# Conversion of Galvano Meter into Voltmeter-Physics-Lab Mannual, Exercises for Physics. Allahabad University

## Physics

Description: This is lab manual for Physics course. It was designed by Sakash Jashith at Allahabad University. It includes: Conversion, Galvanometer, Voltmeter, Ammeter, Working, Principle, Formula, Deflection, Resistance, Shunt
Showing pages  1  -  4  of  4
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Experiment : C
ONVERSION OF
G
ALVANOMETER INTO
V
OLTMETER
/
A
MMETER
Object:
To convert Galvanometer into Voltmeter and Ammeter.
Theory:
Galvanometer: A galvanometer is a device used to find
direction of current and its magnitude. It has small
resistance and is connected in series.
Galvanometer is an electromechanical instrument
which is used for the detection of electric currents
through electric circuits. Being a sensitive
instrument, Galvanometer can not be used for the
measurement of heavy currents.
Working Principle
Galvanometer works on the principle of conversion of electrical energy into
mechanical energy. When a current flows in a magnetic field it experiences a
magnetic torque. If it is free to rotate under a controlling torque, it rotates
through an angle proportional to the current flowing through it.
A
MMETER
: It is a low resistance galvanometer, used to measure current in a circuit. The
current to be measured must pass through it, hence, it is connected in series
combination. An ideal ammeter should have zero resistance.
V
OLTMETER
: It is a high resistance galvanometer, used to measure potential difference
between two points. It is connected across the component’s ends potential
difference across which is to be measured. Hence, it is in parallel to that
component. An ideal voltmeter must have infinite resistance.
C
ONVERSION OF
G
ALVANOMETER TO
A
MMETER
:
Galvanometer can be converted into ammeter by
shunting it with a very small resistance. Potential
difference across the galvanometer and shunt
resistance are equal.
C
ONVERSION OF
G
ALVANOMETER TO
V
OLTMETER
:
Galvanometer can be converted into voltmeter by
connecting it with a very high resistance. Potential
difference (p.d.) across the given load resistance
across galvanometer and p.d. across the high
resistance.
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OBJECTIVE:
To convert galvanometer into voltmeter of range 0-3 volts.
APPRATUS
: A (Weston type) galvanometer, two resistance boxes, a small power
supply, a D.C. voltmeter (of range 3 V), a rheostat, two Switches,
connecting wires, sand paper.
FORMULA USED:
(I) For resistance of galvanometer by half deflection method:
R
g
=
S
R
SR
*
**
………………………. (1)
Where Rg is resistance of galvanometer, R is taken out of resistance box in
series with galvanometer and S is resistance taken out of shunt resistance
box for half deflection [Fig(4)]
(II) For figure of merit K.
K =
θ
)( RgR
E
+………………………. (2)
Where R is resistance connected in series with galvanometer,
θ
is
deflection in galvanometer and E is e.m.f of battery [Fig(5)].
(III) For full scale deflection current Ig:
Ig = nk ………………………….(3)
Where n is total number of divisions on scale of galvanometer.
R
E
R
G
S
K1
Fig.4
R
E
R
G
K1
Fig.5
R
G
R
Fig.6
C
V
K
E
A
B
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(IV) For high resistance Rh that should be connected in series with galvanometer
to convert it into voltmeter of range 0-3 Volt.;
R
h
= …………………………(4)
PROCEDURE :
(I) Make connections according to the circuit shown in figure (4) while keeping
both switches K1 and K2 open. High resistance box (HRB) should connect
in series with galvanometer and battery through switch K1 and low
resistance box (LRB) should be connected in parallel to galvanometer.
(II) First take some resistance (about 5000Ω) from HRB and then close switch
K1. Note deflection in galvanometer. Adjust resistance R in HRB such that
galvanometer shows full scale deflection (of 30 divisions) exactly.
(III) Now close switch K2 without taking resistance from LRB. Deflection in
galvanometer will become zero. Adjust resistance in LRB to such a value
that deflection in galvanometer becomes 15 divisions. Note resistance in
LRB now.
(IV) Open switch K2 (K1 should remain closed) and adjust resistance R in HRB
such that deflection in galvanometer becomes 28 division. Then close K2
and adjust resistance in LRB such than deflection in galvanometer becomes
14 division.
(V) Repeat step (iv) for deflection of 26, 24 and 22 division in galvanometer and
find out S to half deflection in each case.
(VI) Calculate resistance Rg of galvanometer by using relation Rg =
S
R
SR
*
each
of observation. Then find out mean value of Rg
(VII) Connect a high resistance box in series with the galvanometer across a
battery through switch K as shown in figure (5). First take out a high
resistance (e.g.5000Ω) from resistance box and then close switch K. Now
adjust resistance R in the resistance box in such a way that there deflection
of 30 divisions in galvanometer. Measure e.m.f. E of battery by voltmeter
and calculate figure of merit K by using equation(2)
Observation & Calculation
(i) Resistance “Rg” of Galvanometer by Half Deflection method:
S.No
Resistance ‘R’
In H.R.B.
Deflection in
Galvanometer
Θ div
Half deflection
θ/2 div
Resistance ‘S’ for
Half deflection
SR
RS
Rg
=
1
2
3
4
5
6
Mean Value of Rg =
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(ii) For figure of Merit ‘K’
e.m.f. of cell or power supply = E = ------- Volts
S.No
Resistance
‘R’
Deflection in Galvanometer
Θ div Θ+
=
)( RgR
E
k
A
1
2
3
Mean Value of ‘K’ = -------------Ampere per division
(iii) Calculation of High Resistance R
h:
Number of division on scale of galvanometer = n = 30
Current for full scale deflection Ig= nk = ------------A
Range of conversion = V = 3.0 Volts
Required High resistance =
h g
g
V
R R
I
= −
=
(iv) Verification
One small division of galvanometer scale after conversion = V/n = 3/30 =0.1 V
(v) Verification by Half deflection method
S.No
Reading of Shunted Galvanometer Voltmeter
A
Error = V- V
A
In division ‘x’
In Volts V= V*x/n
1
2
3
4
5
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