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Contents:
1.
MOVING COIL GALVANOMETER.
2.
AMMETER AND VOLTMETER
3.
POTENTIOMETER
4.
WHEATSTONE BRIDGE
5
EQUATIONS
7.
DIMENSIONS
8.
SHORT QUESTIONS AND ANSWERS.
A Galvanometer is an electromechanical instrument which is used for the
detection of electric currents through electric circuits. We can measure very small current by using
galvanometer but a galvanometer is used for detection of electric current not the measurement of
current. A galvanometer instrument used to determine the presence, direction, and strength of an
electric current in a conductor.
Principle of Galvanometer:
A moving coil galvanometer works on the principle that a current carrying
coil kept in a uniform magnetic field experiences a torque.
Construction:
It is consists of N number of turns bound on a non-magnetic field and Area A
of the coil. The coil is suspended between two poles of a permanent magnet
Magnetic Field B, which are concave in shape. The coil is suspended by a phosphur
bronze strip and this strip is connected to the terminal T1 of the galvanometer and
other end of the coil is connected to hair spring which is connected to terminal T2 .A
peace of the soft iron is the placed within the frame of the coil. A concave mirror is
attached to the suspension to note the deflection of the coil using length and scale
arrangement.
Mathematical derivation:
When current , I is passed through the coil, it experiences a deflecting torque,
 = N I A B Sin,
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which rotates the galvanometer needle. The Galvanometer needle becomes stationary
when the torque due to the spring equals the torque due to the current. Here  = angle
between area vector and magnetic field
Due to this torque, the coil starts deflecting which produces a twist in the
suspension fiber. This twist produces a restoring torque, in the suspension fiber due
to rigidity. The restoring torque is directly proportional to the angle of twist ().
Restoring torque  Twist
Restoring torque = c 
Where “c” is constant known as “couple per unit twist” and is equal to the couple required for
unit radian twist of suspension of fiber. Its value depends on the length radius and elasticity of the
material of the wire.
Deflecting torque = restoring torque
NBA=c
 c 
I= 
θ
 N B A 
 c 
Where, 
is constant.
 N B A 
Therefore,
 = constant (  )

The larger the current flowing through the coil, the larger the deflection
There are two commonly methods of observing the angle of deflection of coil:
a: Lamp scale method
b: Pivoted coil galvanometer.
(a): Lamp Scale method:
It is sensitive galvanometer, the angle of deflection is observed by
means a small mirror attached to the coil along with a lamp and scale
arrangement. A beam of light from the lamp is directed towards the mirror of
galvanometer. After reflection from the mirror it produces a spot o a
translucent scale placed usually at distance of meter from the galvanometer.
The mirror along with the coil rotates the spot of light moves along the scale. The displacement of the
spot of light on the scale shows coil motion.
(b): Pivoted coil galvanometer:
In this type of galvanometer, a magnetic needle is deflected
by the presence of an electric current in a nearby conductor. When an
electric current is passing through the conductor, the magnetic
needle tends to turn at right angles to the conductor so that its
direction is parallel to the lines of induction around the conductor
and its north pole points in the direction in which these lines of
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induction flow. In general, the extent to which the needle turns is dependent upon the
strength of the current. Modern galvanometers are of this movable-coil type and are
called d'Arsonval galvanometers. If a pointer is attached to the moving coil so that it
passes over a suitably calibrated scale, the galvanometer can be used to measure
quantitatively the current passing through it. Such calibrated galvanometers are used
in many electrical measuring devices.
i ) Voltmeter:
A voltmeter is an instrument used for measuring the electrical potential difference between two
points in an electric circuit. A moving coil galvanometer can be used as a voltmeter by inserting a
resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong
magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and
compresses a small spring. The angular rotation is proportional to the current through the coil. For
use as a voltmeter, a series resistance is added so that the current through the meter, and, therefore,
the angular rotation of the needle, becomes proportional to the applied voltage.
Mathematical Derivation:
Suppose a galvanometer has resistance “Rg” and current “g” is passing through it of
potential “Vg” across it. And the high resistance also draws same current “g”, and
potential “Vx” across resistor “Rx”. The desired potential “V” is to be determined.
Hence,
V = Vg + Vx
V = Rg g + Rx g
V = g [ Rg + Rx ]
V
= Rg + Rx 
Ig
 V
  - R g = R x An ideal voltmeter has an infinite resistance.
 Ig 
ii ) Ammeter:
An ammeter is a measuring instrument used to measure the electric current in
a circuit. Electric currents are measured in amperes, hence the name.
To measure larger currents, a resistor called a shunt is placed in parallel with the
meter. Most of the current flows through the shunt, and only a small fraction
flow through the meter. This allows the meter to measure large currents. The meter used with a shunt has
a full-scale deflection of, so shunts are typically designed to produce a voltage
drop of when carrying their full rated current.
Mathematical Derivation:
The shunt of galvanometer is ‘Rs“and the resistance of the galvanometer coil is “Rg”, which carries the
current g. The value of Rs is choose according to what full scale deflection is desired through which (
 – g) current is passing.
