IMPROVEMENTS IN APPARATUS FOR MEASURING ELECTRIC CURRENTS AND ELECTRICAL PRESSURE OR POTENTIAL
Year: 1883

page_text: LETTERS PATENT to Rookes Evelyn Bell Crompton of the Mansion House
Buildings Queen Victoria Street in the City of London Engineer and Gisbert
Kapp of Arc Works Chelmsford in the County of Essex Engineer and in the
employment of the said Rookes Evelyn Bell Crompton for an Invention of
IMPROVEMENTS IN APPARATUS FOR MEASURING ELECTRIC CURRENTS AND
ELECTRICAL PRESSURE OR POTENTIAL
PROVISIONAL SPECIFICATION left by the said Rookes Evelyn Bell
Crompton and Gisbert Kapp at the Office of the Commissioners of Patents
on the 13th April 1883.
ROOKES EVELYN BELL CROMPTON of the Mansion House Buildings, Queen
Victoria Street in the City of London, Engineer, and GISBERT KAPP of Arc Works,
Chelmsford in the County of Essex, Engineer, and in the employment of the said
Rookes Evelyn Bell Crompton, "IMPROVEMENTS IN APPARATUS FOR MEASURING
ELECTRIC CURRENTS AND ELECTRICAL PRESSURE OR POTENTIAL."
Our invention consists in improvements in apparatus for measuring electric
currents and electrical pressure or potential. The action of such apparatus as
commonly used, is based upon a balance being obtained between the mechanical
forces developed by the current passing through the instrument and some counter-
acting force as for instance the tension of a spring or the attractive force of a
permanent magnet or some other passive force which is independent of the said
current. As long as this passive counter-acting force remains constant or as long as
its precise magnitude can be accurately known, at the moment when a current is
being measured; such measurement will be correct; but it is well known that the
tension of springs and the attractive power of so called permanent magnets are
subject to variations and it is therefore necessary to frequently calibrate such
instruments.
Our invention has for its object the production of measuring instruments which
after having once been correctly calibrated are entirely free from subsequent
variations and give at all times accurate indications.

page_text: This object we obtain by employing for a counteracting force,-not the passive
force of a spring or of a permanent magnet, both of which are independent of the
current, but the active force of an electro magnet excited by the very current or
part of the very current which is to be measured, and thus we obtain that the above
mentioned counteracting force is a definite quantity for each strength of current
and not depending upon any other influence but that of the said current.
We use a short magnetic needle either hung on a fine flexible thread or delicately
supported on centres similar to needles commonly used for tangent galvanometers
and as in such an instrument, surround the said needle with a coil or coils of
insulated wire wound in a direction at right angles to the plan of oscillation of the
needle. We further provide a directing electro magnet the opposite poles of which
stand in a line parallel to the said coils and passing through the centre of the
needle. The tendency of the current circulating in the said coil will be to set
the needle at right angles to the plane of the coils whilst the tendency of the
magnetic force of the Electro magnet will be to set the needle parallel to this
plane; the needle will therefore set at a certain angle to the plane of the coils and
it is found by experiment that with an instrument made according to our invention
this angle varies in a definite manner with the strength of the current.
This action can be explained in the following way.
The magnetic moment of an electro-magnet or as it is sometimes called the
strength of the magnetic field between the poles of an Electro-magnet depends on
the size and shape of the iron core and upon the exciting power of the current.
By exciting power we mean the product of strength of current with the number of
turns of wire wound round the core. As long as the exciting power is small in
comparison to the size of the core, the magnetic moment can be considered as
simply proportional to the current; but when the exciting power is considerable
in comparison to the size of the core, or as it is commonly called, when the Electro-
magnet is nearly saturated, then an increase or decrease in current does not result in
a proportionate increase or decrease of the magnetic moment, but in a much smaller
increase or decrease. In our instrument we make the electro magnet of such pro-
portions that at the weakest current which is intended to be measured the electro
magnet becomes powerfully excited. Then if the current be increased to many times
this minimum strength, the magnetic moment can only increase by a comparatively
small amount and therefore the force which tends to set the needle pointing straight
across from pole to pole can also only increase by a comparatively small amount.
The force exerted by the galvanometer coils being as in an ordinary tangent
galvanometer simply proportional to the strength of the current, for all currents, it
follows that the larger the current, the larger must be the angle formed by the
magnetic axis of the needle with the plane of the galvanometer coils.
.
If the diameter of the coils were very large in comparison to the length of the
needle and if sufficient wire were wound on to the electro magnet so as to make its
strength of field practically constant for all the currents to be ineasured, then the
geometrical tangent of this angle would as in an ordinary tangent galvanometer be
a measure for the current. We find it however not necessary to make the galvano-
meter coils very large in comparison with the length of the needle nor to saturate
the Electro-Magnet so much. By winding a reasonable though not an excessive
quantity of wire round a comparatively thin core we find that the needle although
not following strictly the above mentioned law of tangents, still gives a definite
deflection for a definite current.
