Improvements in or relating to Electric Measuring and Indicating Apparatus.
Year: 1899

page_text: Date of Application, 21st Aug., 1899
Complete Specification Left, 14th May, 1900-Accepted, 18th Aug., 1900
PROVISIONAL SPECIFICATION.
Improvements in or relating to Electric Measuring and Indicating
Apparatus.
We, HORACE DARWIN, Scientific Instrument Manufacturer, of The Orchard,
Cambridge, ERNEST HOWARD GRIFFITHS, Fellow of Sidney College, of 12, Park-
side, Cambridge, and WILLIAM CECIL DAMPIER WHETHAM, Fellow of Trinity
College, of 5, St. Peter's Terrace, Cambridge, all in the County of Cambridge,
do hereby declare the nature of this invention to be as follows:-
-
This invention relates to electric measuring an indicating apparatus and has
for its object the provision of means whereby electrical measurements such as
those of resistance or voltage are automatically indicated and, if desired,
recorded.
Apparatus according to this invention is adapted for use in the various
methods of electrical measurement commonly known as "null" methods; for
example the measurement of potential differences by the "potentiometer" method
or of resistance by the Wheatstone bridge and its numerous modifications.
The essential feature of this invention is the employment of a circuit capable
of motion when a current passes through it-for example the coil of a D'Arsonval
galvanometer or of an electro-dyanometer-such circuit being so arranged that
it automatically sets itself into a position where balance is obtained, that is
where no current flows through it, thus indicating the amount of some electrical
quantity.
The following is a description of one construction of apparatus according to
this invention adapted by way of example to measure electrical resistances as,
for instance, in the measurement of temperatures by electric pyrometers.
<
"
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The movable circuit is in the form of the coil of a D'Arsonval galvanometer
and is preferably suspended by a pivof through which the current enters. The
current leaves the coil through two long pointers or booms suspended from
its lower end, these "booms" being placed in connection with a "bridge wire
corresponding in its functions to the wire of a metre bridge. The connections
are so arranged that if a "boom" is in contact with the bridge wire at any point
except that where balance" exists, the current flowing through the coil will
tend to cause it to turn and move the “boom” along the wire until the balancing
point is reached.
"
"
It is not essential to the operation of the apparatus that more than one "boom
should be used, but the employment of two greatly facilitates the working of
the device. The "booms" are placed, one above, the other below the "bridge
wire and if quite free to swing are out of contact with it. Bars of non-con-
ducting material are placed above and below the bridge wire extending preferably
throughout its length, and these bars are moved towards and away from the
wire alternately say by a clock or other motor. Each bar in its turn presses
the end of a boom into contact with the bridge wire, and one "boom" is
not released by its bar until the other "boom" is in contact with the wire; all
danger of sparking being thus avoided.
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39
Suppose now the top boom to be in contact with the bridge-wire at the

page_text: balancing point the bottom "boom" being free; a change of resistance in one
arm of the bridge will cause a current to flow through the coil which accordingly
turns against the slight couple exerted by the suspension of the top "boom
and carries the bottom boom towards the new balancing point. This point may
be reached when the bottom boom in its turn is fixed by its bar but if not, the
action will be continued, the coil carrying the released top "boom" towards the
balancing point.
The position of the "booms" may be read upon a scale which may be graduated
to give, directly, measurements of resistance, temperature, etc., or light mirrors
may be mounted upon the moving coil or its appendages and used in the well-
known way to reflect a beam of light upon a fixed scale. If it is desired to
record the indications, any of the well known recording devices, such as a
tracing point moving over a travelling chart etc., may be used in conjunction
with the apparatus above described. The record might, for example, be made
by causing a tracing point carried by the boom to come into contact with the
travelling paper at the same time that the boom is pressed against the bridge
wire, or again the record may be made photographically.
