Improvements in Instruments for Measuring and Regulating Electric Currents and Differences of Potential, Partly Applicable to Telegraphic Purposes.
Year: 1888

page_text: Date of Application, 25th Oct., 1888
Complete Specification Left, 23rd July, 1889-Accepted, 28th Sept., 1889
PROVISIONAL SPECIFICATION.
Improvements in Instruments for Measuring and Regulating Electric
Currents and Differences of Potential, Partly Applicable to
Telegraphic Purposes.
I, PHILIP CARDEW, Captain in H.M. Corps of Royal Engineers, and of Brompton
Barracks, Chatham, in the County of Kent, do hereby declare the nature of this
invention to be as follows:-
This invention has for its object improvements in the class of instruments for
measuring and regulating electric currents and differences of potential described in
my Patent No. 623 5th February 1883 and generally known by my name, and more
particularly to enable suitable indications to be obtained from a shorter length of
conductor and in a more convenient manner than has hitherto been the case, thereby
rendering the instruments more portable and lessening the necessary expenditure of
power.
Having described the objects of the improvements effected I will now proceed to
more particularly describe the improved patterns.
There are two principal patterns of instrument in which the requisite movement of
the index for a very small extension of the wire is provided for.
In the one I shall describe first the conductor whose extension is to be measured,
and which is in the form of a thin wire, several thin wires, or a thin strip of metal, is
stretched by means of a flat spring formed of a metal having an equal co-efficient of
expansion for heat to that of the conductor. This spring is slightly bent by the
pull of the stretched conductor as a bow by its string, only that in this case the bow
is as it were cut in half at the middle, the other end of the spring being fixed and the
stretched conductor forming the chord of the arc or string of the bow does not meet
the spring again but is extended to the requisite length and then attached to a piece
of metal insulated from the spring means being provided for adjusting the tension of
the conductor by winding it on a peg or by slow motion screws so as to alter the length
of the conductor or move the point of attachment of its end slightly as will be well
understood.
Now with this arrangement, and the curvature of the spring being slight a large
motion of its free end will be produced by a slight extension of the conductor, and
this motion of the end will be still further increased if the conductor be not attached
to the end, but to a point on the spring at some distance from the free end.
In order to cause this motion to be indicated on a sufficiently large scale the
following devices may be resorted to.
(a). A long and light pointer may be attached directly to the end of the spring
forming a prolongation thereof and indicating over an arc.
(). A thin wire or thread may be attached to the end of the spring and led round
a grooved pulley or screw thread on an axle to which is attached an index moving
over a dial, the end of the wire or thread being either fastened to the pulley or screw
thread after one or more turns or part turns round it, and the wire kept stretched
by means of an ordinary hair spring attached to the axle, or the end may be brought
away from the axle and attached to a spiral spring by which it is kept stretched. In
either case the pull of this thread or wire will usually be in such a direction as to
assist the stretched conductor in bending the spring.
(c). A thin wire or thread attached as before but with the other end attached to one
of Professors Ayrton and Perry's magnifying springs described in their Patent
No. 2156 of 1883 an indicating needle being also attached to this spring moving over
a dial, and the other end of the magnifying spring being fixed.
It will be understood that the length and disposition and also the material of the

page_text: ment is to be put, whether for the measurement of Volts or Ampères, and the extr
pansion is to be measured depend upon the use to which the ins
range required.
For instance in an instrument-for-measuring, only low volts or sinal currents
thin wire in one length only will be used, this wire heing, of low conductivity if
the measurement of volts and generally of higher conductivity if for the measurem
of current: If however. high volts are to be measured a long wire is necessary, a
to avoid the use of an inconveniently. long spring or instrument this wire will
passed backwards and forwards through pulleys or blocks but so that the whole leng
forms one insulated conductor, and the pulleys or blocks will be so attached to t
fixed or moving parts as to cause either the extension of the whole length of wire
actuate the spring, or to cause portions of the wire lying parallel or nearly so to eac
other to operate together by their simultaneous expansion and thus to increase th
pull on the spring and the forces at work as will be easily understood similar device
being now employed in existing patterns of my instruments.
