# 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
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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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