Improvements in and relating to the Calibration of Electric Measuring Instruments.
Year: 1895

page_text: Date of Application, 7th Sept., 1895
Complete Specification Left, 8th June, 1896-Accepted, 25th July, 1893
PROVISIONAL SPECIFICATION.
Improvements in and relating to the Calibration of Electric
Measuring Instruments.
1, SYDNEY EVERSHED, of Woodfield Works, Woodfield Road, Harrow Road,
London, in the County of Middlesex, Electrical Engineer, do hereby declare the
nature of this invention to be as follows:-
This invention relates to electric measuring instruments, such as volt meters and
ammeters, and consists in a novel system of calibration of such instruments, and in
the adaptation to the instruments of an invariable pre-determined indicating scale
for specific purposes (such as can be cheaply produced in quantities by printing or
otherwise) in conjunction with current indicating operative parts of such a character,
as hereinafter more fully described, which can be relied upon to give invariable
readings for specific current values, subject only to an initial zero adjustment, and
the easy adjustment of a controlling counter-balance weight or spring.
It having been found, that the usual practice of calibrating electric measuring
instruments, by ascertaining and marking the indicating scale on each instrument
by direct experiment, is an expensive method of production, I reverse this method
of calibration by providing pre-determined indicating scales, similar for each
instrument for a like purpose, and which can thus be produced cheaply in quantities
by printing or otherwise, and I employ in conjunction therewith such operative
parts as are readily adaptable to such pre-determined scales.
The characteristics of such operative parts, as may be used by me, in combination
with my pre-determined indicating scales, are as follows:-
They will consist of two repelling soit iron surfaces or cores, placed longitudinally
in the axis of the coil, and parallel with or in the lines of the magnetic flux, and
hence of the same polarity to one another.
One such surface or core, herein called "the fixed core" is fixed, when in use,
but capable of initial adjustment in the coil. The other surface or core herein
called "the movable core" is capable of movement round an arbor in the axis of
the coil, carries the indicating pointer, and an adjustable counterbalance weight,
and has its motion in a plane either obliquely transverse, or at right angles, to, the
lines of the magnetic flux, producing the repulsion of the "movable surface or
core" from the "fixed surface or core.'
"
When the operative parts of an electric measuring instrument fulfil the above
conditions, my pre-determined indicating scale and my novel system of calibration
may be used with them, with great economical advantage in the cost of production
and of final calibration.
To carry my invention into effect, I construct and calibrate my instrument as
follows:-
Having chosen a particular form of operative mechanism fulfilling the afore
mentioned conditions, and prepared the tools, gauges, and templates necessary in
order to make the several parts, in large quantities, of identical forins and sizes, so
that respective parts of the operative mechanism shall be so far as possible inter-
changeable, I complete a few instruments with such operative parts; and by means of
hese instruments I determine once for all the relation between the magnetising force
of the working coil of the instrument, and the deflection of the pointer. In other
words, I determine the "law" of the particular operative mechanism I have chosen.

page_text: The use of more than one instrument for this determination is not essential, but it
is desirable, to determine the average law for several, say six, instruments, in order
that the effect of any slight differences unavoidably introduced in course of
manufacture of the operative mechanisms may be eliminated, and an accurate
average law ascertained.
The maximum magnetising force should be the same for ammeters of all ranges,
and will be assumed herein to be 500 ampere turns. Volt meters may have the
same maximum magnetising force as ammeters, but it is usually desirable to employ
a small force in order to avoid undue heating of the instrument. As a volt meter
is usually required to have an open scale, separate laws must be determined for volt
meters and ammeters, all the former being adjusted for one law, and the latter to
another law.
I next draw a curve expressing the average law, in the well known manner, and
from the curve I obtain, by inspection, the exact angles corresponding to given
values of magnetising force. There will be a different set of angles and magnetising
forces for each range of amperes or volts, for which scales are required; and to
make this description as clear as possible I will assume that the scale required is
one, reading from zero to 10 ampères, and that each ammeter to which the scale is
to be applied has 50 turns of wire on its working coil, so that 10 ampères corre-
sponds to a magnetising force of 500 ampère turns. Since the law for ammeter
scales was determined from instruments, adjusted to give the maximum deflection
of the pointer for a magnetising force of 500 ampère-turns, it is clear that the
deflections on the afore mentioned curve, corresponding with 50, 100, 150, etc., up
to 500 ampère turns of magnetising force, will be those required to be marked on
the scale for 1, 2, 3, etc., up to 10 ampères. I therefore write out a table of these
values of the deflections containing every point required on the scale. By means
of a dividing engine, adapted to make circular scales, I divide as many identical
scales for ammeters, reading to 10 ampères, as I require, taking the angles for the
several divisions from the table of deflections. A cheap and convenient way of
making large numbers of such identical scales, is to engrave a scale on a brass or
copper plate by means of the dividing-engine and table of deflections, and use the
engraved plate to print the scales on paper or card. For this purpose the deflections
taken from the table must be read off backwards on the scale of degrees marked
on the dividing engine, so that the scale may be printed in the right direction.
The scale so prepared having been attached to their correspon ling instruine its,
the operative mechanisms are finally adjusted as follows:-Assuming, as before,
that it is required to adjust an ammeter reading to 10 ampères, I first alter the
position of the fixed core, until the ammeter reads an equal percentage too high or
too low over the whole scale; that is to say, until it reads in accordance with the
law already determined for ammeters.
Next I add to or subtract from a weight or spring, used as a controlling force,
until the ammeter reads correctly at any point on the scale. It will now read
correctly over the whole scale. I then permanently fasten the fixed core, and the
weight or spring in place by soldering or otherwise, and the instrument is complete.
The whole of the operating mechanism having been made accurately to gauges,
the fixed core can be placed almost exactly in the right position for an ammeter
(or volt meter) scale, during manufacture; and the final adjustment required will
be very small, and the whole process of adjustment, just described, will only occupy
a few ininutes.
Dated this 7th day of September 1895.
FELL & WILDING,
1 Queen Victoria Street, London, E.C.