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V = Rg g
V = Rs (  –  )
Rs ( –  g ) = Rg g
And
Therefore,
Rs =
R g Ig
I - I 
An ideal ammeter has a zero resistance.
g
iii) Ohmmeter:
An ohmmeter is an electrical instrument that measures
resistance, the opposition to an electric current.
The original design of an ohmmeter provided a small battery to
voltage to a resistance. It used a galvanometer to measure the
current through the resistance. The scale of the galvanometer was
ohms, because the fixed voltage from the battery assured that as
decreased, the current through the meter would increase.
electrical
apply a
electric
marked in
resistance
The potentiometer is an instrument, which can be to measure the emf of a
source without draw current from the source. It balances an unknown potential difference against an
adjustable potential difference.
Principle:
The potential difference across the length of wire is directly proportional to the length of the wire
provided uniform current passing through it.
Construction:
A potentiometer consists approximately four-meter
long wire of uniform area of cross section. It is fixed on wooden board
of four parallel loops.
Working:
A resistance wire AB is permanently connected to the terminals of
source of emf. A sliding contact S is connected through the
galvanometer G to a second source whose emf E2 is to be measured. Contact "S" is moved along the
wire until a position is found at which the galvanometer shows no deflection. If we then separate the
expression for VAB for two paths between these points , we have
Mathematical Derivation:
VAB = 1 RAB
Resistance wire path.
VAB = ( rg + r ) 2 - ( - E2 ) galvanometer path.
Where 2 = 0,we have adjusted current to be equal to zero. Therefore, VAB = E2
Hence, RAB is exactly equal to the emf E2 and it can be computed, if “” and “RAB” are known. No
correction need to made for the “ r” term, since the current in E2 is zero.
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An unknown resistance value can be accurately measured using a circuit known as
a Wheatstone bridge. The fundamental concept of the Wheatstone bridge is two
voltage dividers, both fed by the same input, as shown to the right. The circuit output
is taken from both voltage divider outputs
Construction:
This circuit consists of the one unknown resistance in one leg, three known
resistances and one of them is a calibrated variable resistor in separate three kegs. A
galvanometer connected at the pair ends while a battery on remaining pair ends.
Working:
In the circuit on the one resistor Rx is the unknown resistance to be measured; R1, R2 and R3
are resistors of known resistance and the resistance of R2 is adjustable. A battery of potential V is
connected at the AC ends, while at other pair ends an ammeter is connected BD makes a bridge. If
R 
the ratio of the two resistances in the known leg  2  is equal to the ratio of the two in the unknown
 R1 
R 
leg  x  , then the voltage between the two midpoints (B and D) will be zero and no current will
 R3 
flow through the galvanometer Vg then the bridge is said to be balanced.
Mathematical derivation:
R 
R 
At the point of balance, the ratio of  2  =  x 
 R1 
 R3 
Application 1. One very common application in industry today is to monitor sensor devices such as
strain gauges. Such devices change their internal resistance according to the specific level of strain
and serve as the unknown resistor Rx.
2. A second application is used by electrical power distributors to accurately locate breaks in a power
line. The method is fast and accurate, and does not require a large number of field technicians.
01. I =
c
NA B
GROUP TIME ------------- GROUP NO:- - - - --
R g Ig
02. R s =
 I - Ig 
 V
03 R x =   - R g
 Ig 
04.
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R
R1
= 3
R2
R4
PHYSICAL QUANTITY & SYMBOL
DIMENSION
2
-2
-1 -1
 M L T   A T 
V
=  M L2 T -3 A -2 
Resistance R =
I
A
Potential V = R I
Electric Current
I
 M L2 T -2   A -1T -1  =  M L2 T -3 A -1 
A
UNIT
-1
JC
= ohm
Amp
J C-1
amp.
Q:No. 1 How does a moving coil galvanometer work?
Answer:
Instrument used to detect and measure electrical current. A coil of wire pivoted between the
poles of a permanent magnet is turned by the motor effect of an electric current by which a force acts
on a wire carrying a current in a magnetic field. The extent to which the coil turns can then be related
to the magnitude of the current.
The sensitivity of the instrument depends directly upon the strength of the permanent magnet used,
the number of turns making up the moving coil, and the coil's area. It depends inversely upon the
strength of the controlling springs used to restrain the rotation of the coil. By the addition of a suitable
large resistor, a moving-coil galvanometer can be adapted to read potential difference in volts.
Q:No.2 What is the difference between an ammeter and voltmeter with regard to its external
connection?
Answer:
An Ammeter connects low impedance on the test points, so the equivalent of that is a "short
circuit" between the test points. This is done to avoid a drop of current on the tested circuit.
A Voltmeter connects high impedance on the test points, so the equivalent of that is an "open
circuit" between the test points. This is done to avoid a drop of voltage on the tested circuit.
Q: No: 3 What is the principle of moving coil galvanometer?
Answer:
When a current carrying coil is placed in magnetic field, it is experience a torque.
