which the force
experienced by the said soft iron mass, in the direction of the force, diminishes.
Thus with one and the same weight, currents deviating by 10 or 20 per cent or
more, above or below the magnitude of that which balances the instrument in its
middle position, are measured with sufficient approximation for most practical
purposes, by means of the pointer attached to the balance, and a scale of divisions
graduated to correspond to equal differences of current, or to stated fractions, for.
example each division to correspond to one per cent, of the current which gives
the instrument its middle position. This convenient condition is fulfilled by the
same scale whatever be the weight which is hung on the balance.

page_text: The method of graduation which I prefer for the current meters bereinbeforo
described, with reference to Figures 1 to 5 (inclusive), and which is also applicable
to electrometers constructed in the manner set forth in my Complete Specification
No. 4617 A.D. 1883 and represented at Figures 30 and 31 of the drawings attached
to that Specification, is as follows. For the graduation of the scale it is only
necessary that a constant current be kept flowing through the instrument during
the observations, or, in the case of electrometers, that a constant difference of
potential be maintained between its two terminals, and the absolute value in
amperes or volts need not be known. For the purposes of the graduation of either
of these last mentioned forms of instrument I prepare a set of weights-say five,
bearing to each other the following relative magnitudes; No. I of X milligranimes
No. II of X milligrammes, No III of X milligrammes, No. IV. of X milli-
grammes and No V of X milligrammes. Let it be supposed as an example
that represents 1440 then No. I equals 1440 milligrammes, No. II equals
360 milligrammes, No III equals 160 milligramines, No IV equals 90 milligrammes
and No. V. equals 576 milligramines. To mark the divisions on the scale of either
of the current meters hereinlast mentioned, the instrument is connected in series
with any convenient form of indicator or measurer which is capable of shewing
very small variations in a current flowing through the circuit at any time. There
must also be in the circuit a convenient arrangement for throwing in or taking out
resistance, so as to provide a means of adjusting the strength of the current to
maintain it constant with a current of constant amount flowing through the electro-
magnetic coil. Marks are made on the scale at the positions indicated by the
pointer when each of the separate weights are successively and progressively hung
on the bottom hook of the plunger. The positions so ascertained are marked on
the scale, and numbered respectively 10, 20, 30, 40 and 50, taking the weights in the
order I to V. Another series of points on the scale are found, by using a current
of such amount as brings the pointer to the position marked 40 when the weight V
is on the bottom hook of the plunger. Keeping the current constant at the
amount so found by experiment, the weights IV, III, II and I, are successively
used, and the points on the scale so determined are numbered respectively 32, 24,
16, 8, these numbers being obtained by observing that the value assigned to the
first point determined by, say the weight IV, is to the value assigned to the
second point determined by the same weight, in the proportion of 5 to 4. In
a similar manner beginning with the weight V at division 30, the points 24, 18,
12, 6 may be ascertained. The remaining points necessary to complete the scale
may be filled in by interpolation, from the points already determined by the
procedure herein before set forth; or I may, before completing the scale by inter-
polation, determine one or more points between the 40 and 50 of the sale, by
properly selected weights according to the principle herein before described with
reference to the preparation of weights I to V. The scale so constructed may be
used with any weight which is found convenient for the purpose to which the
instrument is to be applied, whether for measuring milliamperes, amperes or large
multiples of amperes, and the indications on the scale, multiplied by a constant
depending on the weight used, give the measurement of the current in absolute
value. For convenience in practice the magnitude of the weights may be such as
to eliminate the employment of the constant multiplier last referred to, and to
fulfil the condition that the divisions of the scale each indicate the amperes of the
current measured in sub-multiples of ten, units or multiples of ten.
For convenience of reference in respect to the system of graduation herein last
described I have represented at Figures 6 and 7 of the annexed drawings respec-
tively in front and side elevations, an electrometer or electrostatic volt gauge,
constructed in accordance with the principle disclosed in the aforesaid specification
No. 4617 A.D. 1883. The moveable part of the air condenser constituting the
instrument, represented at Figures 6 and 7 of the annexed drawings, consists of a
plate, of aluminium supported by knife edges h³, resting on the V-grooves i³.

