© Javaid Khurshid, PIEAS Page 1 of 4
Experiment : CONVERSION OF GALVANOMETER INTO VOLTMETER / AMMETER
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.
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 across galvanometer and p.d. across the high resistance.
© Javaid Khurshid, PIEAS Page 2 of 4
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.
(I) For resistance of galvanometer by half deflection method:
Rg = 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
+ ………………………. (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.
K E A B
© Javaid Khurshid, PIEAS Page 3 of 4
(IV) For high resistance Rh that should be connected in series with galvanometer to convert it into voltmeter of range 0-3 Volt.;
Rh = …………………………(4)
(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 = 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:
Resistance ‘S’ for
Mean Value of Rg =
© Javaid Khurshid, PIEAS Page 4 of 4
(ii) For figure of Merit ‘K’ e.m.f. of cell or power supply = E = ------- Volts
Deflection in Galvanometer
Θ div Θ+ =
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 Voltmeter
Reading VA Error = V- VA
In division ‘x’ In Volts V= V*x/n