IMPROVEMENTS IN APPARATUS FOR MEASURING AND REGULATING CURRENTS OF ELECTRICITY
Year: 1883

page_text: LETTERS PATENT to Philip Cardew Captain in H.M. Corps of Royal Engineers
and of Brompton Barracks Chatham in the County of Kent for an Invention
of "IMPROVEMENTS IN APPARATUS FOR MEASURING AND REGULATING
CURRENTS OF ELECTRICITY
"
PROVISIONAL SPECIFICATION left by the said Philip Cardew at the Office
of the Commissioners of Patents on the 5th February 1883.
PHILIP CARDEW, Captain in H.M. Corps of Royal Engineers and of Brompton
Barracks Chatham in the County of Kent "IMPROVEMENTS IN APPARATUS FOR
MEASURING AND REGULATING CURRENTS OF ELECTRICITY
"
This Invention has for its object improvements in apparatus for measuring and
regulating currents of electricity produced from any source and has especial
reference to cases where the current or the difference of potential between the two
poles producing the current has to be kept constant within narrow limits.
For this purpose I utilize the heating effect of the current on a conductor or
several conductors through which it is passed, this heating producing an extension
of the conductor, the amount of which being indicated by suitable apparatus
measures the amount of current passing.
This heating effect is produced equally by what are known as continuous or
alternating currents, and the apparatus is equally suitable for use with either.
In the simplest form of apparatus a suitable length of the conductor is enclosed
in a tube of a material of which the coefficient of linear expansion is sensibly the
same as that of the conductor.
To the bottom of this tube is fixed a box of the same material, one side of which
can be easily removed. In the box is a light pointer which is pivotted at the back
of the box and projects through a slit in the front so as to indicate over an arc of a
circle fixed in front of the box.
The conductor whose expansion is to be measured is attached at one end to the
pointer at a small distance from the pivot and at the other end to a metal pin at

page_text: the top of the tube which can be moved up or down to some extent by a screw so
as to allow of adjusting the pointer from time to time to point accurately to zero
at the upper part of the scale. The conductor whose ends are thus attached is
kept stretched either by the weight of the pointer alone, or by an added weight, or
by a spring attached to the bottom of the box. The current to be measured is
passed through the conductor by means of the tube and pin at the top and the
pivots and spring or attached wire at the bottom, care being taken that the tube
and pin is properly insulated from the connection at the bottom, except by the
conductor through which the current is to be passed. The conductor is expanded
by the passage of the current and the pointer consequently drops through a certain
angle. The scale on the arc is calibrated so as to give correct readings in Ampères
or Volts.
When the instrument is required to indicate difference of potential as between
the poles of a dynamo electric machine the conductor will generally consist of a
fine wire of some metal or alloy not easily oxidisable and whose specific resistance
is high. When it is required to indicate the current passing in any circuit it may
consist of a number of wires, either arranged parallel to each other in which case
they would all be fixed at the two ends to two transverse bars from which con-
nection would be made to the adjusting pin and pointer, or plaited or twisted
together.
The number of wires to be used and their specific resistance will be determined
by the mean strength of the currents each instrument is intended to measure.
In some cases where large currents have to be measured the conductor instead of
being simply extended in one line from top to bottom of the tube will be turned
round two or more pulleys some at the top of the tube and some at the bottom of the
box. Those at the top being all connected to the pin and those at the bottom
connected together and to the pointer so that the current divides itself among the
several turns and the total extension produced is measured by the fall of the
pointer. When more convenient a single wire will be used in place of a number of
parallel wires.
In all cases means will be provided for readily renewing the wire or wires in
case of fracture by taking off the cap at the top through which the pin passes
attaching the wire to the pin and dropping it down the tube where it is attached
to the pointer by removing one side of the box for the purpose.
