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.