When large currents are to be measured a number of wires will be laid parallel t
each other and soldered to the spring or a projection from
of metal at the other end so as to be connected in parallel the spring conveying th
current back from the moving end.
at one end and to a stri
The wire or conductor and the spring are conveniently enclosed in a metal tube to
exclude currents of air, and a screw or catch is arranged, so that it can be made to
press against the spring near the free end in order to avoid danger of breaking the
wire when carrying the instrument about.
The above described pattern can be readily caused to act as a regulator, no indicating
apparatus being then generally necessary. The free end of the spring can be caused
when the wire is extended by heat to play between two contact points so as on any
variation in the volts or current to make contact with one or the other, the contact
made when the volts or current actuating the instrument are too high being against a
platinized screw which can be adjusted easily while the contact made when the Volts
or Ampères are too low, which is also of course made when the instrument, is not at
work, being against a platinized contact at the end of a delicate spring also capable of
adjustment which yields when necessary and does not obstruct the contraction of the
working conductor.
The spring and screw contacts may be both mounted on a piece of ebonite or
other non-conductor which may be moved as a whole to adjust the position of a
regulation.
The contacts thus made on any variation of the required Volts or Ampères may bo
used to correct this variation in any of the well known ways for instance by introducing
resistance into the magnetising circuit of a dynamo, or by shunting the coils if series
wound or by varying the position of the brushes, or by adding to or taking from the 4
circuit battery or accumulator cells &c., by means of the electric currents caused to
flow by the making of the contacts.
This arrangement can also be applied to electric signalling and telegraphic purposes
and forms a delicate relay in which case a very fine wire would generally be used,
and the contact against which the spring presses when not in action would generally 45
be omitted. One method I adopt for working a telegraph line with these instruments
consists in causing a steady current to flow through the instrument and then when
ignalling increasing the current for a
for a spacing" signal, this being effected by well known means.
marking" signal and if necessary decreasing
bis is that the sensitiveness is greatly increased by maintaining a high temperature 50
the wire.
The object of
".
The advantage of using an instrument on this principle for telegraphy is that the
If induction is so small as to be negligible and thus a great many instruments may
operated in one circuit without any retardation of signals.
When very small currents are required to be indicated or measured the fine wire 55
d spring are enclosed in a glass tube from which the air may be subsequently
hausted connection being made by means of platinum wires sealed into the glass as

page_text: usual, and the movement of the spring is caused to impart a motion of rotation to a
small mirror which is indicated by the motion of a reflected beam of light on a
scale.
In the second pattern of instrument all pivotting is dispensed with and the
construction in the simplest form is as follows:-
A very thin and light pointer of aluminium has three small holes bored through it
at the wider end which is uppermost, the needle moving generally in a vertical
plane. Through the uppermost hole is passed, and made fast to it, a fine wire which
is led away in a horizontal or nearly horizontal direction and attached at its other
end to a metallic peg by turning which it can be tighted up to any required extent.
Through the next hole is similarly passed and made fast another fine wire led away
in the opposite direction and also attached to a peg for tightening.
Through the lowest hole is similarly passed and made fast another fine wire led
away in the same direction as the first but terminating in a spiral spring.
Now by tightening up the two upper wires the needle is caused to hang in a
position considerably inclined to the vertical and with the spring well extended.
The current to be measured passes along the two upper wires crossing from one to
the other by the needle and by heating them causes an extension which is taken up by
the needle moving into a new position under the pull of the spring until the equilibrium
of forces is again established.
In this way a motion of the needle of 120° can be easily secured riz.-60° on each
side of the vertical, and the motion is practically circular about a virtual pivot between
the two upper holes.
