IMPROVEMENTS IN APPARATUS FOR MEASURING ELECTRIC CURRENTS AND ELECTROMOTIVE FORCE
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
ELECTROMOTIVE FORCE
PROVISIONAL SPECIFICATION left by the said Rookes Evelyn Bell Crompton
and Gisbert Kapp at the Office of the Commissioners of Patents on the
18th September 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 ELECTROMOTIVE FORCE
Our invention consists in Improvements in Apparatus for measuring Electric
Currents and Electromotive force, which improvements refer more especially to
the subject matter of the Provisional Specification filed by us with our Application
for Letters Patent for "Improvements in apparatus for measuring Electric Currents
and Electrical Pressure or Potential" on the 13th day of April 1883. and where a
magnetic needle or a piece of soft iron, pivotted or suspended is brought under the
combined influence of an Electro magnet (I) which tends to make it point
straight or direct from pole to pole and of a coil or coils or part of a coil
or single wire (II) which tends to deflect it from that position. The resultant
of these combined forces acting upon the needle gives it a deflection, the angle
of which is a definite function of and therefore a definite measure for the
current-The directing power of the Electro-magnet is the sum of the magnetic
moment of its soft iron core and of that of the coils (I) of insulated wire on it. If

page_text: a sufficient number of turns be employed and if the mass of iron in the core is,
relatively speaking, small then the core will be saturated and its magnetic moment
will remain nearly constant under varying currents. The magnetic moment of the
coils (I) proper is however directly proportional to their number and to the current
traversing them. The force exerted upon the needle by the deflecting coil (II)
tends to deflect it from its zero position in line with the poles of the electro-magnet
and is proportional to the strength of the current. We find it desirable to so
arrange our instruments that the total directing power of the electro magnet should
vary as little as possible with varying currents, because in this case the geometrical
tangent of the angle through which the needle is turned from its zero position by
the deflecting coil (II) is approximately proportional to the current and a
convenient division of the scale or dial is thus obtained.
One way of attaining this result is described in our above mentional Provisional
Specification bearing date April the 13th 1883 and it consists in extending the
poles of the electro-magnet so that they project to some distance from their
coils (I) and towards the needle-In consequence of this arrangement the needle is
sufficiently far removed from the coils (I) to be only slightly affected by them
whilst it is well under the influence of the soft iron core excited by these coils (I).
In this manner only that part of the total directing force of the electro magnet
which belongs to its soft iron core proper (and which is nearly constant) has a
strong influence upon the needle and that part of the total directing force which
belongs to the coils (I) has only a very small influence upon it
Another way of attaining this result is by surrounding the needle with coils of
insulated wire wound in opposite direction to the coils (I) of the electro magnet
and joined up with them so that the same or a proportional current to that which
traverses the wire of the electro magnet also traverses the wire of this balancing
coil, thereby neutralizing the magnetic moment of the coils (I) of the electro-
magnet. As the balancing coil is close to the needle a comparatively small number
of turns only is required and as it does not immediately surround the core of the
electro-magnet its weakening influence upon the latter is only very slight.
Another method of balancing the directing force of the coils (1) of the electro-
magnet is by placing the deflecting coil (II) not exactly parallel with the zero
position of the needle but at such an angle to it that if the iron core be removed
from the coils (I) of the electro magnet and if the instrument be so placed in the
field of the earth that the needle pointing North South stands at right angles to its
zero position a current traversing the instrument will not deflect the needle from
its position in the magnetic meridian In this case the resultant of the forces from
the directing coils (I) and the deflecting coils (II) acts at right angles to the zero
position of the needle or in other words the directing force of the coil (I) of the
electro magnet is exactly balanced by the angular position of the deflecting
coil (II).
The angle at which the deflecting coil (II) must be set, we determine experi-
mentally in the manner above described. If the coil be fixed at this angle and the
iron core of the electro magnet be replaced an instrument is obtained which
indicates in a similar manner to an ordinary tangent galvanometer and the scale or
dial of which has to be divided in nearly the same manner as that of an ordinary
tangent galvanometer. Sometimes we prefer to fix the deflecting coil (II) at an
angle somewhat larger than that found by the experiment above described and in
this case the effect of the angular position of the deflecting coil (II) is as though
the power of the directing coils (I) of the electro magnet weakened with an
increasing current and therefore the needle will (especially for the larger currents)
give somewhat greater deflections than in the former case. This arrangement we
employ when it is desirable that the greatest currents for which the instrument is
designed should be read with rather more accuracy than the smaller currents.
