Question

To decrease the range of an ammeter, its resistance need to be increased. An ammeter has resistance $ R_0 $ and range I. Which of the following resistance can be connected in series with it to decrease its range to $ \frac{1}{n} $ ?

• A. $ \frac{R_0}{n} $
• B. $ \frac{R_0}{(n - 1)} $
• C. $ \frac{R_0}{(n + 1)} $
• D. None of these

Answer 1

Answer: (D)

A galvanometer can be converted into an ammeter by using a low resistance wire in parallel with the galvanometer. The range of ammeter can be increased but cannot be decreased. The reason is that a series resistor cannot change the current that will produce full deflection.
A shunt connected in parallel is given by $ S = \frac{I_g G}{I - I_g} $
If $ I < I_g $ , then the value of S is negative.
Hence, a galvanometer cannot be converted into an ammeter of range $ I < I_g $