To convert the given galvanometer of known resistance and figure of merit into an ammeter of desired range and verify the same

 

CONVERSION OF GALVANOMETER INTO AMMETER

A galvanometer is an electronic device that is used to detect mild currents in an electrical circuit. In order to measure large currents it is converted into an Ammeter by connecting a low resistance called as shunt resistance in parallel to the galvanometer. Hence, we will consider it as the conversion of a galvanometer into an ammeter.

Aim of the experiment :

To convert the given galvanometer into an ammeter of the required range (0-30mA) and verify the same.

Apparatus required :

1. Connecting wires
2. Galvanometer
3. Battery
4. Rheostat
5. Plug Key
6. Ammeter
7. Resistor

Principle of the experiment :

Galvanometer can be converted into an ammeter by connecting a suitable low resistance in parallel with it, so that very small current flows through the galvanometer.

Formula :

01. Current (in Ampere) required for full-scale deflection in galvanometer, Ig = N*K

Where,

            N - Number of divisions on either side of zero of the galvanometer.

            K - Figure of merit of galvanometer.

02. Theoretical value of shunt resistance (in Ω ohm), S = Ig*G/I-Ig

Where,

            G - Resistance of the galvanometer.

            I - Range of a converted galvanometer.

Circuit diagram :

The circuit diagram for the conversion of the galvanometer into an ammeter is as follows below.

To convert the given galvanometer into ammeter

Procedure :

• The current for the full-scale deflection in the galvanometer, Ig = N*K is calculated.

• The shunt resistance S is calculated using the formula S = Ig*G/I-Ig

• With the given values of radius,r and resistivity, ρ f the wire, the length, l of the wire is calculated using the formula as follows l = Sπr²/ρ

• The wire of a slightly longer length than the calculated length is connected in parallel with the galvanometer and connections are made as shown in the circuit diagram respectively.

• The rheostat is adjusted for 30mA current in the ammeter.

• The length of the wire is adjusted for full-scale deflection in the galvanometer and its length l is measured.

• Shunt resistance S is calculated using the formula, S' = ρl'/πr² 

• S' is equal or nearly equal to S and hence conversion is verified.

Observation :

1. Resistance of the galvanometer, G (given) = ..................................Ω ohm.
2. Figure of merit of the galvanometer, K (given) = ..................................A/div.
3. Number of divisions on either side of zero of the galvanometer scale, N = ..................................A.
4. Current required for producing full-scale deflection of N divisions, Ig = N*K = ..................................Ampere
5. Maximum current to be measured, I (say 30mA) = ..................................Ampere.

Calculation :

Shunt resistance , S = Ig*G/I-Ig =.................................. (in Ω ohm)

Result :

01. The theoretical value of shunt resistance, S =..................................(in Ω ohm)

02. The experimental value of shunt resistance, S' =..................................(in Ω ohm)