The " booms" are preferably balanced by means of small counter-weights and
their vibration may be checked by attaching to them light vanes of mica or other
convenient material. The non-conducting bars are preferably mounted upon a
system of levers pivotted to a rigid portion of a frame or base supporting the
apparatus. The motion of the bars towards and away from the bridge wire may
be positively caused by the clock or other motor or such motor may be used
only to move the bars in one direction, the return motion being caused say by
weights or springs.
The D'Arsonval galvanometer with the bridge wire, and coils forming the
other arms of the bridge, together with the clock or other motor etc., are all
conveniently mounted upon a common base provided with means for adjusting
its level.
Apparatus according to this invention may be used with alternating currents,
but it is then necessary to replace the magnets of the D'Arsonval galvanometer
by coils of wire through which the alternating currents flow, and produce an
alternating magnetic field. The bridge coils etc., are connected with the field
coils and, if desired, a shunt coil is provided which may or may not possess self
induction. The circuits are so arranged that any difference of phase between
the applied alternating electro-motive force and the current in the field coils
shall be the same or approximately the same as the difference in phase between
the electro-motive force and the current in the bridge. The changes of current
in the moving coil thus synchronize with those in the field coils and therefore
measurements of resistance etc., may be made as in the case of direct currents.
It is to understood that various modifications may be made in apparatus of
the kind above described without departing from the spirit of this invention
and that the particular application to the measurement of resistances is merely
given by way of example, the invention being adapted as herein before men-
tioned to the determination of any electrical quantity by a "null" method.
Dated this 21st day of August 1899.
בדבד יחד
COMPLETE SPECIFICATION.
Improvements in or relating to Electric Measuring and indicating
Apparatus.
We, HORACE DARWIN, Scientific Instrument Manufacturer, of The Orchard,
Cambridge, ERNEST HOWARD GRIFFITHS, Fellow of Sidney College, of 12, Park-
Improvements in or relating to Electric Measuring and indicating
Apparatus.
We, HORACE DARWIN, Scientific Instrument Manufacturer, of The Orchard,
Cambridge, ERNEST HOWARD GRIFFITHS, Fellow of Sidney College, of 12, Park-

page_text: side, Cambridge, and WILLIAM CECIL DAMPIER WHETHAM, Fellow of Trinity
College, of 5, St. Peter's Terrace, Cambridge, all in the County of Cambridge,
do hereby declare the nature of this invention and in what manner the same
is to be performed, to be particularly described and ascertained in and by the
following statement:―
This invention relates to electric measuring and indicating apparatus and has
for its object the provision of means whereby electrical measurements such as
those of resistance or voltage are automatically indicated and, if desired,
recorded.
Apparatus according to this invention is adapted for use in the various
methods of electrical measurement commonly known as "null" methods; for
example the measurement of potential differences by the potentiometer method
or of resistance by the Wheatstone bridge and its numerous modifications.
The essential feature of this invention is the employment of a circuit capable
of motion when a current passes through it-for example the coil of a D'Arsonval
galvanometer or of an electro-dyanometer-such circuit being so arranged that
it automatically sets itself into a position where balance" is obtained, that is
where no current flows through it, thus indicating the amount of some electrical
quantity.
66
"
Conveniently the current may leave the movable circuit or coil by a long arm
or boom" suspended from its lower end, this "boom" being placed in con-
nection with a bridge wire corresponding in its functions to the wire of a metre-
bridge. The connections are so arranged that if the boom is in contact
with the bridge wire at any point except that where balance exists, the current
flowing through the coil will tend to cause it to turn and move the "boom
along the wire until the balancing point is reached.
In the accompanying drawings, which illustrate one construction of apparatus
according to this invention adapted by way of example to measure electrical
resistances as, for instance, in the measurement of temperatures by electrical
pyrometers,
Figure 1 is a perspective view of the apparatus, part of the containing case
being removed;
Figure 2 is a diagram illustrating the electrical connections of the apparatus;
Figure 3 is a perspective view of a recording device which may be used in
apparatus according to this invention, and
Figure 4 illustrates diagrammatically a portion of the apparatus adapted for
use with alternating currents.
