11.1 HOT-WIRE INSTRUMENTS
Electrical quantities such as
pressure (voltage) and flow (current), being invisible to the eye, can only be
measured indirectly by observing their effect on other things such as a mechanical
indicating system. Even in the
mechanical field steam or hydraulic pressure is indicated on a pressure gauge,
and flow rate or flow volume on a flowmeter.
In the electrical world the first
‘indirect’ means of measurement used the heating effect of a current passing
through a wire. In Chapter 4 it was
shown that, if a current I amperes flows through a conductor having a
resistance of R ohms, then heat is
generated in that conductor at the rate of I2R
watts.
This heat in the wire raises its
temperature until the rate of radiation away of the heat just balances the rate
of heat generation. At that point the
temperature will stabilise, and it will then be a measure of the current
(strictly, the square of the current) which was causing the heating.
FIGURE
11.1
HOT-WIRE AMMETER
Use of this is made in the
‘hot-wire ammeter’ shown in Figure 11.1.
A short length of platinum-iridium wire is connected electrically in
series with the circuit whose current flow is to be measured. The wire is tensioned until it is fairly
taut, and from its centre is taken a silk thread tensioned by a spring and
whose movement actuates a spindle to which a pointer is fixed.
When no current flows through the
main wire its tautness keeps it almost straight, and the centre thread is at
its left-most position. When current
flows the temperature of the wire rises, and the wire becomes longer and
slacker. The slack is taken up by the
spring pulling on the centre thread and moving it to the right. In doing so the spindle is rotated clockwise,
and the pointer moves over a scale.
Since any given current causes a
given I2R heating rate and therefore a given
elongation of the wire and a given deflection of the pointer, for each current
there is a definite position of the pointer, which depends on the square of the
current. The scale can be calibrated in
current units (amperes), though it will not be an even scale, being crowded at
the lower end.
The instrument
described is the original ‘hot-wire ampere-meter’ (or ammeter) and was used
generally until about the turn of the century.
(It is of interest that it will also indicate alternating currents, as
explained in the manual ‘Fundamentals of Electricity 3’.)
FIGURE 11.2
HOT-WIRE AMMETER AND VOLTMETER
A similar
instrument is used for indicating voltage, since, by Ohm’s Law 
If R is the resistance of the hot wire
(considered fixed), then the current indicated by it will be proportional to V. In
Figure 11.2 a hot-wire ammeter in series with the circuit to be measured is
shown on the left, and on the right is a parallel circuit operating at the same
voltage. A second hot-wire instrument is
connected across this parallel circuit, but one with a high resistance in
series with it. As explained above, the
current, small because of the series resistance, drawn by this second
instrument will still be proportional to V,
and so the instrument acts as a voltmeter and can be scaled directly in
volts. It too will have an uneven scale.
11.2 MOVING-IRON INSTRUMENTS
Whereas the hot-wire instruments
described above depend on the heating effect of a current for its indirect
method of indicating, another class of instruments makes use of the magnetic
effect of the current discovered by Oersted.
Its development was largely due to Lord Kelvin.
FIGURE
11.3
MOVING-IRON INSTRUMENT
Figure 11.3 shows a fixed coil
through which passes the current to be measured. The coil is wound with wire of sufficient
section to carry the current without overheating.
As current flows through the coil
it gives rise to a magnetic field along its axis. In one form of the instrument a small piece
of soft iron is pivoted so that it lies normally well across the coil’s axis
and is held there by a spiral control spring.
When current flows through the coil
the soft iron is magnetised by induction and tries to align itself along
the axis of the coil’s magnetic field, but it is restrained from completely
doing so by the back-pull of the spring.
It takes up a balance position where the magnetic pull of the coil is
just counterbalanced by the spring.
The moving-iron ‘armature’ is
pivoted on a spindle carrying a pointer which moves over the instrument
scale. If the instrument has a
low-resistance coil and is used in series with the circuit whose current is to
be measured, it acts as an ammeter. If
however it has a high-resistance coil and is used in parallel with the circuit,
it acts as a voltmeter. There is
otherwise no basic difference between the movements.
Some such instruments are provided
with two fixed coils in parallel planes and arranged to assist each other
magnetically. The moving-iron armature
is then placed symmetrically on their common axis. In other makes the iron may
be pivoted differently, but it always moves towards the coils’ magnetic axis
against some form of restraint. Various
types of damping are usually added.