This simplest form is modified as required and as will be easily understood by the
use of blocks and pulleys to enable a longer wire or several wires in parallel to
be used.
Just behind the needle a scale is fixed so that it can be easily removed if required
and the whole is enclosed in a box of metal having an equal coefficient of expansion
to that of the wire used and to which the various attachments are made and insulated
as required.
It will be understood that the range of any of these instruments adapted for the
measurement of volts may be extended to any required amount by the addition of
resistances which may be formed of a suitable length of a similar conductor to that
in the instrument I find however that a very suitable method is to insert incandescent
lamps having carbon filaments in circuit with the instrument, and the variation of
resistance of such filaments for temperature being negative tends to exactly compensate
the instrument so that no correction for external temperature is necessary for the
greatest exactitude. In the same way instruments adapted for the measurement of
current may have their range increased by shunting thein, and if the conductor used
as a shunt have a smaller variation of resistance for temperature than that used in the
instrument a similar compensation is obtained.
Dated this with day of October 1888
CARPMAEL & Co.,
Agents for the Applicant.
COMPLETE SPECIFICATION.
Improvements in Instruments for Measuring and Regulating Electrio
Currents and Differences of Potential, Partly Applicable to
Telegraphic Purposes.
1, PHILIP CARDEW, Captain in H.M. Corps of Royal Engineers, and of Brompton
Barracks, Chatham, in the County of Kent, 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 has for its object improvements in the class of instruments for
measuring and regulating electric currents and differences of potential described in
my Patent No. 623 5th February 1883 and generally known by my name, and, more

page_text: conductor and in a more convenient manner than has hitherto been the case, ther
Suable indications to be obtained from a shorter lengt
. rendering the instruments more portable and lessening the necessary expenditure
power.
Having described the objects of the improvements effected I will now proceed
more particularly describe the improved patterns.
There are two principal patterns of instruments in which the requisite movement
the index for a very small extension of the wire is provided for.
In the one I shall describe first the conductor whose extension is to be measure
and which is in the form of a thin wire, several thin wires, or a thin strip
metal, is stretched by means of a flat spring formed of a metal having an equ
coefficient, of expansion for heat to that of the conductor. This spring is slightl
bent by the pull of the stretched conductor as a bow by its spring, only that in th
case the bow is as it were cut in half at the middle, the other end of the spring bein
fixed and the stretched conductor forming the chord of the arc (or string of th
bow) does not meet the spring again, but is extended to the requisite length and
then attached to a piece of metal insulated from the spring, means being provided fo
adjusting the tension of the conductor by winding it on a peg or by slow motion
screws so as to alter the length of the conductor, or move the point of attachment of
its end slightly as will be well understood.
Now with this arrangement, and the curvature of the spring being slight a large
motion of its free end will be produced by a slight extension of the conductor, and
this motion of the end will be still further increased if the conductor be not
attached to the end, but to a point on the spring at some distance from the free
end.
In order to cause this motion to be indicated on a sufficiently large scale the
following devices may be resorted to.
(a) A long and light pointer may be attached directly to the end of the spring
forming a prolongation thereof and indicating over an arc.
(b) A thin wire or thread may be attached to the end of the spring and led 'round
a grooved pulley or screw thread on an axle to which is attached an index moving over
a dial, the end of the wire or thread being either fastened to the pulley or screw thread
after one or more turns or part turns round it, and the wire kept stretched by means
of an ordinary hair spring attached to the axle, or the end may be brought away
from the axle and attached to a spiral spring by which it is kept stretched. In either
case the pull of this thread or wire will usually be in such a direction as to assist the
stretched conductor in bending the spring.
(c). A thin wire or thread attached as before but with the other end attached to one
of Professors Ayrton and Perry's magnifying springs described in their Patent
No. 2156 of 1883 an indicating needle being also attached to this spring moving over 4
a dial, and the other end of the magnifying spring being fixed.