We prefer to use an electro magnet with pole pieces projecting inwards and we
make the distance between the poles not too great in comparison with the length
of the needle, so that when the needle is deflected under the action of a strong
current the distance between its poles and the poles of the electro magnèt is slightly
greater than when deflected to a lesser degree by a weak current. In this manner
the influence of the electro magnet upon the needle [in so far as this influence
depends on the element of distance] decreases at such times as the absolute strength
of the Electro magnet increases and by properly proportioning the length of the

page_text: needle to the distance between the poles of the Electro magnet we obtain a more
convenient division of the scale or dial.
The magnetic moment of the needle itself has, as in an ordinary tangent galvano-
meter no influence upon the indications of the instrument. A needle made of soft
iron would indicate the same angle, but we prefer to use a magnetized steel needle
because its deviation to the right or left shows the direction of the current, also it
has the advantage of making the needle stand straight across between the poles
and the index point to zero when no current is passing.
Another part of our invention consists in employing a ring commutator for the
Jurpose of either sending the current through the successive parts of the instrument
coupled up in series or of splitting up the current into two equal parts which are
sent through the instrument in parallel. This commutator enables us to measure
large and small currents on the same instrument with an equal degree of accuracy.
To measure clectrical pressure or potential, we use a similar instrument, but in
this case we make the galvanometer cuil and the exciting coils on the electro
magnet of very high resistance by employing many turns of a fine wire of copper,
german silver or other suitable metal, and we place the commutator to series if the
potential to be measured is high and to parallel if the potential is low.
If very large currents are to he measured we add to the instrument a circular
coil of very thick copper wire, or very thick copper strip, which coil we place
parallel to the plane of oscillation of the needle and concentric with it, so that a
current circulating in this coil has no tendency to alter the angle of the needle.
We couple up this coil in parallel with the wire in the instrument and having
carefully determined the relative resistances, we determine the. total current by the
well known law, that the current passing the instrument and that passing the
additional thick coil are inversely as their resistances.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Rookes Evelyn Bell Crompton and Gisbert Kapp in the Great Seal
Patent Office on the 12th October 1883.
ROOKES EVELYN BELL CROMPTON of the Mansion House Buildings Queen
Victoria Street in the City of London Engineer and GISBERT KAPP of Arc Works
Chelmsford in the County of Essex Engineer and in the employment of the said
Rookes Evelyn Bell Crompton "IMPROVEMENTS IN APPARATUS FOR MEASURING
ELECTRIC CURRENTS AND ELECTRICAL PRESSURE OR POTENTIAL
"
Our invention consists in improvements in apparatus for measuring electric
currents and electrical pressure or potential.
The action of such apparatus as commonly used is based upon a balance being
obtained between the mechanical forces developed by the current passing through
the instrument and some counteracting force as for instance the tension of a spring
or the attractive force of a permanent magnet or some other passive force which is
independent of the said current. As long as this passive counteracting force
remains constant or as long as its precise magnitude can be accurately known at
the moment when a current is being measured, such measurement will be correct
but it is well known that the tension of springs and the attractive power of so called
permanent magnets are subject to variations and it is therefore necessary to
frequently calibrate such instruments.
Our invention has for its object the production of measuring instruments which
after having once been correctly calibrated are free from subsequent variations and
give at all times accurate indications.
This object we obtain by employing for a counteracting force-not the passive
force of a spring or of a permanent magnet both of which are independent of the
current-but the active force of an electro magnet excited by the very current or
part of the very current which is to be measured and thus we obtain that the above
mentioned counteracting force is a definite quantity for each strength of current
and not depending upon any other influence but that of the said current.
We use a short magnetic needle either hung upon a fine flexible thread or
delicately supported on centres similar to needles commonly used for tangent
galvanometers and as in such an instrument surround the said needle with a coil or
coils of insulated wire wound in a direction at right angles to the plane of
oscillation of the needle. We further provide a directing electro-magnet the
opposite poles of which stand in a line parallel to the said coils and passing
through the centre of the needle.
The tendency of the current circulating in the said coil will be to set the needle
at right angles to the plane of the coils whilst the tendency of the magnetic force
of the electro magnet will be to set the needle parallel to this plane the needle wil
therefore set at a certain angle to the plane of the coils and it is found by experi
ment that with an instrument made according to our invention this angle varies in
a definite manner with the strength of the current.
