A galvanometer having a coil resistance of 60 Ω shows full-scale deflection when a current of 1.0 A passes through it. It can be converted into an ammeter to read currents up to 5.0 A by:

The correct answer is option 3) i.e. putting in parallel resistance of 15 Ω.

CONCEPT:

• Galvanometer: A galvanometer is an instrument used for detecting and indicating the presence of an electric current.
  • ​A galvanometer is a sensitive device and can detect only a very small current of the order of 100 mA.
  • Full-scale deflection of galvanometer refers to the maximum reading available on the meter. Anything beyond this scale cannot be measured by this instrument.
• Conversion of galvanometer into an ammeter: 
  • For measuring a current greater than of the order of 100 mA, a low resistance called a shunt is connected in parallel across the galvanometer.

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• Let I be the total current in the circuit which is the actual value of current, then the current (I - I_g) passes through a shunt (S), where Ig is current for full-scale deflection of the galvanometer.
• Since G and S are parallel to each other, therefore the potential difference across both is the same.

V_G = V_S

I_gG = (I - I_g)S

⇒ \(S = \frac{I_gG}{(I-I_g)}\)

CALCULATION:

Given that:

The resistance of galvanometer, G = 60 Ω 

Current for full-scale deflection of the galvanometer, I_g = 1 A

The total current in the circuit, I = 5 A

A galvanometer is converted into an ammeter by connecting a shunt in parallel across the galvanometer.

The resistance of shunt connected in parallel, S \(=\frac{I_gG}{(I-I_g)}\)\(= \frac{1 \times 60}{(5-1)}\) = 15 Ω