It will be understood that the length and disposition and also the material of the
conductor whose expansion is to be measured depend upon the use to which the
instrument is to be put, whether for the measurement of Volts or Ampères, and
the extreme range required.
For instance in an instrument for measuring only low volts or small currents one
thin wire in one length only will be used, this wire being of low conductivity if for
the measurement of volts and generally of higher conductivity if for the measurement
of current. If however high volts are to be measured a long wire is necessary, and
o avoid the use of an inconveniently long spring or instrument this wire will be 50
passed backwards and forwards through pulleys or blocks but so that the whole.
ength forms one insulated conductor, and the pulleys or blocks will be so attached
o the fixed or moving parts as to cause either the extension of the whole length of
vire to actuate the spring, or to cause portions of the wire lying parallel or nearly so
each other to operate together by their simultaneous expansion and thus to increase 55
he pull on the spring and the forces at work as will be easily understood, similar
evices being now employed in existing patterns of my instruments.
4

page_text: When large currents are to be measured a number of wires will be laid parallel to
each other and soldered to the spring or a projection from it at one end and to a strip
of metal at the other end so as to be connected in parallel the spring conveying the
current back from the moving end.
The wire or conductor and the spring are conveniently enclosed in a metal tube to
exclude currents of air, and a screw or catch is arranged so that it can be made to
press against the spring near the free end in order to avoid danger of breaking the
wire when carrying the instrument about.
The above described pattern can be readily caused to act as a regulator, no
indicating apparatus being then generally necessary. The free end of the spring
can be caused when the wire is extended by heat to play between two contact
points so as on any variation in the volts or current to make contact with one or
the other, the contact made when the volts or current actuating the instrument are
too high being against a platinized screw which can be adjusted easily while the
contact made when the Volts or Ampères are too low, which is also of course made
when the instrument is not at work, being against a platinized contact at the end of
a delicate spring also capable of adjustment which yields when necessary and does
not obstruct the contraction of the working conductor.
The spring and screw contacts may be both mounted on a piece of ebonite or
other non-conductor which may be moved as a whole to adjust the position of
regulation.
The contacts thus made on any variation of the required Volts or Ampères may be
used to correct this variation in any of the well known ways for instance by intro-
ducing resistance into the magnetising circuit of a dynamo, or by shunting the coils
if series wound or by varying the position of the brushes, or by adding to or taking
from the circuit battery or accumulator cells &c. by means of the electric currents
caused to flow by the making of the contacts..
This arrangement can also be applied to electric signalling and telegraphic purposes
and forms a delicate relay in which case a very fine wire would generally be used, and
the contact against which the spring presses when not in action would generally be
omitted. One method I adopt for working a telegraph line with these instruments
consists in causing a steady current to flow through the instrument and then when
signalling increasing the current for a "marking" signal and if necessary decreasing
it. for a
spacing signal, this being effected by well known means. The object of
this is that the sensitiveness is greatly increased by maintaining a high temperature in
the wire.
..
22
The advantage of using an instrument on this principle for telegraphy is that the
self induction is so small as to be negligible and thus a great many instruments
may be operated in one circuit without any retardation of signals.
When very small currents are required to be indicated or measured the fine wire
and spring are enclosed in a glass tube from which the air may be subsequently
exhausted connection being made by means of platinum wires sealed into the glass as
usual, and the movement of the spring is caused to impart a motion of rotation to a
small mirror which is indicated by the motion of a reflected beam of light on a
scale.
In the second pattern of instrument all pivotting is dispensed with and the con-
struction in the simplest form is as follows:-
A very thin and fight pointer of aluminium has three small holes bored through it
at the wider end which is uppermost, the needle moving generally in a vertical plane.
Through the uppermost hole is passed, and made fast to it a fine wire which is led
away in a horizontal or nearly horizontal direction and attached at its other end to a
metallic peg by turning which it can be tightened up to any required extent. Through
the next hole is similarly passed and made fast another fine wire led away in the
opposite direction and also attached to a peg for tightening.