Another part of our invention consists in the employment of a single, straight, or
Bemicircular or otherwise curved conductor instead of a complete deflecting coil (II)
and also in the employment of two parallel magnetic needles instead of one

page_text: We arrange these needles with their similar poles pointing the same way and we
suspend them on a centre pivot somewhat in the manner of compass needles.
We also employ sometimes a pair of astatic needles (fixed parallel to each other
but with their similar poles in opposite directions) These needles we fix at such
distance from each other, that whilst the one is in the centre and under the influence
of the deflecting coils (II) and between the poles of the electro-magnet (I) the
other is sufficiently far removed to bo out of this influence-The object of this
arrangement is to make the instrument independent of its position with regard to
the magnetic field of the earth.
Another part of our invention consists in combining with any of the above
arrangements a comparatively heavy core or cores of soft iron so placed with regard
to the deflecting coil (II) that their power over the needle is thereby increased and
the employment of a stronger electro-magnet (I) is thus made possible-The object
of this arrangement is to make the instrument more independent of disturbing
external influences as for instance the magnetism of the earth or neighboring
currents masses of iron dynamo-electric machines or magnets.
Another part of our invention has more especial reference to the measurement of
electric currents which although variable as regards their intensity are always
flowing under a constant or nearly constant electromotive force as for instance the
current obtained from self regulating dynamo machines or those supplied under
constant electrical pressure to private customers in a system of public supply
In those cases we prefer to excite the electromagnet not by the whole of the
current which is to be measured but by a very small part thereof which is taken
direct from the positive and negative wires at points between which the electro
motive force is constant or nearly constant.
We can combine with any of the above arrangements of instruments for
measuring electromotive force a resistance coil which by a suitable key or switch
can be inserted into the circuit thereby increasing the total resistance of the instru-
ment by a convenient multiple. If for example the resistance of the additional
coil be nine times that of the electro-magnet and deflecting coils then the
maximum electromotive force which can be measured with the additional resistance
in circuit will be ten times that which can be measured with the resistance short
circuited
We also sometimes combine with our instruments any of the well known commu-
tating devices by which the direction in which the current traverses the instrument
can be reversed and we also furnish sometimes the commutator with a handle or
index which indicates at which of the terminals the current enters
We can also combine with our instruments for measuring electric currents any
of the mechanical time integrating devices by which the quantity of electricity
which has passed through the instrument in a given time is indicated.
We can also surround our instruments with a casing of soft iron suitably placed
and arranged so as to shield them from external disturbing influences as for
instance neighboring electric currents, masses of iron dynamo electric machines or
magnets.

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 Patent Office
on the 12th March 1884.
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 ELECTROMOTIVE FORCE."
Our invention consists in improvements in apparatus for measuring Electric
Currents and Electromotive force, which improvements refer more especially to the
subject matter of the specification filed by us in pursuance of Letters Patent
granted to us for "Improvements in apparatus for measuring Electric currents
and Electrical Pressure or Potential" dated the 13th day of April 1883. and
where a magnetic needle or a piece of soft iron, pivotted or suspended is brought
under the combined influence of an electro magnet (I) which tends to make it
point straight or direct from pole to pole, and of a coil or coils, or part of a coil, or
single wire (II) which tends to deflect it from that position. The resultant of these
combined forces acting upon the needle, gives it a deflection, the angle of which is
a definite function of, and therefore a definite measure for the current. The
directing power of the Electro-magnet is the sum of the magnetic moment of its
soft iron core and of that of the coils (I) of insulated wire on it. If a sufficient
number of turns be employed, and if the mass of iron in the core is, relatively
speaking, small, then the core will be saturated and its magnetic moment will
remain nearly constant under varying currents. The magnetic moment of the
coils (I) proper is however, directly proportional to their number and to the current
traversing them. The force exerted upon the needle by the deflecting coil (II)
tends to deflect it from its zero position in line with the poles of the electro-magnet
and is proportional to the strength of the current. We find it desirable to so
arrange our instruments, that the total directing power of the electro-magnet
should vary as little as possible with varying currents, because in this case the
geometrical tangent of the angle through which the needle is turned from its zero
position by the deflecting coil (II) is approximately proportional to the current and
a convenient division of the scale or dial is thus obtained.
One way of attaining this result is described in the before-mentioned specification
of Patent bearing date April 13th 1883 and it consists in extending the poles of
the electro-magnet so that they project to some distance from their coils (I) and
towards the needle.