This action can be explained in the following way-
The magnetic moment of an electro magnet or as it is sometimes called th
strength of the magnetic field between the poles of an electro magnet depends o
the size and shape of the iron core and upon the exciting power of the current
By exciting power we mean the product of strength of current with the number o
turns of wire wound round the core. As long as the exciting power is small i
comparison to the size of the core the magnetic moment can be considered as simpl

page_text: proportional to the current; but when the exciting power is considerable in com-
parison to the size of the core or as it is commonly called when the electro-magnet
is nearly saturated then an increase or decrease in current does not result in a
proportionate increase or decrease of the magnetic moment but in a much smaller
increase or decrease. In our instrument we make the electro magnet of such pro-
portions that at the weakest current which is intended to be measured the electro
magnet becomes powerfully excited. Then if the current become increased to many
times this minimum strength the magnetic moment can only increase by a com-
paratively small amount and therefore the force which tends to set the needle
pointing straight across from pole to pole can also only increase by a comparatively
small amount.
The force exerted by the galvanometer coils being as in an ordinary tangent
galvanometer simply proportional to the strength of the current for all currents,
it follows that the larger the current the larger must be the angle formed by the
magnetic axis of the needle with the plane of the galvanometer coils.
If the diameter of the coils were very large in comparison to the length of the
needle and if sufficient wire were wound on to the electro magnet so as to make its
strength of field practically constant for all the currents to be measured then the
geometrical tangent of this angle would as in an ordinary tangent galvanometer be
a measure for the current. We find it however not nece-sary to make the galvano-
meter coils very large in comparison with the length of the needle nor to saturate the
electro-magnet so much. By winding a reasonable though not an excessive quantity
of wire round a comparatively thin core we find that the needle although not
following strictly the above mentioned law of tangents still gives a definite deflection
for a definite current.
We prefer to use an electro magnet with pole pieces projecting inwards and we
make the distance between the poles not too great in comparison with the length
of the needle so that when the needle is deflected under the action of a strong
current the distance between its poles and the poles of the electro magnet is slightly
greater than when deflected to a lesser degree by a weak current.
In this manner the influence of the electro-magnet upon the needle (in so far as
this influence depends on the element of distance) decreases at such times as the
absolute strength of the electro inagnet increases and by properly proportioning the
length of the needle to the distance between the poles of the electro-magnet we
obtain a more convenient division of the scale or dial.
The magnetic moment of the needle itself has as in an ordinary tangent galvano-
A needle made of soft
meter no influence upon the indications of the instrument.
iron would indicate the same angle but we prefer to use a magnetized steel needle
because its deviation to the right or left shows the direction of the current also it
bas the advantage of making the needle stand straight across between the poles
and the index point to zero when no current is passing.
In the accompanying Drawing Figure 1 shews one method of arranging the
electro magnet and the deflecting or galvanometer coil; C. C. C. is the soft iron
core of the electro magnet; P P are its poles which project inwards so as to
come within a moderate distance to the magnetic needle N S without its being
necessary to bring the coils of the electro magnet themselves so near to the needle.
In this manner the needle is mostly under the influence of the magnetism proper
of the iron core of the electro magnet and only slightly or not at all under the
influence of its coils which is an advantage as the directive power tending to bring
the needle back to its zero position is thus more nearly made a constant. GG is
a gun metal frame upon which the deflecting coils D D are wound for convenience
these coils are arranged in two halves leaving sufficient space between each other
for the insertion of the needle and its pivots. The current enters at terminal I
goes round the deflecting coils thence into the coils of the electro magnet as shown
by the arrows and finally out of the instrument by terminal II
Figure 2 is a section of the instrument taken through the centre of the magnetic
neɩdle and the poles PP of the electro magnet. The winding is left out,

page_text: In addition to the parts figured and mentioned above is a spindle to which
the needle N S is firmly attached. a. a. are bearings in which the spindle can
revolve and p. p. is a light pointer also rigidly attached to the spindle. The:
position of the needle is thus shewn by the pointer on the dial d. d. The top
bearing a is supported by the narrow bridging piece b. b. and g. g. is a glass cover.
Figure 3 shews a side view of the galvanometer frame G. G. The two halves
are connected on the top by a piece e which contains an opening into which the
casing f. f. (shewn in Figs 2 and 5) containing the needle is inserted.
If the current to be measured should be too large for the size of wire with which
the instrument is wound we shunt a portion of it through a coil of thick copper
wire or copper bar of accurately known resistance. This wire or bar shewn by a
dotted line in Figure 1 is connected to the terminals of the instrument and fixed
in a plane coinciding with the plane of oscillation of the needle. The current
which circulates in this coil has therefore no influence upon the position of the
needle and only that portion of the total current which passes through the galvano-
meter coil and electro magnet is directly measured by the instrument. But as the
two currents passing through these alternate circuits are inversely proportionate
to the resistances of these circuits it follows that by suitably arranging these
resistances the total current can be made to be a convenient multiple of the current
which passes the instrument proper. Thus for instance if the resistance of the
thick shunt or bye pass coil be one ninth of the resistance of the wire in the
instrument the total current will be ten times that indicated by the instrument.