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Q: No: 4 A wire is covered with a carpet how will you find whether any current is
passing through it or not?
Answer:
A current carrying wire is covered with a carpet. A wire either current carry or not is
determined by magnetic compass.
Q: No: 5
An ammeter and voltmeter of suitable ranges are to be used to
measure the current and voltage of electric lamp. What would happen if by mistake
the meters were interchanged?
Answer:
By a mistake two meters were interchanged, the voltmeter has very high resistance in series, no
current flows through the circuit. And in the ammeter a low resistance in parallel and it doesn’t give
any deflection.
Q: No: 6 Why must an ammeter be always connected in series and voltmeter in parallel?
Answer:
All the current, which flows through the circuit, also passes from ammeter. Hence, it measures
the current of load, when connected in series.
A voltmeter measures the potential difference of a load between two points. Hence it is
connected in parallel.
Q: No: 7 What is shunt resistance? Explain how is it used to convert a galvanometer into an ammeter?
Answer:
A shunt is a low resistance connected with galvanometer in parallel, to measure accurate current.
Thus galvanometer is converted into ammeter.
Q: No: 8 What is voltmeter? Explain how galvanometer is converted into voltmeter?
Answer:
It is an instrument used for the measurement of potential across ends of a circuit. A high
resistance connected with galvanometer in series, to measure accurate potential difference. Thus,
galvanometer is converted into voltmeter.
Q: No: 9
A voltmeter cannot read the exact emf of a cell .Why?
Answer:
A voltmeter has high resistance in series, it draws some current from the circuit and the potential
difference across the terminals decreases. Thus, a voltmeter cannot read the accurate emf of the cell.
Q: No: 10 When the bridge is balanced why does no current flow through the galvanometer?
Answer:
When a bridge is balanced then the potential difference across the terminals is zero. Hence,
no current flows through the galvanometer.
Q: No: 11
Why is a voltmeter connected parallel to the points between which the voltage is to
be unmeasured?
Answer:
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Because, a voltmeter is an instrument used for the measurement of potential
difference between two terminals of a load. Therefore, it must be connected in parallel
to the points, thus required voltage is to be measured.
Q: No: 12 What is the function of the concave pole pieces and the coaxial soft iron
cylinder in the moving coil galvanometer?
Answer:
The function of the concave pole pieces in the moving coil galvanometer is, that to
produce uniform and strong magnetic field. And the function of coaxial soft iron cylinder is used in
the moving coil galvanometer that it doesn’t become magnet.
Q: No: 13 We want to convert a galvanometer into a) an ammeter b) a voltmeter. What do we need
to do in each case?
Answer:
a) When a galvanometer is to be converted into an ammeter a shunt (a low resistance) is to be
connected in parallel with galvanometer.
b) When a galvanometer is to be converted into voltmeter a high resistance is to be connected in
series with galvanometer
Q: No: 14 Which is the more accurate instrument a meter bridge or post office box?
Answer:
A post office box is more accurate instrument than the meter bridge.
Q: No: 15
What is an Avometer?
Answer:
An instrument used for the measure the current, potential and resistance of different values. .
Q: No: 16
What is ohmmeter?
Answer:
An instrument used for the measure he resistance in ohms.
Q: No: 17
What is potentiometer?
Answer:
An instrument used for measuring the potential difference or voltage between two terminals of a
circuit or electromotive force of a current source.
Q: No: 18
What is Galvanometer?
Answer:
An instrument used for the detection of current and its direction.
Q: No: 19 What is Wheat stone bridge?
Answer:
An instrument used for measure the unknown resistance.
Q: No. 20 Why Cylindrical iron core is made of soft iron ?
Answer:
Soft iron has very high magnetic permeability and attracts a large number of magnetic lines of
force towards itself. This increases the magnetic field on the coil and increases the torque which again
results in the increased sensitivity of the galvanometer.
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Q: No. 21 Why hair spring is used in the galvanometer made of Phosphor bronze
Answer:
Phosphor bronze hair spring is used in the galvanometer which connects
the galvanometer needle with the coil. Phosphor bronze is used because:
i.
It is highly elastic
ii.
It has less elastic after effect
iii.
It is light
iv.
It is a good conductor
v.
It provides least counter torque per unit deflection increasing the sensitivity
of the galvanometer.
Q: No. 22 Which galvanometer usually has greater sensitivity aluminum pointer and lamp scale
type? Why.
Answer:
A lamp scale type galvanometer has greater sensitivity than aluminum pointer. Because the angle of
deflection is observed at very small current by means a small mirror attached to the coil along with a
lamp and scale arrangement
Q: # 23 What necessary conditions must a voltage measuring device safety?
Answer:
A voltage measuring device says voltmeter is connected in parallel with the circuit.
Q: No. 24 What is wheat stone bridge and how it is used for measuring an unknown resistance?
Answer:
A Wheatstone bridge It is measuring instrument used to measure an unknown electrical
resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown
component. Its operation is similar to the original potentiometer except that in potentiometer circuits
the meter used is a sensitive galvanometer.
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