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Conversion of Galvanometer into Voltmeter Experiment (Class 12)


CONVERSION OF GALVANOMETER INTO VOLTMETER EXPERIMENT (CLASS 12)

Aim: To convert a pointer type galvanometer of known resistance and figure of merit into a voltmeter to read 1 mV per scale division and to calibrate it.

Apparatus: An accumulator, pointer type galvanometer of known resistance and figure of merit, a resistance box of range 1 to 10,000Ω, a high resistance rheostat, a standard millivoltmeter, key etc.

Theory: A galvanometer can be converted into a voltmeter to read any desired value of voltage V per scale division, a suitable high resistance R is connected in series with it. If ig is the current sensitiveness (Figure of merit) and G is resistance of the galvanometer, then,
V = ig(R +G)
Therefore, R = (V — igG)/ ig

Procedure:

(i) To convert the galvanometer into voltmeter

The galvanometer resistance G and its figure of merit ig are noted. The high resistance R to be connected in series with the galvanometer to convert it into a voltmeter to read 1 mV/division is calculated by the equation,
R = (V — igG)/ig, where V = 1 mV = 10-3 V.
A resistance RΩ is unplugged in the resistance box and it is connected in series with the galvanometer. Now the galvanometer is converted into a voltmeter to read 1 mV/div.

(ii) To calibrate the 'galvanometer converted' voltmeter

Connections are made as shown in the figure. The rheostat is used as a voltage divider. V is a standard millivoltmeter.
The rheostat is adjusted to get a defection of 2 divisions in the galvanometer-converted voltmeter. Now its reading is V0 = 2 mV. The standard voltmeter reading V is noted. The correction in the converted voltmeter reading, V — Vo, is calculated.
The experiment is repeated for different values of the converted voltmeter readings. A calibration curve is drawn with the correction V — Vo, along the y-axis and Vo, along the x-axis.

Observations and Readings

 

(i) To convert the galvanometer into voltmeter

 

G = ..... Ω

ig = ..... A/div

V = 1 mV/division

R = (V - igG)/ig = ..... Ω

 

(ii) To calibrate the converted voltmeter:

 

Deflection in galvanometer

Converted voltmeter Vo (mA)

Standard voltmeter V (mA)

Correction, V – Vo(mA)

2

2

 

 

3

3

 

 

4

4

 

 

5

5

 

 

-

-

 

 

-

-

 

 


Result

(i) The galvanometer is converted into a voltmeter to read 1 mV per scale division
(ii) The calibration curve is drawn.

Note:

(i) The correction (V — V0) may be positive or negative
(ii) The calibration curve is drawn by joining the points on the graph with straight lines.

Viva Questions for Conversion of Galvanometer into Ammeter and Voltmeter:

I. What is a Voltmeter?

Voltmeter is a device used to measure p.d. between two points


2. How is a galvanometer converted into a voltmeter?

The galvanometer is converted into a voltmeter by connecting a high resistance in series with the galvanometer.


3. Define resistance of an ideal voltmeter?

The resistance of an ideal voltmeter is infinity.


4. Distinguish between voltmeter and voltameter.

Voltmeter is a device used to measure pal. between two points whereas voltameter is used to carry the process of electrolysis.


5. Define e.m.f. of a cell.

The e.m.f. of a cell is the maximum p.d. between two electrodes of a cell once the cell is in open circuit.


6. Define terminal potential difference.

The potential difference between the two electrons of a cell in a closed circuit is called terminal potential difference. The terminal potential difference should always be less than the electromotive force of the cell.


7. Define relaxation time.

Relaxation time is the time interval between two successive collisions of a ‘free’ electron with the atoms in a metal (10-14s). 

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