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Conversion of Galvanometer into Ammeter Experiment


Aim: To convert a galvanometer of known resistance and figure of merit into an ammeter to read, say 0.05 ampere per division and to calibrate it.

Apparatus: A sensitive Weston (pointer type) galvanometer, a lead accumulator, a rheostat (100Ω), a uniform copper wire of known resistance per unit length, ammeter, key etc.

Theory: To convert a galvanometer into an ammeter to measure any desired value of current, a suitable shunt S is connected to the galvanometer. (A shunt is a low resistance connected in parallel across the terminals of the galvanometer).

If Ig is the figure of merit (current sensitiveness) of the galvanometer, i.e.,the current required to produce a deflection of one scale division, the shunt S required to convert the galvanometer to read i A per division is given by,

ig x G = (i - ig) x S;
Therefore, S = ig x G/(i - ig)
(The shunt S will be a low resistance. Such a low resistance can be obtained from uniform copper wire).


(i) To convert the galvanometer into an ammeter to read 0.05 A/div

The shunt S required to convert the galvanometer into ammeter to read i = 0.05 A/div is calculated using the equation

S = (ig x G)/(i — ig);

Here ig, the figure of merit and G, the resistance of the galvanometer are given. If p is the resistance per unit length of the uniform copper wire, length l required for the shunt is calculated. (l = S/ρ).
The copper wire of length l is cut and connected across the terminals of the galvanometer. Now, the galvanometer is converted into an ammeter to read 0.05 A per division.

(ii) To calibrate the `galvanometer-converted' ammeter

A lead accumulator, a rheostat of resistance about 100Ω, the converted galvanometer, a standard sensitive ammeter (0.1 A/div) and a key are connected in series. The circuit is closed and the rheostat is adjusted to get a deflection of, say, 4 division is the galvanometer. The galvanometer reading is Io = 0.05 x 4 = 0.2 A. The standard ammeter reading 1 is noted. The correction in the converted galvanometer reading, I — Io, is calculated. (The correction may be the positive or negative).

The experiment is repeated for the galvanometer deflections 7, 8, 10, ... (i.e.,for 0.3, 0.4, 0.5, ... A). A calibration curve is drawn with (I — Io) along the y-axis and Io along the x-axis. (Here the points on the graph are joined by straight lines.)


(i) The galvanometer is converted into an ammeter to read 0.05 A/div.
(ii) The calibration curve is drawn.

Conversion of Galvanometer into Ammeter Experiment Viva Questions and Answers

1. What is an ammeter?

Ammeter is an instrument used to determine the strength of current.

2. What is meant by shunt?

Shunt can be called as a low resistance which is connected in parallel with galvanometer or ammeter.

3. What are the uses of shunt?

i. Shunt is used to protect the galvanometer from strong current.

ii. Shunt is used for converting a galvanometer into an ammeter.

iii. Shunt is used to increase the range of ammeter.

4. Which one has more resistance, galvanometer or ammeter?


5. Why a galvanometer is having more resistance than an ammeter?

Galvanometer has more resistance than ammeter because ammeter is a shunted galvanometer.

6. Why is the coil wrapped on a conducting frame in a galvanometer?

To make the galvanometer dead beat.

7. Why is the coil of a moving coil galvanometer wound on an aluminium frame?

Because aluminium being a very light metal, does not add significantly to the inertia of the moving coil, also aluminium induces eddy current.

8. Why is soft iron core used in a moving coil galvanometer?

To make the magnetic field radial and to increase the strength of the magnetic field.

9. Why are pole pieces of galvanometer made concave?

Concave poles produce strong, consistent and radial magnetic field.

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