Because the magnetic pull of a coil
on a piece of iron magnetised by induction is proportional to the square
of the current, the movement of a moving-iron instrument is proportional to I2
just as was the case with the hot-wire instrument where the heating depended on
I2. The scale of a
moving-iron instrument, calibrated in amperes or volts, is therefore uneven and
is crowded towards the lower end. Indeed it is the crowding of the scale that
makes it possible to tell a moving-iron from a moving-coil instrument at a
glance.
11.3 MOVING-COIL INSTRUMENTS
The moving-coil type of instrument
is magnetic in principle and depends on the interaction between a coil and a
magnet. As shown in Figure 11.4, the
magnet is fixed and takes the form of a permanent magnet; the ‘armature’ is a
small pivoted coil to which connections are made by light flexibles. Normally the axis of the coil lies well
across the axis of the permanent magnet field and is held there by a spiral
control spring.
FIGURE 11.4
MOVING-COIL INSTRUMENT
The circuit current to be measured
(or a known proportion of it) flows through the coil, causing it to produce an
electromagnetic field along its axis.
This field reacts with the field of the fixed permanent magnet and
causes the coil to try to align its own axis with it, but it is restrained from
completely doing so by the back-pull of the spring. It takes up a balance position where the
magnetic pull on the moving coil is just counterbalanced by the spring.
The coil is pivoted on a spindle
carrying a pointer which moves over the instrument scale. If the instrument has a coil of fairly low
resistance and is used in series with the circuit whose current is to be
measured, it acts as an ammeter. But
here it is necessary to point out that the moving coil is necessarily of light
construction, and, with its flexible connections, there is a limit to the size
of wire that can be used. For heavier
currents therefore such an ammeter would be used with a shunt (see para. 11.5). If
the coil has a high resistance and is used in parallel with the circuit, it
acts as a voltmeter. There is otherwise no basic difference between the
movements.
In a moving coil operating in a
permanent magnet field, the magnetic pull on the coil depends only on the
current in that coil, not on the square of the current as in the moving-iron
type where the magnetisation is induced.
Therefore the movement of the coil is almost linear with the current,
and the scale of a moving-coil instrument is fairly even over its whole range,
not crowded at its lower end. It is also
reversible - that is, negative
currents cause negative movement - and,
if a ‘set-up zero’ is used, they can be scaled to show the reverse currents; a
centre-zero ammeter is an example.
In order that a moving-coil
instrument shall retain its accuracy, it is important that the permanent magnet
retain its original strength and not weaken with the passage of time or with
vibration or shock. Therefore the
magnets used in these instruments are made from specially chosen iron of high
retentivity. They then undergo a
thorough artificial ‘ageing’ process and are given rigorous tests to prove that
they are truly magnetically stable. It
should seldom be necessary to change a magnet.
Moving-coil instruments can only be
used with d.c.; they will not function on a.c., as explained in the manual
‘Fundamentals of Electricity 3’.
11.4 DYNAMOMETER INSTRUMENTS
The dynamometer instrument is
really a special case of the moving coil. There is a moving coil as already
described, but the permanent magnet is replaced by a fixed coil (usually a
pair, for reasons of symmetry, since they produce a nearly uniform field).
(Figure 11.5.)
FIGURE
11.5
DYNAMOMETER
INSTRUMENT CONNECTED AS WATTMETER
If such an instrument is used as an
ammeter or voltmeter, both fixed and moving coils are in series, and the torque
depends on the current in each. So the total
torque depends once again on the square of the current, giving an uneven
scale. Being more costly than
moving-iron, they therefore have little advantage over the moving-iron
type. They are also inferior to the simple
moving-coil instruments and are not much used in this application.
However, one application is
important. The actual torque on the
moving coil depends, as stated, on both the current in the fixed coils and that
in the moving coil; it is therefore proportional to their product. If the fixed coils are connected in series
with the circuit (so carrying the current) and the moving coil is connected in
parallel with the circuit (so carrying a current proportional to the voltage),
the instrument will indicate I x V, or d.c. watts. This dynamometer
instrument, so connected, then becomes a wattmeter. (It is of interest that this instrument will
also register a.c. watts, as explained in the manual ‘Fundamentals of
Electricity 3’.)
