Conversion of Galvanometer into Voltmeter Experiment


CONVERSION OF GALVANOMETER INTO VOLTMETER EXPERIMENT

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.

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:

1. What is the principle of shunt?
2. An ammeter is of low resistance and a voltmeter is of high resistance. Explain why?
3. How will you convert a galvanometer into an ammeter?
4. How will you convert a galvanometer into a voltmeter?
5. What is the resistance of an ammeter?
6. What is the resistance of a voltmeter?
7. Can you use a Weston galvanometer to measure ac?

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