Like letters indicate like parts throughout the drawings.
Within a case A which may conveniently be provided with a glass top and
front (not shown) is a permanent magnet B secured as by screws 13¹ to the
back of the case. A block C of magnetic material is fixed between the poles
of the magnet B and in the magnetic fields between the poles of the magnet B
and the block C a coil D is suspended. Fixed to the upper part of the coil D
is a ring D into which a screw D2 is inserted, this screw having an agate cup
at its lower end. The cup is supported upon a pivot D³ carried by a bracket D
attached to the case A. Current is led into the coil 1 through a fine wire 15
which is connected to the bracket D. The magnet B with its coil D forms a
galvanometer of the well-known D'Arsonval type.
"
Rigidly attached to the lower end of the coil D is a horizontal arm or
boom" E and a torsion conductor F in the form of a fine metal strip connects
the bottom end of the coil and the "boom E with another boom" G, the
'booms' E and G being joined by links H. Counterweights E¹ and G¹ are
provided for the "booms E and G respectively. The lower boom" G
serves as is hereinafter described as a conducting arm to make connection between
the coil D and a bridge wire J supported within the case A. A light spring G²
is attached to and extends above the "boom" G, and the end of that "boom
is arranged to move over a scale L and thus indicate the position of the balanc-
"

page_text: .
ing points upon the bridge wire and the magnitude of the electrical quantity
measured.
In order that the "boom" G shall alternately make contact with and be free
to travel over the bridge wire J a bar K of non-conducting material is pro-
vided, supported by arms K' from a pivotted bar or rock-shaft K2 which is
rocked automatically by means of pins M attached to one member of a clockwork
train, these pins coming into contact as the wheel rotates with a downwardly
extending lever K³ attached to the rock-shaft K².
The positions of the bar K and the "booms" E and G are such that when
the bar is in its lowest position as shown in Figure 1, the spring G² is com-
pressed and the "boom G pressed firmly against the bridge wire J, the end
of the upper "boom"E being then slightly above the top of the bar K. When,
owing to the rocking of the shaft K2, the bar K is raised, its upper edge comes
into contact with the end of the "boom" E before it has ceased to press upon the
spring G; that is to say there is no interval of time during which both
booms" E and G are free to move, and for a short interval both are in contact
with the bar K.
By reference to Figure 2 it will be seen that the bridge wire J is equivalent,
in the scheme of electrical connections there represented, to the bridge wire of
a metre-bridge, the "booms" E and G forming what may be termed an auto-
matically moving key in the galvanometer circuit.
At one end of the bridge wire J, Figure 2 is a resistance N and another
resistance N is connected to the other end of the bridge wire. Resistances ()
and preferably equal to one another form the other arms of the bridge, a
battery P being connected in the usual way across opposite points of the
circuit.
If the apparatus is used in the electrical measurement of temperatures, the
resistance N would preferably be in the form of a platinum pyrometer, while
the resistance N¹ would be formed of, say, manganin or some other substance
having a very small coefficient of increase of resistance with temperature, the
two resistances N and N¹ being equal at some given temperature. At that
temperature if the resistances 0 and 0¹ are equal, balance will of course be
obtained when the boom" G is in contact with the middle point of the bridge
wire J.
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65
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The coil D is wound in such a direction that if a current flows through it
owing to there being a difference of potential between its two ends it will turn
in the direction which will move the boom G towards the balancing point.
Suppose now that a change of one of the resistances N occurs when the
apparatus is in the position shown in Figure 1, the coil will turn against the
slight couple exerted by the torsion conductor F and will carry the top
"
boom E towards the balancing point. The bar K on rising, frictionally
engages the end of the
boom" and keeps the coil in its new position, and
then releases the lower "boom G which through the action of the torsion
conductor F and the links H moves along until it comes beneath the top
"boom"E. On its next descent the bar K makes contact between the "boom" G
and the bridge wire and the current again flows through the coil D if the
balancing point has not been reached. This action goes on until the coil 1)
has set itself so that no current flows through it when contact is made between
the "boom" G and the bridge wire J.