Since the dynamometer instrument
indicates the product of V and I, then if I reverses, the torque also reverses and the instrument indicates
backwards. If its zero is ‘set up’, it
can be scaled to indicate both normal (forward) and reverse power that is to
say, it is directional. Whenever
it is desired to take direction into account (such as for some relays in
protective schemes), a dynamometer or ‘wattmetric’ type of movement is always
used.
If the currents to be measured are
too high for the coils of the instrument, the series (fixed coil) current can
be taken through a shunt (see para. 11.5 below), thereby using only a known
proportion of the circuit current.
Similarly the voltage coil (moving coil) can be fed through a ‘dropping’
resistance of known value, so that only a known fraction of the circuit voltage
is applied.
11.5 SHUNT-CONNECTED INSTRUMENTS
Most instruments receive an input
from the circuit whose current is to be measured, but, due to their
construction, they can in most cases accept only a small, but accurately known,
proportion of that current. It is
therefore necessary to divide the circuit current into two parts: the main part
carrying the majority of the current, and the other part a known fraction of it
- in many cases only an
infinitesimal fraction, perhaps one-thousandth part.
FIGURE
11.6
INSTRUMENT
WITH SHUNT
In Figure 11.6 the line current is I and is shown in heavy line. It passes through a ‘shunt’ which usually for
the heaviest currents takes the form of two brass blocks connected together by
strips of conducting metal of very small, but not negligible, resistance. In parallel with this shunt is the
instrument’s current coil with a relatively high resistance, perhaps many
thousand times that of the shunt. The
line current will divide in inverse proportion to the resistances. For example, if the shunt resistance were 0.001
ohm and that of the instrument coil 1.0 ohm, a current of, say, 100A would
divide approximately 99.9A through the shunt and 0.1A through the coil. The instrument would then actually measure
only 0.1A, but it would be scaled one thousand times up to read ‘100A’.
When instruments receive their
current input through a shunt, it is important that they be calibrated by the
maker with their own shunt and with the actual connecting leads with which they
will eventually be installed. Because of
the great disparity between the resistance of the shunt itself and of the
parallel instrument circuit, small errors in the latter could cause
considerable errors in the instrument reading; hence the need to calibrate all
together.
When an instrument is being
installed with its shunt, the leads must never be altered or shortened; any
extra length must be left in and flaked out.
Shunt-operated instruments should all be marked with their associated
shunt serial number and used with no other.
For example, in the case quoted, one-tenth of an ohm cut out of the
shunt leads would lead to a 10% error in the instrument’s reading.
11.6 TERMINOLOGY
The words used with indicating
instruments and similar devices are often misused, and below is a summary of
the terms and their correct usage:
‘Indicating Instrument’ or simply ‘Instrument’. An indicating device to show the
instantaneous value of the quantity being measured. Note that, although the terms ‘voltmeter’,
‘ammeter’, ‘wattmeter’ etc. are generally used, the generic term ‘meters’
should never be used for indicating instruments.
‘Integrating Meter’. A
device for integrating a quantity over a period of time. Usually associated
with watts and vars, the total quantity shown by dial or digitally as
watt-hours (Wh) or var-hours (varh).
Such integrating devices are known collectively as ‘meters’ (as distinct
from ‘instruments’ above).
‘Recording Instrument’ or ‘Recorder’. A
device for continuously recording the instantaneous value of a quantity, the
record being made by pen-on-paper or similar device, or digitally on paper or
tape.
‘Set-up Zero’. When an instrument is carrying no current,
the position taken up by its pointer under the influence of the control spring
is its ‘zero’, and it is usually at the extreme left-end of the scale. However, instruments can be arranged so that
the zero is not at the extreme end but at some distance to the right; it is
then said to have a ‘set-up zero’. This
allows the instrument to register, in part at least, in the negative or reverse
direction. It can only be applied to
moving-coil or dynamometer instruments.
A centre-zero ammeter showing battery charge and discharge is an example
of a set-up zero.
Each group of devices has a
separate British Standards symbol.
 | |
The letter within shows the
quantity being measured, for example A (amperes), Wh (watthours), V (volts),
etc.
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