page_text: COMPLETE SPECIFICATION.
Improvements in and relating to the Calibration of Electric
Measuring Instruments.
I, SYDNEY EVERSHED, of Woodfield Works, Woodfield Road, Harrow Road,
in the County of Middlesex, Electrical Engineer, 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 instruments such as volt meters and
ammeters and consists in a novel system of calibration of such instruments, and in
the adaptation to the instruments of an invariable pre-determined indicating scale
for specific purposes (such as can be cheaply produced in quantities by printing or
otherwise) in conjunction with current indicating operative parts, of such a
character, as hereinafter more fully described, which can be relied upon to give
invariable readings for specific current values, subject only to an initial zero
adjustment, and the easy adjustment of a controlling counterbalance weight or
spring.
It having been found that the usual practice of calibrating electric measuring
instruments by ascertaining and marking the indicating scale on each instrument
by direct experiment, is an expensive method of production, I reverse this method
of calibration by providing pre-determined indicating scales, similar for each
instrument for a like purpose, and which can thus be produced cheaply in quantities
by printing or otherwise, and employ in conjunction therewith such operative
paris as are readily adaptable to such pre-determined scales.
The characteristics of such operative parts, as may be used by me, in combination
with my pre-determined indicating scales, are as follows:-
They will consist of two repelling soft iron surfaces or cores, placed longitudinally
in the axis of the coil and parallel with, or in the lines of, the magnetic flux, and
hence of the same polarity to one another.
One such surface or core, herein called the "fixed core," is fixed, when in use, but
capable of initial adjustment in the coil. The other surface or core, herein called
"the movable core," is capable of movement round an arbor in the axis of the coil,
carries the indicating pointer, and an adjustable counterbalance weight, and has
its motion in a plane either obliquely transverse, or at right angles to, the lines of
the magnetic flux, producing the repulsion of the movable surface or core from the
fixed surface or core.
When the operative parts of an electric measuring instrument fulfil the above
conditions, my predetermined indicating scale and my novel system of calibration
may be used with them, with great economical advantage in the cost of production
and of fiual calibration.
And in order that my invention may be the better understood, I now proceed to
describe the same with reference to the drawing hereto annexed, and to the letters
and figures marked thereon. Like letters refer to similar parts in each of the
figures.
Figures 1, 2, and 3 are diagrammatical sections through the operative parts of
known types of electric measuring instruments, which are adapted to my improved
method of calibration.
33
In each of the sectional diagrams, A is the index axle, lying concentric to the
enveloping brass tube T, and also to the usual external working coil (not shown),
and carrying the "movable core or iron M, a rod R on which the weight W is
capable of adjustment, and an index finger H, ranging over a scale, which with a
gravity controlling force as shown, may extend over nearly 90°. The "fixed core
or iron F, may be either within or without the brass tube T, and is attached to a
split ring S, sprung inside or outside the tube T. When S is inside T, a small
"