If more convenient the tube may be dispensed with altogether and a rod of the
same metal or of a metal of similar expandibility used instead the wire being
protected by a casing of wood or similar arrangement. But in all cases it should
be arranged that the material between the pivot of the pointer and the upper
attachment of the wire shall be such as to expand and contract for temperature to
the same extent as the wire. In some cases two parallel wires may be used a short
distance apart to which the pointer is attached and between which is attached to
the pointer a spring pulling against the wires. The current would then be passed
through one wire alone. In place of indicating by a simple arrangement of pointer
as above the motion may be multiplied by levers or the wire may be made in
expanding to coil round a small pulley by a spring on the axle and in this way the
pointer may be easily caused to travel over an angle of 360°. If necessary the
wire used may be protected from oxidation by any covering or by being coated
with paint or oxide. For instance if a steel or iron wire be used it may be coated
with magnetic oxide by the well known process.
To protect the wire from being over heated the pointer may be caused to short
circuit it when depressed through a certain angle by means of a wire attached to it
dipping in mercury.
This apparatus can also be utilised to regulate the potential between the poles of
a dynamo machine or the current furnished by it.
For instance when the potential is required to remain constant or nearly so as
in incandescent lighting the apparatus would be connected to the two poles of the
machine

page_text: To the pointer will be attached at same point a conductor of suitable resistance
such as a piece of carbon this will dip more or less into mercury and will
form a shunt on the magnetizing coils the proper position for the mercury being
found by trial.
If now the speed increases the potential will tend to increase and will cause a
fall in the pointer but this will have the effect of reducing the resistance of the
shunt on the magnetizing coils and thus the potential will be prevented from
increasing to the extent it otherwise would.
Or instead of this the pointer may be caused when fallen below a certain limit
to put in circuit more lamps and when above this limit to break the circuit of some
lamps.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Philip Cardew in the Great Seal Patent Office on the 2nd August
1883.
PHILIP CARDEW Captain in H.M. Corps of Royal Engineers and of Brompton
Barracks Chatham in the County of Kent "IMPROVEMENTS IN APPARATUS FOR
MEASURING AND REGULATING CURRENTS OF ELECTRICITY
"
This Invention has for its object improvements in apparatus for measuring and
regulating currents of electricity produced from any source and has especial
reference to cases where the current or the difference of potential between the two
poles producing the current has to be kept constant within narrow limits.
For this purpose I utilize the heating effect of the current on a conductor or
several conductors through which it is passed, this heating producing an extension
of the conductor the amount of which being indicated by suitable apparatus
measures the amount of current passing.
This heating effect is produced equally by what are known as continuous or
alternating currents, and the apparatus is equally suitable for use with either.
As the amount of heat produced depends on the resistance offered by the con-
ductor, and this resistance varies in all conductors with temperature, those in
which this variation is least as metallic alloys are preferable.
In the simplest form of apparatus, shewn in elevation and section in Figures 1
and 2, a suitable length of the conductor is enclosed in a tube of a material of
which the co-efficient of linear expansion is sensibly the same as that of the
conductor.
A brass tube is found to answer well when the conductor used is either of
German silver or platinum silver or even of copper.
But if the conductor be of
iron or steel an iron tube should be used.
To the bottom of this tube is fixed a box of the same material, one side of which
can be easily removed. In the box is a light pointer which is pivotted at the back
of the box and projects through a slit in the frout so as to indicate over an are of
a circle fixed in front of t' fox.
The conductor whose sion is to be measured is attached at one end to the
pointer at a small distance .rom the pivot and at the other end to a metal pin at
the top of the tube which can be moved up or down to some extent by a screw so
as to allow of adjusting the pointer from time to time to point accurately to zero
at the upper part of the scale. The conductor whose ends are thus attached is
kept stretched either by the weight of the pointer alone, or by an added weight, or
by a spring attached to the bottom of the box. The current to be measured is
passed through the conductor by means of the tube and pin at the top and the pivots
and spring or attached wire at the bottom, care being taken that the tube and pin
is properly insulated from the connection at the bottom except by the conductor
through which the current is to be passed. The conductor is expanded by the
passage of the current and the pointer consequently drops through a certain angle.
The scale on the arc is calibrated so as to give correct readings in Ampères or
Volts.
In Figure 1 which is a front elevation of the instrument, A is a brass tube 6 feet
long down the centre of which passes the conductor, B is the adjusting pin and
screw at the top passing through a moveable cover, C a brass box at the bottom
which can be opened by unscrewing D, E the scale with glass front over which the
index F noves, G a wooden base and H the terminals.