In consequence of this arrangement the needle is sufficiently far removed from
the coils (I) to be only slightly affected by them whilst it is well under the
influence of the soft iron core excited by these coils (I). In this manner only that
part of the total directing force of the electro-magnet which belongs to its soft iron
core proper (and which is nearly constant.) has a strong influence upon the needle,
and that part of the total directing force which belongs to the coils (I) has only a
very small influence upon it.
Another way of attaining this result is by surrounding the needle with coils of
insulated wire wound in opposite direction to the coils (I) of the electro-magnet
and joined up with them, so that the same or a proportional current to that which
traverses the wire of the electro-magnet also traverses the wire of this balancing
coil thereby neutralizing the magnetic moment of the coils (1) of the electro

page_text: DESCRIPTION OF THE DRAWINGS.
In the accompanying drawing, Fig 1. C. C. C. is the core of the electro-
magnet. W. W. are the coils exciting it and s n the magnetic needle; b b are
coils shewn in section surrounding the needle on either side, and joined up with the
coils W. W. These coils bb we call the balancing coils, and we arrange the amount
of wire on them, their position, and their size so that their effect on the needle is
about equal or slightly larger but in sense opposed to the magnetic effect of the
coils W. W. D. D. are the deflecting coils already described in our former
specification.
Another method of balancing the directing force of the coils (I) of the electro-
magnet is by placing the deflecting coil (II) not exactly parallel with the zero
position of the needle, but at such an angle to it that if the iron core be removed
from the coils (I) of the electro-magnet, and if the instrument be so placed in the
field of the earth that the needle pointing North-South stands at right angles to
its zero position a current traversing the instrument will not deflect the needle
from its position in the magnetic meridian. In this case the resultant of the forces
from the directing coils (I) and the deflecting coils (II) acts at right angles to the
zero position of the needle, or in other words, the directing force of the coil (I) of
the electro-magnet, is exactly balanced by the angular position of the deflecting
coil (II).
The angle at which the deflecting coil (II) must be set, we determine
experimentally in the manner above described,. If the coil be fixed at this angle
and the iron core of the electro-magnet be replaced, an instrument is obtained
which indicates in a similar manner to an ordinary tangent galvanometer, and the
scale or dial of which has to be divided in nearly the same manner as to that of an
ordinary tangent galvanometer. Sometimes we prefer to fix the deflecting coil (II)
at an angle somewhat larger than that found by the experiment above described
and in this case the effect of the angular position of the deflecting coil (II) is, as
though the power of the directing coils (I) of the electro-magnet, weakened with
an increasing current and therefore the needle will (especially for the larger
currents) give somewhat greater deflections than in the former case. This
arrangement we employ when it is desirable that the greatest currents for which
the instrument is designed, should be read with rather more accuracy than the
smaller currents.
.
In Fig. 2. D. D. are the deflecting coils set at an angle as shewn and for the
purpose above described.
Another part of our invention consists in the employment of a single straight, or
semi-circular, or otherwise suitably curved conductor instead of a complete deflecting
coil (II) and also in the employment of two parallel magnetic needles instead
of one.
We arrange these needles with their similar poles pointing the same way and we
suspend them on a centre pivot somewhat in the manner of compass needles.
Figs. 5. 6. 7. 8. 9. 10 show this arrangement. The deflecting coil is here replaced
by a single metallic strip D. D. set at an angle and passing underneath the
needles 8. n. 8. n¹. when two needles are employed as shown in Figs 8. 9. and 10.
or beneath the needle n. 8. if only one needle is employed as in Figs. 5. 6 and 7.
The current, after entering at the flat electrode a divides itself between the
winding w. w. of two electro-magnets and returns by way of the deflecting strip
to the other flat electrode b where it leaves the instrument.
The double needle 8. n. s'. n. is suspended on a point p in the manner of compass
needles.

page_text: -
We also employ sometimes a pair of astatic needles (fixed parallel to each other,
but with their similar poles in opposite directions.) These needles we fix at such
distance from each other, that whilst the one is under the influence of the deflecting
coils (II) and between the poles of the electro-magnet (I) the other is sufficiently
far removed to be out of this influence. The object of this arrangement is to make
the instrument independent of its position with regard to the magnetic field of
the earth.
In the accompanying drawings Fig. 11 shows the arrangement of astatic needles.
