ELECTRIC MEASURING INSTRUMENTS
Year: 1885

page_text: We JAMES SWINBURNE of 9 Manor Road, Brockley in the County of Kent, and
SYDNEY EVERSHED of Kenley, in the County of Surrey Electricians do hereby
declare the nature of our invention for ELECTRIC MEASURING INSTRUMENTS to be
as follows:-
To make an instrument according to our invention coils may be used as in the
instrument well known as Weber's dynamometer, where the current'in a fixed coil
acts on a moveable coil also with current, so that the current or currents may be
measured by measuring the force or forces acting on the moveable coil. The force
tending to move the moveable coil may be opposed by the force of gravity. Thus
the moveable coil may he mounted on an arbor whose axis is not vertical, and the
centre of gravity of the system may be arranged so as not to be in the axis of
the pivot.
The moveable coil will then tend to come to one position of rest. One fixed coil
may be used & may be arranged to be either outside or inside the moveable coil,
but it will probably be better to use two fixed coils much larger than the moveable
coil, and placed one on each side of the moveable' coil, so as to allow the axis,
& connections to be easily arranged. Thus two large fixed coils may be placed
side by side, so that their axes are one straight line, with the moveable coil between
them so that its axis is in the same plane as their axis, and at right angles to the
axis of its arbor. The centre of gravity of the moveable coil may be arranged to
tend to keep the axis of the moveable coil at right angles to the axis of the fixed
coils. On electricity being passed through, say all three coils in series the moveable
coil will be turned on its pivots, & the angle it turns through may be measured by
means of an index and dial, so that current may be known. As such an instrument
always reads in the same direction it may be better to arrange the moveable .coil
so that when there is no electricity passing it comes to rest, not at right angles to
the fixed coils but at some other angle say 45°. The instrument may thus be made
to have about twice the reading angle. This principle of arranging the controlling
force at a smaller angle than a right angle may be applied to other forms than that
just described. Thus an instrument in which the coil acts on a needle controlled
by a permanent magnet may have the permanent magnet so arranged that the
needle itself moves from say 45° with the plane of the coil on one side to 45° with

page_text: it on the other, thus moving through a right angle & giving accurate readings all
the way.
The fixed coils & the moveable coil should be so proportioned that the
calibration curve is continuous, so that there may not be two points of rest for one
current value. The dial may be itself slanting so as to be at right angles to the
axis of the moveable coil arbor. The dial may be marked in degrees or divisions,
& a table of reference given from which to get the current readings, but we much
prefer to make the instrument direct reading by actual calibrations all through the
scale. To make circuit with the moveable coil two mercury cups may be arranged
in the axis of the arbor at any convenient points. Thus the upper end of the
arbor may be a very small cylindrical pivot working in a jewel, or a hole in hard
metal. The lower end of the arbor may end in a point, say of iridium, resting in
& jewel cap.
And below again may be the two mercury cups arranged in the same
line with suitable contact pieces dipping in. One or both of these may have vanes
to make the instrument dead beat.
If the instrument is to be used as a potential galvanometer, or volt meter,
the two fixed coils may be wound of high resistance & arranged in series with the
small moveable coil, which latter may be wound with aluminium wire, which is
very light,
For accurate work corrections would have to be made for error due to alteration
of the resistances of the coils from change of temperature. The change of tem-
perature may be due to variations of temperature of the atmosphere or to heating
of the instrument due to the expenditure of electrical energy. The error due to
external changes may be approximately compensated by using indexes made of
two metals side by side, so that they curl one way or the other on alterations
of their temperature. These may be made by electro deposition of one metal on
another with a different expansion coefficient, & then filing out the indexes.
To give such an index more range it may go round the arbor once or twice spirally
before joining it. Either in conjunction with this method, or instead of it, in a
voltmeter a resistance may be arranged in shunt to the small coil & be made of
any metal with a larger temperature resistance alteration than that used in the
small coil,
Thus if the large coils are platinum silver, and the small one also platinum
silver, a copper resistance may be arranged in shunt to the small coil so that
though the current in the instrument lessens when the temperature rises, the
current in the small coil is increased so that the reading is still the same..
If the instrument is used as a current measurer or ammeter all the coils may be
made of copper wire, arranged preferably in series.
If the instrument is used to measure power or as a watt-meter the fixed coils
may be in shunt to the part of the circuit where the power is to be measured and
'the moveable coil may be in series.
The current in the fixed coils will then be proportional to the E, M, For E &
that in the small coil to the current, & the index may read directly in watts,
electrical horsepower, lamps per horsepower &c &c. The E. M. F or E coil may be
corrected for external change of temperature by arranging another coil in series with
it & another in shunt to it of metals with suitable temperature resistance alterations.
This method may be applied to other forms of instrument, as to the galvanometer
above mentioned with the unsymmetrical controlling magnet. To make a watt-
meter theoretically accurate with the above arrangements the E coil would have
to be of infinite resistance or the current coil, or C. coil of no resistance; for the
E, coil must either be in shunt to the part of the circuit to be measured, say to a
glowlamp, or to the glowlamp & to the C. coil in series. In the latter case its
current is proportional to the E. on the lamp plus the E on the current coil; but
if it is in shunt to the lamp alone, the C coil gets not only the lamp's current but
the E. coil's current in addition. An extra compensating coil may be arranged.
Suppose the E. coil in shunt to the lamp alone. A coil is wound backwards round
the C. coil & connected in series with the E. coil so as to compensate. If the

page_text: E. coil is in shunt to the lamp & C. coil a compensating coil is wound backwards
round the E. coil & put in shunt to the C. coil.
In installation work it is advisable to have instruments to give wide readings
about the E. or C. generally in use such instruments may be arranged by making
the moveable coil come nearly at right angles to the fixed coils & then putting a
stop, so that the instrument does not begin to read till the normal E, or C. is nearly
reached.
For calibrating or other purposes it may be desirable to get a given reading
with one tenth or ten times the usual-E or C. To attain this object ten wires are
wound together, preferably cable fashion on a coil, & nine of them are thin & one
thick, and their resistances are properly adjusted.
.
Thus in an E. coil there might be nine thin wires of 9900 ohin's in series &
another thick wire of 100 ohms making 10,000 ohms in series; if the thick wire
alone be used, a given field will be got from that coil with one tenth of the E.
The range of the instrument may be altered by altering the inclination of the
arbor.