We prefer to make the connection between one of the terminals and the thick
shunt or bye pass coil detachable so that the same instrument can serve for currents
of greatly varying intensity.
Another method for making the same instrument suitable for measuring strong
and weak currents with the same degree of accuracy is by the employment of a
ring commutator shown in Figures 6. 7 and 8. Figure 6 shows the commutator
put to parallel. D is a disc of insulating material which can revolve on a centre O
within four terminals 1. 2. 3. 4.
One of the terminals is shown in detail in Figure 8. It consists of a gun metal
casting into which is fixed one of the wires W leading to the instrument and a
screw S by which the edge of the disc can be pressed down upon the metal face f
Part of the edge of the disc is faced with curved metal facing pieces G which are
shewn in dotted lines in Figures 6 and 7. The object of the screw S is to establish
good electrical contact between the terminals and the facing pieces G. If the disc
be placed in the position. shewn in Figure 6 and if the screws S of the terminals 1.
2. 3 and 4 be screwed down tightly terminals 1 and 2 will be in good electrical
connection and terminals 3 and 4 will also be in good electrical connection. One
half of the wire within the instrument (which is not shewn in the drawing) is
contained between the branches a and c and the other half between the branches b
and d, the current entering at terminals I and. 1 will therefore divide itself into
two parts, one part flowing out of branch a into the instrument and returning by
branch c the other part flowing out of branch b into the instrument and returning
by branch.d.
In this way the wire composing the instrument has only to carry one half the
total current. If the commutator be placed as shown in Figure 7 then the total
current. will flow out of branch a into the instrument, traverse one half of the wire
therein, return to the commutator by branch o to terminal 3 then flow along the
facing piece G to terminal 2 from there through branch b again into the instrument
traverse the other half of the wire therein, return to the commutator by branch d
and finally leave the commutator through terminals 4 and II.
Our instrument as above described for the measurement of currents can also be
.adapted for the measurement of clectrical pressure or difference of potential. The
general features of the invention remain the same, but instead of winding the
galvanometer frame G G and the electro magnet with comparatively few turns of
thick wire.we.wind them with a great number of turns, of, fine wire having a

page_text: sufficiently high resistance to allow only a very small current to pass through the
instrument if its terminals be connected to the two points of which the difference
of potential is to be measured.
By Ohm's law this current passing is proportional to the difference of potential
and hence with a suitably graduated dial the difference of potential can be directly
read off the instrument.
Figures 4 and 5 shew one form of our invention when adapted for tho
measurement of difference of potential. Figure 4 is a horizontal section through
the centre of the needle and Figure 5 is a vertical section on a plane passing
through the centre of the needle and the poles of the electro-magnet. The latter is
however not shewn in section. N. S. is the magnetic needle attached to the
spindle which also carries pointer p. p. a. a. are the bearings the lower one being
supported by the bottom of casing f. f. the upper one by a narrow bridge piece b. b.
in a similar manner to that shewn in Figure 2. The pointer p. p. sweeps over a
dial d. d. upon which the deflection of the needle can be read off.
We prefer to engrave the dial directly in Volts but any other unit might be used.
C. C. is a casing supporting a glass cover g. g. G. G. are the galvanometer frames
upon which the deflecting coils D D are wound; C is the core and P P the poles of
the electro magnet which is wound to a thickness W with a large number of turns
of fine wire. The direction of the current is shewn by the arrows. The thickness
of wire and the number of turns we employ depends upon the electrical pressure up
to which the instrument has to be used. But if we desire to use the same instru-
ment for greatly varying pressures we fit it with a commutator similar to that
shown in Figure 6, but with all the parts smaller; as the currents transmitted aro
so much smaller.
For low pressures we put the commutator to parallel and for high pressures to
series.
Having thus described our invention and means adapted for carrying the same
into practical effect we would have it understood that we do not limit ourselves to
the precise construction of apparatus shown in the accompanying Drawings by
way of example to make our invention moro clearly understood but what we do
claim is :-
First The employment of an electro magnet or electro magnets excited by the
whole or part of the current to be measured in lieu of the permanent steel magnets
which hitherto have been used for counterbalancing the deflecting force of the
galvanometer coils in instruments for measuring electric currents substantially as
described.
Second-The employment of an electro magnet or electro magnets excited by
many turns of fine wire of high resistance in lieu of a permanent steel magnet
hitherto used for counterbalancing the deflecting force of galvanometer coils of
many turns of fine high resistance wire in instruments for measuring electrical
difference of potential substantially as described.
Third In apparatus for measuring electric current or electrical pressure or
potential the employment of electro magnets with pole pieces projecting towards
the needle further than their exciting coils substantially as described.
In witness whereof I the said Rookes Evelyn Bell Crompton have hereunto
set my hand and seal this ninth day of October One thousand eight hundred and
eighty three