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Experiment No. Phy-
Object: TO CONVERT GALVANOMETER INTO VOLTMETER AND AMMETER.
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. However we can measure very small currents by using galvanometer but the primary purpose of galvanometer is the detection of electric current not its measurement.
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.
AMMETER: 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.
VOLTMETER: 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.
CONVERSION OF GALVANOMETER TO AMMETER: Galvanometer can be converted into ammeter by shunting it with a very small resistance. Potential difference across the galvanometer and shunt resistance are equal.
CONVERSION OF GALVANOMETER TO VOLTMETER:
Galvanometer can be converted into voltmeter by connecting it with a very high resistance. Potential difference (p.d.) across the given load resistance
is the sum of p.d across galvanometer and p.d. across the high resistance.
Object 1: To convert Galvanometer into Ammeter.
Apparatus: A (Weston type) galvanometer (of about 130 Ω resistance), two
resistance boxes, a cell or power supply, a voltmeter, an ammeter (of range ≥ 1A), a rheostat, two keys, screw gauge, meter rod, connecting wires, sand paper, shunt wire.
Formula Used:
(I) For resistance of galvanometer by half deflection method:
Rg = R S
R S
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(1)] (II) For figure of merit K.
K = ( R Rg ) θ
E
Where R is resistance connected in series with galvanometer,^ θ^ is deflection in galvanometer and E is e.m.f of battery [Fig(2)]. (III) For full scale deflection current Ig: Ig = nk ………………………….(3)
R
E
R G
K
S
K
Fig.
R
E
R G
K Fig.
BAT
R G
R
K
Fig.
A
K
(IX) Calculate current Ig that should pass through galvanometer for full scale deflection by using equation (3) and then from equation (4) determine shunt resistance Rs that has to be connected in parallel to galvanometer to convert it into ammeter. Put I = 1 Amp in this equation. (X) Measure diameter d of given shunt wire at different position (about 6 readings) and find out mean diameter of shunt wire and then calculate its length l which has required resistance Rs by using equation (5). (XI) Cut length of the shunt wire 2 cm more than the calculated length I. connect the wire between terminals of galvanometer such that length of wire between two terminals is exactly equal to I. (if there is insulation on wire then rub sand paper on its ends before connecting it across galvanometer.) (XII) Make connections as shown in fig (3). Close key K and take different readings of currents in galvanometer and ammeter by changing resistance e in circuit with the help of rheostat. Find current that produces deflection of 1 small division of galvanometer after its conversion into ammeter by dividing maximum current (1 A in this experiment) with total number of divisions on either side of zero of its scale. Multiply it with number of divisions by which pointer is deflected for given reading to get value of current. If calculated value of current and reading of ammeter are not equal, there is error which must be very small.
Observation & Calculation
(i) Resistance “Rg” of Galvanometer by Half Deflection method:
S.No
Resistance ‘R’ In H.R.B. Ω
Deflection in Galvanometer Θ div
Half deflection θ/2 div
Resistance ‘S’ for Half deflection Ω
R S
RS Rg −
Mean Value of Rg = (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 =^ ( R + Rg )Θ
E k A 1 2 3
Mean Value of ‘K’ = -------------Ampere per division
(iii) Calculation of Shunt Resistance Rs: Number of division on scale of galvanometer = n = Current for full scale deflection Ig= nk = ------------A Range of conversion = I = 1A
Required Shunt resistance = = ------------------Ω
(iv) Calculation of Length of wire:
Where d is diameter of the wire, and ρ is the resistivity of the wire.
(v) Verification One small division of galvanometer scale after conversion = I/n =
(vi) Verification by Half deflection method
S.No Reading of Shunted Galvanometer In division ‘x’ In Ampere I = I*x/n
Ammeter Reading “IA” Error = I- IA
1 2 3 4 5
OBJECT : To convert galvanometer into voltmeter of range 0-3 volts.
APPRATUS : A( Weston type) galvanometer , two resistance boxes, a cell L.T
power supply , a D.C. voltmeter (of range ≥ 3 V) , a rheostat , two keys, connecting wires, sand paper.
FORMULA USED:
(I) For resistance of galvanometer by half deflection method:
Rg = R S
R S
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.
RgIg Rs I Ig
2
4
d Rs l
ρ
that deflection in galvanometer becomes 15 divisions. Note resistance in LRB now. (IV) Open key K2 (K1 should remain closed) and adjust resistance R in HRB such that deflection in galvanometer becomes 28 division. Then close K 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 = R S
R S
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 key K as shown in figure (5). First take out a high resistance (e.g.5000Ω) from resistance box and then close key 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. Ω
Deflection in Galvanometer Θ div
Half deflection θ/2 div
Resistance ‘S’ for Half deflection Ω
R S
Rg RS −
Mean Value of Rg = (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 =^ ( R + Rg )Θ
E k A 1 2 3
Mean Value of ‘K’ = -------------Ampere per division
(iii) Calculation of High Resistance Rh: 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 In division ‘x’ In Volts V= V*x/n
Voltmeter Reading VA Error = V- VA 1 2 3 4 5
