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Moving Charges And Magnetism

Question
CBSEENPH12037598

A galvanometer coil has a resistance of 15 Ω and the metre shows full scale deflection for a current of 4 mA. How will you convert the metre into an ammeter of range 0 to 6 A?

Solution
Given, a galvanometer coil.
Resistance on galvanometer, G = 15 Ω
Current pasing through the galvanometer,I
g = 4 mA = 4 x 10–3 A
Current across the ammeter, I = 6 A 

Using formula, 
                       S = Ig . GI-Ig 

Putting values,
                        S = 4 × 10-3 × 156-0.004    = 60 × 10-35.996      = 10 × 10-3Ω 

                         S = 10 m Ω 

Therefore, a resistance of 10mΩ should be connected so as to convert the galvanometer into an ammeter.