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:
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).
0 on: "Conversion of Galvanometer into Voltmeter Experiment"