The two needles 8. n and n¹. 8. are mounted parallel but at some distance from
each other upon the same spindle. The needle 8. n is between the poles of the
electro-magnet and under the influence of the deflecting strip D. D. whilst the
needle n'. 8. of opposite polarity is sufficiently removed from either to be practically
unaffected. In this manner only the needle 8. n. is affected by the passage of a
current, and any external disturbing force like the magnetic field of the earth
affecting both needles equally but in opposite sense cannot produce a deflection.
In order to increase the power and precision of this instrument we sometimes
surround the second needle n¹ 8. by a second electro-magnet and a second
deflecting coil so joined up that upon the passage of a current both needles are
deflected in the same sense.
Another part of our invention consists in combining with any of the above
arrangements a comparatively heavy core or cores of soft iron, so placed with
regard to the deflecting coil (II) that their power over the needle is thereby
increased, and the employment of a stronger electro-magnet (I) is thus made
possible. The object of this arrangement is to make the instrument more
independent of disturbing external influences, as, for instance the magnetism of
the earth, or neighbouring currents, masses of iron, dynamo electric machines, or
magnets.
In the drawing, these comparatively heavy cores are shewn in dotted lines
within the deflecting coils D D in Fig 4.
Another part of our invention has more especial reference to the measurement of
electric currents, which, although variable as regards their intensity are always
flowing under a constant or nearly constant electromotive force, as for instance
the current obtained from self-regulating dynamo machines or those supplied under
constant electrical pressure to private customers in a system of public supply, or
similarly supplied.
In those cases we prefer to excite the electro-magnet, not by the whole of the
current which is to be measured but by a very small part thereof which is
taken direct from the positive and negative wires at points between which the
electro-motive force is constant or nearly constant.
Figs 12 and 13 show such an arrangement. The double electro magnet W. W¹.
W. W. is wound by many turns of fine high resistance 'wire and excited by a
current taken as a shunt from their positive and negative mains, which are supposed
to be kept at a constant or Dearly constant difference of potential; b. b. are
balancing coils as explained above. The needle 8. n. which is mounted in the usual
way, is deflected by the main current passing along the conductor D. D. which is
placed under it and in this case not at an angle to but parallel with the line joining
the poles N. S. of the electro magnet; T. T. are terminal screws, but where very
large currents have to be measured and where it would be inconvenient to cut the
main cable for the purpose of inserting the instrument into the circuit, we prefer to
use the main cable itself instead of the conductor D. D. by simply placing the
instrument over the main cable, and fixing the latter to the case of the instrument
so as to bring it within the proper distance and position with reference to the
needle
We can combine with any of the above arrangements of instruments for
measuring electro-motive force, a resistance coil which by a suitable key or switch
can be inserted into the circuit thereby increasing the total resistance of the
instrument by a convenient multiple. If for example the resistance of the

page_text: additional coil be nine times that of the electro-magnet and deflecting coils then
the maximum electro-motive force which can be measured with the additional
resistance in circuit will be ten times that which can be measured with the
resistance short circuited.
We also sometimes combine with our instruments any of the well known
commutating devices by which the direction in which the current traverses the
instrument can be reversed and we also furnish sometimes the commutator with a
handle or index which indicates at which of the terminals the current enters.
Figs 2 & 3 show an arrangement of additional resistance drum R. R. short
circuiting key K by which this can be cut out of the circuit and commutator C by
which the direction of the current through the instrument can be reversed. The
wires by which these different devices are joined up are not shewn as this will
be clear without further explanation to any practical electrician.
We prefer
to arrange the connections in such manner that the handle h on the commutator
points to the positive terminal screw when the pointer stands over the scale.
We can also surround our instruments with a casing of soft iron, suitably placed
and arranged so as to shield them from external disturbing influences as for
instance neighboring electric currents, masses of iron, dynamo-electric machines
or magnets.
Such an arrangement of an iron surrounding shield or casing is shewn in dotted
lines in Fig. 3. at I I.
Having thus described the nature and objects of 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 herein
described and shewn in the accompanying Drawings by way of example to enable
our invention to be more clearly understood and practised, but what we do claim
as our invention is :-
First. The improvements in apparatus for measuring electric currents and electro-
motive force, substantially as, and for the purpose set forth.
Second. The arrangement or combination of means for exciting the electro
magnets of these instruments by a shunt, and for utilizing the main cable itself as
a deflecting agent in cases where it is inconvenient to cut a main or such like cable
for the purpose of inserting a measuring instrument, substantially as set forth.
In witness whereof we the said Rookes Evelyn Bell Crompton and Gisbert
Kapp have hereunto set our hands and seals this eleventh day of March in the
year of our Lord One thousand eight hundred and eighty-four.
D
ADALOTION