page_text: We JAMES SWINBURNE of 34 Holford Square, London, & SYDNEY EVERSHED
of Kenley, Surrey, Electricians do hereby declare the nature of our invention
for ELECTRIC MEASURING INSTRUMENTS and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement: 1
Several ways of making instruments according to our invention are described in
our provisional application; we will here describe an instrument embodying what
we have so far found the best method.
We use fixed and movable coils as in the well known Weber's dynamometer, but
we oppose the action of the coils by mounting the moving coil on an arbor whose
axis is slanting, arranging the centre of gravity out of the axis so that the moving
coil and arbor always tend to come to rest in one position.
We make the fixed coils large, in the form of rings, and the movable coil is
embraced by them so that it is in a strong field. The two fixed coils have, of
course, a common axis, and the small coil may be turned so that its axis coincides
with it. The axis of the arbor is of course at right angles to the axes of the coils.
The small coil is weighted so as to come to rest not at right angles to the large
coils but at about half a right angle to it, so that when acted on it turns through
about 90° easily. This gives a large reading and spreads the middle of the scale
out. The arbor is supported at the top by a pivot and jewel and at the bottom it
ends in an iridium point which rests in a sapphire cap. Below this in a line with
the axis are two mercury cups, and insulated connections dip into these to make
circuit through the moving coil. In one of these cups a small vane may be
submerged to make the instrument dead beat. An index is fixed to the arbor at
the top and it reads over a slanting dial-plate. Means of shewing that the
instrument is level are attached, as it is important that the moving arbor be
always at the same inclination The moving coil is preferably wound with
aluminium wire. The instrument may of course be wound as a voltmeter, ammeter,
or wattmeter.
For accurate work it is important that the readings should not be affected by
change of temperature, or by the error inherent in wattmeters as commonly made.
The temperature error may be due to change of temperature of the room or to
the heating of the coils by the electricity flowing. To compensate for temperature
alterations in the room, we arrange a coil in series with the instrument, this coil
being of wire with a small temperature alteration, and in shunt to the instrument
we arrange a coil of wire of high temperature co-efficient. These coils must be
calculated for the instrument so that the increase of the shunt resistance is so great
that the current in the instrument is the same, though the current in the whole
system is less, when the coils are heated. If the wires are respectively German
This arrange-
silver, or platinum silver, and copper, the resistances work out well.
ment may of course be applied to any voltmeter or in any such case where uniform
current is wanted with uniforin electric pressure. If our instrument is wound as a
voltmeter with its coils in series, the fixed coils may be, say, platinum silver and in
series with the moving coil, which may be of platinum silver or German silver.
A coil of copper may then be arranged in shunt to the moving coil and should be

page_text: of such resistance that the product of the currents in the movable and fixed coils
remains the same when the temperature of the room varies.
The change of resistance may also be due to the heating of the coil by the
electricity flowing. To make the field constant when the temperature varies, the
coil may be wound with 2 wires, say platinum silver, and copper.
These wires
are in parallel and are wound opposite ways, so that the resulting field is the
difference of the fields due to each. The resistances and fields are so calculated
that this difference is constant.
In wattmeters or even in voltmeters and ammeters arranged to measure the
power spent in a part of a circuit there is an error due to the pressure coil, if in
shunt to the work and to the current coil, measuring the sum of the pressures in
the work and due to the resistance of the current coil, or to the current coil
measuring the sum of the currents in the work and pressure coil if the pressure
coil is in shunt to the work only. If the pressure coil is in shunt to the work and
current coil we pass a few turns of the current circuit round the pressure coil
backwards so that there is no deflection when the power is nothing, that is, when
it is short circuited out. If the pressure coil is in series with the current coil and
in shunt to the work, we pass a few coils of the pressure circuit backwards round
the current coil so that there is no deflection in the second case, of the power
being nothing, that is when the work circuit is open so as to have infinite
resistance.
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. An electro-dynamometer for measuring electromotive force current or power,
substantially as described.
2. An electric measuring instrument with fixed and moving coils, the moving
coil being mounted on an oblique arbor with its centre of gravity out of the axis,
substantially as described.
3. An electric measuring instrument in which gravity acts on a slanting arbor
bearing a coil substantially as described.
4. Compensating the errors in electric measuring instruments by means of wires
of different temperature resistance co-efficients after the manner explained.
5. Compensating a dynamometer substantially as herein explained.
6. Arranging a resistance of small temperature coefficient in series with a coil
and a resistance of large temperature coefficient in shunt to it so as to get constant
current in it substantially as set forth.
7. Arranging 2 coils of which one moves in series and putting a resistance of
large temperature coefficient in shunt to one of them so that the product of their
currents may be constant, after the manner described.
8. In a measuring instrument winding with two wires of different temperature
resistance coefficients so as to get a constant field with varying temperature
substantially as explained.
9. In measuring power with the current coil in series with the pressure coil
winding some of the pressure circuit backwards round the current coil as indicated.
10. In measuring power with the pressure coil in shunt to the current coil,
passing the current circuit round the pressure coil backwards as indicated.