page_text: In Figure II which is a section through the same instrument the same letters
apply. I is a block of ebonite to which the brass piece containing the pivotting
arrangement is screwed, K the pivotting piece for the index and L an insulated
lead connecting K to one terminal. The other terminal is connected to the brass
box and tube.
When the instrument is required to indicate difference of potential as between
the poles of a dynamo electric machine the conductor will generally consist of a
fine wire of some metal or alloy not easily oxidisable and whose specific resistance
is high.
The alloy of platinum and silver is very suitable. The finer the wire the better
suited it is for the purpose 003 inches diameter does well or even finer wire may
be employed.
I find a length of 6 feet of 003 inch diameter wire suitable to indicate up to
50 Volts. When the instrument is required to measure greater differences of
potential I employ a resistance coil of platinum silver or German silver wire
having four times the resistance of the conductor which is put in circuit by means
of a plug. When it is required to indicate the current passing in any circuit the
conductor may consist of a number of wires either arranged parallel to each other
in which case they would all be fixed at the two ends to two transverse bars from
which connection would be made to the adjusting pin and pointer or plaited or
twisted together.
The number of wires to be used and their specific resistance will be determined
by the mean strength of the currents each instrument is intended to measure.
Instead of several wires a thin strip of the metal may be used.
In some cases where large currents have to be measured the conductor instead of
being simply extended in one line from top to bottom of the tube will be turned
round two or more pulleys some at the top of the tube and some at the bottom of
the box. Those at the top being all connected to the pin and those at the bottom
connected together and to the pointer so that the current divides itself among the
several turns and the total extension produced is measured by the fall of the
pointer. When more convenient a single wire will be used in place of a number of
parallel wires.
The reason for using a number of small wires or turns of wire or a thin strip of
metal in place of a thick wire is that the time taken up in heating a conductor by
any current to its full value, that is to say, the time before the full value of the
deflection for any current is obtained on these instruments is lessened by increasing
the surface of conductor exposed compared with its volume.
In all cases means will be provided for readily renewing the wire or wires in
case of fracture by taking off the cap at the top through which the pin passes
attaching the wire to the pin and dropping it down the tube where it is attached
to the pointer by removing one side of the box for the purpose.
If more convenient the tube may be dispensed with altogether and a rod of the
same metal or of a metal of a similar expandibility used instead, the wire being
protected by a casing of wood or similar arrangement. But in all cases it should
be arranged that the material between the pivot of the pointer and the upper
attachment of the wire shall be such as to expand and contract for temperature to
the same extent as the wire. In some cases two parallel wires may be used a short
distance apart to which the pointer is attached and between which is attached to
the pointer a spring pulling against the wires. The current would then be passed
through one wire alone.
In place of indicating by a simple arrangement of pointer as above the motion
may be multiplied by levers or the wire may be made in expanding to coil round a
small pulley by a spring on the axle and in this way the pointer may be easily
caused to travel over an angle of 360°.
An arrangement of this sort is shewn in Figures III IV and V. In this case
instead of one tube 6 feet long, two tubes A side by side each about 3 feet long are
emploved
The win
D 20
11

page_text: the top, and down the other tube. The tubes are fixed to a wooden box C at the
bottom with a metal dial over which moves an index turning on an axle. On an
enlargement on this axle is cut a screw thread (female) of four turns.
This axle is pivotted and supported so as to move with great freedom carrying
the index with it. One end of the wire is brought to the centre of the screw
thread and is bent round for two complete turns in the thread. The wire is then
soldered in the thread at one point so as to prevent all possibility of slip.
After passing round the screw the end of the wire is fastened to a German silver
or other metallic spiral spring D which passes to the bottom of the box and is fixed
there. This spring serves to keep the wire stretched and also to conduct the
current from the wire. It should be just strong enough to keep the wire stretched
without putting an undue strain on it. The screw thread should be of such
dimensions that the length of wire required for one complete turn of the thread
should be about inch the other end of the wire is fixed to a pin E in the box to
which a slow motion can be imparted by a screw to allow of correcting the zero as
in the other form of instrument. The support of this pin and the spiral spring are
each connected to a terminal forming the two terminals of the instrument.