Through the lowest hole is similarly passed and made fast another fine wire led
away in the same direction as the first but terminating in a spiral spring.

page_text: Now by tightening up the two upper wires the needle is caused to hang in a position
considerably inclined to the vertical and with the spring well extended. The current
to be measured passes along the two upper wires crossing. from one to the other by
the needle and by heating them causes an extension which is taken up by the needle
moving into a new position under the pull of the spring, until the equilibrium of
forces is again established.
In this way a motion of the needle of 120° can be easily secured viz. 60° on each
side of the vertical, and the motion is practically circular about a virtual pivot between
the two upper holes.
This simplest form is modified as required and as will be easily understood by the
use of blocks and pulleys to enable a longer wire or several wires in parallel to be
used.
Just behind the needle a scale is fixed so that it can be easily removed if required
and the whole is enclosed in a box of metal having an equal co-efficient of expansion
to that of the wire used and to which the various attachments are made and insulated
as required.
It will be understood that the range of any of these instruments adapted for the
measurement of volts may be extended to any required amount by the addition of
resistances which may be formed of a suitable length of a similar conductor to that in
the instrument. I find however that a very suitable method is to insert incandescent
lamps having carbon filaments in circuit with the instrument, and the variation of
resistance of such filaments for temperature being negative tends to exactly compensate
the instrument so that no correction for external temperature is necessary for the
greatest exactitude.
In the same way instruments adapted for the measurement of
current may have their range increased by shunting them, and if the conductor used
as a shunt have a smaller variation of resistance for temperature than that used in the
instrument a similar compensation is obtained.
In order that my said Invention may be fully understood and readily carried into
effect 1 will proceed to describe the drawings hereunto annexed.
DESCRIPTION OF THE DRAWINGS.
Figure 1 is an elevation partly in soction of a measuring instrument constructed in
accordance with my Invention. a is the case of the instrument, it is glazed in front
so that the indications may be seen. bis a metal spring made fast by one end to the
base of the case. c is a wire attached to the free end of the spring band also to the
base of the case where the wire can be wound upon a pog to give a suitable flexture
of the bow in the original adjustment of the instrument. d is a screw which is only
used to relieve the wire from tension when the instrument is out of use. e and fare
conductors which lead the current into and from the instrument, the flow being from
one of these conductors by the bow and string to the other conductor. g is a scale
graduated to accord with a standard. For an instrument for measuring difference of
potential the resistance of the wire c should be high, and a wire of platinised silver
say 0025 of an inch in diameter is usually suitable; an inch of this wire should be
allowed for each volt of the instruments maximum. The bow should be of German
Silver as the expansion of this metal by heat is about the same as that of platinised
silver. If the bow b is of a metal which expands at a rate different from that of the
wire the zero of the instrument will alter with any change in the external temperature
-where the instrument is used for nieasuring current it is desirable that its resistance
should be small and then silver is a suitable material for both the wire c and the
spring b. If the wire be 003 of an inch in diameter it will be suitable to allow one
ampere per wire.
Figure 2 differs from Figure 1 in that the bow spring b is twisted
one quarter round near the base and this confines its resilient action to the lower part
An auxilliary spring his then with advantage applied near the end of the bow.
Figure 3 differs from figure 2 in that the bow is prolonged or carries a pointer
The extremity of this pointer moves over an arc.larger than that traversed by the end

page_text: of the bow, consequently the graduations of the scale ġ may be made larger and they
are then more readily seen.
In Figure 4 a thread i is represented passing from the end of the bow around an
axis on which is a pointer k. The cord is kept in tension by a coiled spring attached
by one end to the axis and by the other to a stationary support. g is the scale with
the larger graduations.
Figure 5 differs from Figure 4 only in the manner in which the spring 7 is applied;
it is attached by one extremity to the casing and by the other to a thread wound upon
the axis.