page_text: This plate g, with its pointer 1, and the lower knife edge ³, is counterpoised so
that the centre of gravity is as nearly as practicable in a line coincident with the
bottom of the V-grooves, the knife edge k³, having its length in a plane
perpendicular to the plane in which the plate moves. The fixed part of the
condenser consists of two plates 1, of brass or other suitable metal, shaped preferably
according to the form represented at Figures 6 and 7. These plates, are
metallically connected together with their adjacent surfaces parallel and at equal
distances on each side of the plane in which the pivoted plate g³, moves; the
relative positions of the moveable plate g³, and fixed plates P, when under no
electrification, being as represented at Figure 6. An arrangement for checking the
vibrations of the pointer, corresponding to that herein before described with
reference to Figures 1 and 2, is represented as applied to the instrument, and need
not here be more particularly referred to, the same letters of reference indicating
corresponding parts upon these last mentioned drawings. The weights necessary
in the action of the instrument are suspended upon the knife edge *. The form
given to the pivoted plate g, represented upon the annexed drawings, is such that
the divisions of the scale, representing equal differences of potential, shall be
approximately equal over a large part of the scale.
In order to minimise risk of damage to the instrument when it is placed in
electrical connection with a dynamo or a voltaic battery or other source of high
potential, I use glass siphon tubes suspended from each terminal.
Each siphon
tube contains a little water in the bend, and a wet thread passed through it, one
end of which is secured to the corresponding terminal of the instrument, and the
other is free to provide a means of connecting to one of the two terminals of the
circuit between which the potential is to be measured. I sometimes secure the
free end of the thread, of each siphon tube, to a subsidiary terminal provided on
the instrument, the two subsidiary terminals so constituted being in this case for
direct connection to the aforementioned terminals of the circuit.
Having now particularly described and ascertained the nature of my said
Invention, and in what manner the same is to be performed, I declare that what
I claim is
First. The instruments for determining the strength of an electric current by
measuring the attraction of a globe, or cube, or oval, or disc, or short bar, of soft
iron in a non uniform field of electro-magnetic force due to the current, the
proportion of greatest to least diameter of the piece of soft iron being so small,
that there is but little effect of residual magnetism, and the magnetism is very
nearly in simple proportion to the strength of the current as hereinbefore
described.
Second. The reversing key, substantially as hereinbefore describe 1 with reference
to Figures 3, 4, 5, 5a, and 56, in connection with the current meter, for, as it were,
shaking out any retained magnetism that may have been induced by an over-
strong current at any time.
Third. The obtaining a non-uniform field of electro-magnetic force by the several
instruments substantially as hereinbefore described.
Fourth. The obtaining indications of different strengths of current, by means of
a force on the soft iron, increasing from zero at its position corresponding to zero
current, sub-tantially as hereinbefore described with reference to Figures 1 and 2
of the annexed drawings.
Fifth. The obtaining indications of different strengths of current, by means of
the difference of position in the electro-magnetic field, which gives approximately
equal resultant forces on the piece of soft iron, substantially as bereinbefore
described with reference to Figures 3 to 56 inclusive of the annexed drawings.
Sixth. The graduating of the current meters which is also applicable to the
graduation of my electrostatic volt gauge herein before described with reference to
Figures 6 and 7 of the annexed drawings in which, while the instrument is kept

page_text: under the influence of a constant current or constant potential, weights are
successively hung upon it, inversely as the squares of the currents or of the
potentials to be indicated in the inspectional use of the instruments as set forth.
Seventh. The combination of the current meter with adjustable resistance coils,
to constitute a long range potential meter, substantially as herein before described 5
with reference to Figures 3, 4 and 5 of the annexed drawings.
J. H. JOHNSON, DAY & JOHNSON,
Applicant's Agents.