page_text: "
pin P, fixed to S, is provided to render adjustment and movement of S easy. F the
<< fixed core "and M the "movable core are preferably of equal length, and have
their corresponding ends in the same plane normal to the axis. As a general rule,
the closer F and M can be brought to the central axis, the better are the operative
parts adapted to my improved method of calibration.
I proceed as follows;-Having chosen a particular form of operative mechanism,
fulfilling the aforementioned conditions, such as either of those shown in Figures 1,
2 and 3, or equivalents and having prepared the tools, gauges, and templates,
necessary, in order to make the several parts in large quantities of identical forms
and sizes, so that respective parts of the operative mechanism shall be so far as
possible, interchangeable, I complete a few instruments with such operative parts;
and by means of these instruments, I deterinine once for all the relation between
the magnetizing force of the working coil of the instrument and the deflection of
the pointer. In other words, I determine the "law" of the particular operative
mechanism I have chosen. The use of more than one instrument for this deter-
mination is not essential, but it is desirable to determine the average law for
several, say six, instruments, in order that the effect of any slight differences of
the operative mechanisms, unavoidably introduced in course of manufacture may be
eliminated, and an accurate average law ascertained.
The maximum magnetizing force should be the same for ammeters of all ranges,
and will be assumed herein to be 500 ampere turns. Volt meters may have the
same magnetizing force as ammeters, but it is usually desirable to employ a small
force in order to avoid undue heating of the instrument. As a voltmeter is usually
required to have an open scale, separate laws must be determined for volt-meters
and ammeters, all the former being adjusted for one law, and the latter to
another law.
I next draw a curve expressing the average law in the well known manner, and
from the curve I obtain by inspection the exact angles corresponding to given
values of magnetizing force. There will be a different set of angles and
magnetizing forces for each range of amperes or volts for which scales are
required, and to make this description as clear as possible, I will assume that the
scale required is one reading from zero to 10 amperes, and that each ammeter to
which this scale is to be applied has 50 turns of wire on its working coil, so that
10 amperes corresponds to a magnetizing force of 500 ampere turns. Since the
law for ammeter scales was determined from instruments adjusted to give the
maximum deflection of the pointer for a magnetizing force of 500 ampere-turns, it
is clear that the deflections on the aforementioned curve corresponding with 50,
100, 150 etc. up to 500 ampere-turns of magnetizing force, will be those required to
be marked in the scale for 1, 2, 3, etc. up to 10 amperes. I therefore write out a
table of these values of the deflections containing every point required on the
scale. By means of a dividing engine adapted for making circular scales I divide
as many identical scales for ammeters reading to 10 amperes as I require, taking
the angles for the several divisions from the table of deflections. A cheap and
convenient way of making large numbers of such identical scales, is to engrave a
scale on a brass or copper plate by means of the dividing engine, and table of
deflections, and use the engraved plate to print the scales on paper or card. For
this purpose the deflections taken from the table must be read off backwards on
the scale of degrees marked on the dividing engine so that the scale may be
printed in the right direction.
The scales so prepared having been attached to their corresponding instruments,
the operative mechanisms are finally adjusted as follows;-Assuming, as before,
that it is required to adjust an ammeter reading to 10 amperes, I first alter the
position of the "fixed core "F, until the amineter reads an equal percentage too
high or too low over the whole scale; that is to say, until it reads in accordance
with the law already determined for ammeters. Next I add to, or subtract from,
a weight W or spring, used as a controlling force, or adjust the weight W at a
greater or less distance from the axle A,-until the ammeter reads correctly at any

page_text: point on the scale. It will now read correctly over the whole of the scale. I then
permanently fasten the fixed core F, and the weight W or spring in place, by
soldering or otherwise, and the instrument is complete.
"
The whole of the operative mechanism having been made accurately to gauges,
the fixed core I can be placed almost exactly in the right position for an ammeter
(or volt meter) scale, during manufacture, and the final adjustment required will be
very small, and the whole process of adjustment, just described, will only occupy a
few minutes.
Having now particularly described and ascertained the nature of this said inven-
tion and in what manner the same is to be performed declare that what I
claim is :-
1st. In electric measuring instruments of types specified, a system or method of
calibration consisting of the adaptation of an invariable predetermined indicating
scale (cheaply produced in quantities by printing or other duplicating process,) the
initial adjustment of the "fixed core or iron, and the final adjustment of a
controlling weight or spring upon the "movable core or iron substantially as
described.
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2nd. In electric measuring instruments of types specified, the combination of a
mechanically duplicated or printed invariable scale, with an inductive "fixed"
core, capable of initial adjustment and remaining stationary after adjustment;
movable core fixed to indicating axle, and adapted to move concentrically to
"fixed core," and to external coil; and an adjustable controlling weight or spring
on said movable indicating axle, to adapt said operative and indicating parts, to
my method of calibration, substantially as described.
Dated this 30th day of May 1906
FELL & WILDING,
1 Queen Victoria Street, London, E.C., Agents for the Applicant.