Figure 6 shows another way of causing the movements of the end of the bow to be
repeated upon a larger scale.
Here an Ayrton and Perry magnifying spring m is fixed by one end to the bow and
from its other end a thread passes to the case of the instrument. n is a pointer
attached to the magnifying spring. The movement of the bow causes the spring m to
be more or less extended, and from the nature of the spring this produces a certain
amount of coiling and uncoiling of the spring; in which movement the spring carries
the pointer n around with it and causes the end of the pointer to traverse along the
scale g which in this instrument is at right angles to the plane in which the end of the
bow moves.
Figure 7 is similar to Figure 1 except that a number of parallel wires are substituted
for a single wire forming the string of the bow. This arrangement reduces the
resistance of the instrument and adapts it to receive a large current through it.
Figure S is similar to Figure 2 except that the wire is carried over guide
pulleys o o in place of passing direct from the end of the bow, to the point of attach-
ment to the case.
This arrangement allows of the length of the wire being increased without incon-
venience, thereby adapting the instrument to a higher voltage.
The pulleys if of metal must be insulated from the plate on-which-they are carried
and this should be of German Silver to avoid changes in the zero of the
instrument.
Figure 9 is a current or potential regulator; it is similar to Figure 1 or Figure 7 but
is without a scale and the end of the bow is provided with a contact piece p which
stands between two other contact pieces and r. When the current passing
through the bow increases or diminishes the contact piece p is brought
When is in contact a
against one or other of the contact pieces and r.
current passes in a circuit of which 7 and b form part and similarly when ŕ is in
contact a current passes in another circuit of which r and 6 form part. Known
apparatus in these electric circuits effect the regulation or indication required.
Figure 10 is similar to Figure 8 and shows the arrangement for ringing an electric
bell when the current flowing through the instrument becomes too large.
Figure 11 shows an indicator of another form.
is a case glazed in front and
containing a light pointer t. u u are wires attached to the pointer and w is a spring
which keeps the wires strained. is a graduated scale along which the extremity of
the pointer traverses when a current is first passed through the wires u u or a change
in the current occurs.
8
Electric regulators may be made in this way in which case the pointer t will be
provided near its free end with a contact piece similar to that marked p in Figure 9
and this by the movement of the pointer t is brought against other contact pieces
similar to those marked q and r in Figure 9. Thus as already described in respect
to Figure 9 a current is caused to pass in one or other of two circuits and known
apparatus in these circuits effects the regulation required.
In very delicate instruments the pointer is replaced by a small mirror which causes
a beam of light to traverse along a scale in a well known way.
For telegraphic use the arrangement which I prefer is practically the same as shown
in Figure 10, a telegraphic instrument being substituted for the bell. This instrument
will work equally well with an alternating current which will not affect the ordinary

page_text: telegraph instruments which may be used in conjunction with it thus increasing the
working capacity of the line.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is :-
1. An instrument substantially as described for measuring regulating or indicating
electric current or difference of potential; having as its operative mechanism a bow
and string arrangement in which the string is a wire or conductor heated and
elongated by the passage of current and in which the position of the extremity of the
bow determines the indication or regulation required.
2. In an instrument substantially as described for measuring regulating or indicating
electric current or difference of potential, the combination with the bow and string
mechanism of multiplying gear which causes the movement of the bow to be repeated
upon a larger scale.
3. An instrument substantially as described for measuring, regulating or indicating
electric current; having as its operative mechanism a pointer or lever held suspended
between two wires attached to it at different points and passing from it in opposite
directions to fixed points of attachment with a spring operating upon the pointer or
lever in such manner as to hold the wires in tension; the said wires being heated and
elongated by the passage of current and the position of the pointer or lever deter-
mining the indication or regulation required.
4. Instruments substantially as represented by the annexed drawings.
CARPMAEL & Co.,
A gents.