When
the instrument is required to measure higher differences of potential than 50 Volts
either the spring or pin is connected to one plate of an ordinary peg commutator
and the terminal to the other plate, between these plates is connected a resistance
coil as described of 4 times the resistance of the conductor and they can be short
circuited by a peg.
By this means differences of potential up to 250 Volts can be measured-this
form of instrument is much lighter and more portable than the other which is
intended to be fixed in one position as in the engine room of an electric light
installation.
If necessary the wire used may be protected from oxidation by any covering or
by being coated with paint or oxide. For instance if a steel or iron wire be used
it may be coated with magnetic oxide by the well known process.
To protect the wire from being over heated the pointer may be caused to short
circuit it when depressed through a certain angle by means of a wire attached to it
dipping in mercury. Or in the circuit may be included a short piece of wire of
smaller section calculated to fuse and break the circuit before damage is done.
This apparatus can also be utilised to regulate the potential between the poles of
a dynamo machine or the current furnished by it.
For instance when the potential is required to remain constant or nearly so as in
incandescent lighting the apparatus would be connected to the two poles of the
machine.
To the pointer will be attached at some point a conductor of suitable resistance
such as a piece of carbon, this will dip more or less into mercury and will form
a shunt on the magnetizing coils, the proper position for the mercury being found
by trial.
If now the speed increases the potential will tend to increase and will cause a
fall in the pointer but this will have the effect of reducing the -resistance of the
shunt on the magnetizing coils and thus the potential will be prevented from
increasing to the extent it otherwise would. Or instead of this the pointer may
be caused when fallen below a certain limit to put in circuit more lamps and when
above this limit to break the circuit of some lamps.
The general principles to be observed are as follows:-
When a current of electricity is passing through a conductor heat is developed at
the rate of C2 R units of heat per unit of time, where C is the current flowing
and R the resistance of the conductor. This heat raises the temperature of the
conductor above that of the surrounding medium and the temperature will con-
tinue to rise if the current be maintained constant until the flow of heat from
the conductor to the medium by radiation is equal to the rate at which heat is
being developed by the current at which point the temperature will remain constant
so long as there is no change in the other quantities. As this rise in the

page_text: temperature of the conductor produces a proportional increase in its length it is
evident that by measuring this increase of length we obtain data for determining
the amount of current passing.
In order to construct a practical instrument on this principle the following
points require attention.
1. The effect of atmospheric changes of temperature must be as far as possible
eliminated.
2. The current of heat from the conductor due to a certain rise of temperature
must be always the same.
3. To indicate varying currents the final temperature due to any current must
be arrived at instantaneously.
4. The energy absorbed by the instrument must generally be a minimum.
5. The conductor must be able to take a high temperature without danger of
oxidation or alteration of molecular structure.
The first two conditions have been satisfied in the instruments described by
enclosing the conductor to be heated in a tube of a substance having the same
co-efficient of expansion for temperature which compensates atmospheric changes of
temperature and causes the current of heat to be always that due to the difference
of temperature between the conductor and the tube without being affected by the
absolute temperature of either. The conductor is completely protected from
contact with the external air and no amount of draught on the tube affects the
indications since it only alters the absolute temperature of the combination.
The third condition is best satisfied by giving the conductor sufficient surface for
radiation combined with a small sectional area.
The fourth condition is met by the use of as small a mass of conductor as
possible.
The fifth condition is best met by the use of platinum silver wire.
Having thus described the nature of my said invention and the manner of
performing the same I would have it understood that I claim my improved
instrument for measuring currents of electricity or difference of potential in which
the passage of a current heats a conductor and thereby causes an extension thereof
which in turn causes movement of an index substantially as described.
I also claim the combination with my measuring instrument of regulating
appliances as described by which an approximately constant difference of potential
is maintained the resistance in a shunt circuit being automatically varied by the
instrument.
In witness whereof I the said Philip Cardew have hereunto set my hand and
seal this first day of August in the year of our Lord One thousand eight hundred
and eighty three.
PHILIP CARDEW (L.S.)