When the two "booms" are joined as shown in Figure 1 by a link it is not
absolutely essential that the conductor F should be a torsion wire or strip, for
the link will serve to cause the lower "boom" to follow the motion of the upper
"boom.
Further where a torsion strip is used as the suspension for the lower
"boom
it is not essential that a link should be employed, conveniently how-
ever both a torsion conductor and a link are used as in the construction illus-
trated in Figure 1.
If it is desired to preserve a record of the various positions of the "boom
"
G

page_text: relatively to the bridge wire J, a device similar to that shown in Figure 3 may
be used, a tracing point G3 being secured to the "boom" G so that when the
latter is depressed by the bar K the tracing point is brought into contact with a
travelling record strip R.
Apparatus according to this invention may be used with alternating currents,
but it is then necessary to replace the magnets of the D'Arsonval galvanometer
by coils of wire through which the alternating currents flow and produce an
alternating magnetic field; such an arrangement is diagrammatically represented
in Figure 4, the coils B2 being the field coils.
The bridge coils are connected with the field coils and if desired a shunt coil
is provided which may or may not possess self-induction. The circuits are sc
arranged that any difference of phase between the applied alternating electro-
motive force and the current in the field coils shall be the same or approxi-
mately the same as the difference in phase between the electro-motive force
and the current in the bridge. The changes of current in the moving coils thus
synchronise with those in the field coils and therefore measurements of resis-
tance, etc., may be made as in the case of direct currents. These particular
arrangements, however, form no part of the present invention but are mentioned
as showing that the apparatus according to this invention may be used with
alternating currents.
"
Although in the construction herein described and illustrated only one of the
"booms
serves as a conducting arm, yet both "booms may serve that purpose,
if desired. In that case, however, the "booms" would preferably be placed one
above and the other below the bridge wire and two bars, similar to the bar K,
would be used, one operating to press the lower "boom" up against the bridge
wire, and the other bar to press the upper
"boom
down upon the wire, these
actions taking place alternately, the one
" boom not being released until the
other is in contact with the bridge wire.
"
The scale L may be graduated so as to give directly measurements of resistance
temperature or other quantity which is measured electrically by the apparatus,
and instead of a pointer moving across a scale other means such as a beam of
light reflected from a mirror mounted upon the "booni" may be used to indi-
cate the position of the balancing point.
It is to be understood that various modifications may be made in apparatus
of the kind above described without departing from the spirit of this invention,
and that the particular application to the measurement of resistances is merely
given by way of example, the invention being adapted as hereinbefore men-
tioned to the determination of any electrical quantity by a null" method.
Having now particularly described and ascertained the nature of our said
invention and in what manner the same is to be performed, we declare that
what we claim is:-
"
1. In electrical measuring and indicating apparatus the combination with a
bridge wire of a coil or circuit having a conducting arm which makes contact
with the bridge wire and when current flows is moved automatically to the
balancing point by the action of the coil or circuit substantially as described.
2. In electrical measuring and indicating apparatus the combination with a
conducting arm or boom which is moved automatically over a bridge wire
of mechanism for intermittently causing the boom to make contact with the
bridge wire substantially as described.
3. In electrical measuring and indicating apparatus the combination with
a movable coil or circuit of booms such as E and G connected thereto and
mechanism operating substantially as described to alternately check the motion
of each boom.
4. In electrical measuring and indicating apparatus the combination with a
movable coil or circuit of linked booms such as E G arranged and operating
substantially as described.

page_text: 5. The complete measuring and indicating apparatus substantially as described
or illustrated in Figure 1 of the accompanying drawings.
Dated this 12th day of May 1900.
HORACE DARWIN.
ERNEST HOWARD GRIFFITHS.
"
"
WILLIAM CECIL DAMPIER WHETHAM. ..
Boult, Wade & Kilburn,
Agents for the Applicants.