Download Meter moving coil

The moving coil meter
A very common use of the forces on a coil in a
magnetic field is that of the moving coil meter,
shown in Figure 1.
The coil is suspended between the poles of a
magnet on jewelled bearings and is held in
place by two finely coiled springs (S1 and S2)
through which the current to be measured
passes in and out of the coil.
magnet
soft iron armature
spring (S1)
coil
The pole pieces are shaped so that the
magnetic field is radial thus giving the maximum
and constant torque on the coil whatever its
position (Figure 2). There is a soft iron armature
in the centre of the coil, and this further
concentrates the magnetic field through it due to
the high value of the relative permeability of the
material of the core.
The coil is supported on a light metal frame and
induced
currents
in
this
frame
give
electromagnetic damping of the movement.
When it is in equilibrium with a current passing
through it, the torque on the coil produced by
the magnetic field is balanced by an opposing
torque due to the rigidity of the springs. Clearly
the more delicate the springs the bigger the
deflection for a given current.
pointer
Figure 1
armature
N
S
Some meters are of the suspended coil type;
the coil is mounted on a fine phosphor bronze
wire and the deflection measured with a lamp
and scale. A small mirror mounted above the
Figure 2
coil reflects a beam of light on to a scale, the
angle of twist of the light beam being double the
angle of rotation of the coil. Such instruments have a framework of brass or aluminium.
Meters are often made with a protective series resistor since too large a current will burn out the
springs or the coil. When the correct range for the meter has been found this protective resistor
may be shorted out.
The moving coil analogue meter is used less and less these days having been replaced in many
applications by the digital meter.
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Sensitivity of a meter
The amount of deflection of the pointer in a meter for a certain current depends on the design of
the meter. The amount of angular twist for a unit (1A) current is called the current sensitivity of
the meter.
Consider a coil of N turns and cross-sectional area A, carrying a current I in a field of flux
density B. The torque C due to the magnetic field is given by
Torque (C) on the coil = BANI
for a radial field, and the opposing torque due to the twist of the suspension is k where k is the
torsion constant for the wire and S is the angle of twist.
In equilibrium k = BANI
The angular twist per unit current (/I) is called the current sensitivity of the meter and is defined
by the formula:
Current sensitivity (/I) = BAN /k
To increase this (that is, to make the meter more sensitive) we require:
(a) large magnetic flux density (B), that is, the gap between the poles as small as possible,
(b)a coil with a large area (A),
(c) a large number of turns (N), and
(d) a small value of k that is, a very thin wire or one with a very low rigidity.
Unfortunately (b) and (c) tend to make the coil both bigger and heavier and so cause problems
with (a) and (d). A compromise has to be reached.
The radial field is important since for this arrangement B sin  is constant and therefore the
angle of twist  is directly proportional to the current I for all positions of the coil.
Example
The coil of a lamp and scale galvanometer has an area of 4 cm 2 and 200 turns.
A torque of 2x 10-7 Nm causes it to twist through 180o against the torsion of the suspension. If the
field acting on the coil is 0.2 T, find the current that will cause the spot of light on a scale 1 m away to
be deflected through 1 mm. Original torque = 2x10 -7 = ko.
Therefore k = 2x10-7/o.
C
New angle () is given by: /I = [0.2 x 4x10-4 x 200 x /2x10-7
Angle of deflection = 10-3/1= 10-3 radians
Therefore current I = 2 x 10-7 x 10-3/0.2 x 4x10-4 x 200 x 
= 2 x 10-10/5.02 x10-2 = 4 x 10-9 A = 4 x 10-3 A
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Conversion to ammeter or voltmeter
A typical moving coil meter such as those used in schools may have a full scale deflection (fsd)
of 100 A and a resistance of 1000 . This means that the pointer will deflect right across the
scale when a current of 100 A is passed through the meter When this occurs the p.d. between
the terminals of the meter will be 100 A x 1000  = 100 mV. Clearly this is quite inadequate
when measurements of currents of say 5 A or voltages of 12 V are required. External resistors
may be used to extend the range of the meter and these are known as shunts and series
resistors. (see Figure 2)
Ammeter
Voltmeter
119 k
5A
100 A
1000 
100 A
1000 
12 V
0.02
Figure 2
So that as little current as possible is drawn from a circuit under test a good voltmeter should
have a resistance of at least 1000  per volt - for example a meter designed to read. voltages
up to 10 V should have a resistance of 10 k. This also requires a galvanometer with a high
current sensitivity
The moving coil galvanometer can also be used as an ohmmeter and a wattmeter.
There has been a considerable increase in the use of direct-reading digital meters in the last few
years. These rely on a totally different principle, that of the integrated circuit, for their operation
and have no moving parts. The digital voltmeter has a very high resistance (of the order of 10
M on d.c.) but it does need a small internal battery to power the instrument. The input voltage
to be measured is compared with a steadily rising voltage produced by a ramp generator (an
application of the op amp). The time taken for the rising voltage to reach that of the input voltage
is measured and this time is directly proportional to the voltage. The output reading is scaled to
give a direct reading in volts.
Measurement of alternating current
It should be clear that none of the preceding moving coil instruments are suitable for the
measurement of alternating current or voltage.
The coil would tend to oscillate between a positive and a negative reading. In Britain, however,
the frequency of the mains is 50 Hz and the inertia of a coil will prevent it from moving far before
the current reverses, so it simply vibrates slightly.
The following types of meter can be used to measure alternating currents:
moving iron instruments - using the repulsion between two metal rods in the field
hot wire instruments - using the expansion of a wire rectifier instruments
dynamometer instruments electrostatic voltmeter
diode valve voltmeter
cathode ray oscilloscope
digital meter
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