To convert the given galvanometer (of known resistance and figure of merit) into an ammeter of desired range and to verify the same.
What is a Galvanometer?
A galvanometer is a device used to detect feeble electric currents in a circuit. It has a coil pivoted (or suspended) between concave pole faces of a strong laminated horse shoe magnet. When an electric current passes through the coil, it deflects. The deflection is proportional to the current passed. The galvanometer coil has a moderate resistance (about 100 ohms) and the galvanometer itself has a small current carrying capacity (1 mA).
What is an Ammeter?
An ammeter is a device used for measuring large electric currents in circuits. For this purpose, it is put in series with the circuit in which the current is to be measured.
How to convert a Galvanometer into an Ammeter?
A galvanometer can detect only small currents. Thus, to measure large currents it is converted into an ammeter. It can be converted into an ammeter by connecting a low resistance called shunt resistance in parallel to the galvanometer.
Let G be the resistance of the galvanometer and Ig be the current for full scale deflection in the galvanometer, the value of the shunt resistance required to convert the galvanometer into an ammeter of 0 to I ampere is,
S = I g × G I − I g
Ig is calculated using the equation, Ig = nk, where n is the number of divisions on the galvanometer and k is the figure of merit of galvanometer.
What is figure of merit of a galvanometer?
The figure of merit of a galvanometer is defined as the current required in producing a unit deflection in the scale of the galvanometer. It is represented by the symbol k and is given by the equation,
Where E is the e.m.f. of the cell and θ is the deflection produced with resistance R.
Let ‘l’ be the length of the resistance wire required for a resistance of S ohm,
where, r is the radius of the wire and ρ is the resistivity of the material of the wire.
The student learns the following concepts:
Real Lab Procedure:
G- the resistance of the galvanometer.
I- the range of desired ammeter
Ig = nk, the current required for full scale deflection in the galvanometer,
where, n- total number of divisions in the galvanometer
k- the figure of merit of the galvanometer.
Then, the length of the wire required for shunt can be calculated using the formula,
Where, ρ- the resistivity of material of the wire
r- the radius of the wire, which can be measured using a screw gauge.
Repeat the experiment by changing the rheostat resistance.
Simulator procedure (as performed through the Online Labs):
Resistance of the galvanometer, G = ............ohms
Figure of merit of the galvanometer, k = ............... amp./div.
Number of divisions in the galvanometer scale, n = ................
Current for full scale deflection, Ig = nk =............amp.
Desired range of the converted ammeter (I) ---------mA