Conversion of a Galvanometer in Ammeter

An ammeter is a device used to measure electric current, and it is always connected in series in a circuit. For a device to serve as an ideal ammeter, it must offer zero resistance, so that it does not alter the current it is measuring.

A galvanometer, although capable of detecting current, cannot be used directly as an ammeter because it possesses appreciable internal resistance. Connecting it directly into a circuit would reduce the current flowing through it, giving inaccurate readings.

To convert a galvanometer into an ammeter capable of measuring larger currents, a low-resistance shunt (S) is connected in parallel with the galvanometer coil (resistance G).

• The galvanometer carries only its safe full-scale current I_g
• The remaining current (I − I_g) passes through the shunt
• This protects the galvanometer while allowing measurement of large currents

Since G and S are in parallel, the potential difference across both is equal:

Rearranging for shunt resistance:

Effective Resistance of the Ammeter: RA = \[\frac {GS}{G+S}\]

A galvanometer of resistance G = 15 Ω gives full-scale deflection for I_g = 4 mA. Convert it into an ammeter of range I = 6 A.

Solution:

Correct Answer: S ≈ 0.01 Ω