To convert the given galvanometer (of known resistance & figure of merit) into an ammeter of desired range & to verify the same

Aim

To convert the given galvanometer (of known resistance & figure of merit) into an ammeter of desired range & to verify the same.

Apparatus

A Weston type galvanometer whose resistance & figure of merit are given, a constantan or manganin wire, a battery, one-way key, a rheostat, a milli-ammeter, connecting wires, sand paper etc.

Formulae Used:

To convert a galvanometer which gives full scale deflection for current I_Ginto an ammeter of range O to I_Oamperes, the value of required shunt is given by: S= [I _G/I_O-I _G]G

Required shunt resistant S is made using a uniform wire whose, specific resistance is σ (known) & its length:

ι=πr ₂S/σ

Observation:

Given resistance of galvanometer, G = 70.8 Ω

Given value of figure of merit, k = 2.19 x 10 ^-5A div _-1

Current for full scale deflection, I_G= N_Ox k = 6.57 x 10 ^-4A

*Required range of converted ammeter, I_O= 3A

*Value of shunt resistance, S=[I_G/I_O-I_G]x G=0.0155Ω

*Computing the length of the wire to make resistance of 0.155Ω

Pitch of screw gauge, p = 1 mm

No. of division of circular scale = 100

Least count, a = 0.01 mm

Zero error, e = 0.0 mm

Diameter of the wire = 0.98 mm, Radius = 0.049 cm

Table for Verification

Verification:

Checking the performance of the converted ammeter: Current indicated by full scale deflection (N _O) of converted ammeter. I _O= 3A

Least count of converted ammeter,k' = I_o/N_o= 0.1A/div.

Result:

• Current IG for full scale deflection = 6.57 x 10 ^-4A
• Resistance of shunt required to convert the galvanometer into ammeter, S=0.0155Ω
• Required length of wire, ι=60.8 cm
• As error ι’–ι is very small, conversion is verified.

Precautions & Sources of Error:

• All connections should be neat & tight.
• The diameter of the wire for making shunt resistance should be measured accurately for diameter is taken in two mutually perpendicular directions.
• The terminal of the ammeter marked positive should be connected to positive pole of the battery. Also ammeter should be in series with circuit.

Mean k= 2.19 x 10 ^-5 A/div.

Result:

1. Resistance of Galvanometer by half – deflection method: G= 70.8Ω
2. Figure of merit, k= 2.19 x 10 ^-5 A/div.

Precautions & Sources of Error:

1. All connections should be neat & tight.
2. The diameter of the wire for making shunt resistance should be measured accurately for diameter is taken in two mutually perpendicular directions.
3. Initially a high resistance from the resistance box (R) should be introduced in the circuit. Otherwise for small resistance, an excessive current will flow through the galvanometer or ammeter & damage them.