Galvanometer → Ammeter / VoltmeterJEE Main

Shunts and multipliers — interactive Physics simulation for IIT-JEE.

Concept

A galvanometer deflects fully at a tiny current $I_g$ . To read big currents, add a small shunt in parallel — it bypasses the excess current. To read voltages, add a big multiplier in series — it soaks up the excess voltage. A good ammeter has ~zero resistance; a good voltmeter, ~infinite.

Key formula

S = \frac{I_g G}{I - I_g} \;(\text{parallel}), \qquad R = \frac{V}{I_g} - G \;(\text{series})

Derivation

Ammeter: shunt and coil share the same voltage: $I_gG = (I - I_g)S$ . Solve for S — tiny when $I \gg I_g$ .

Voltmeter: at full deflection the series pair carries $I_g$ across the target V: $V = I_g(G + R)$ .

Effective resistances: ammeter $= G\parallel S \approx S$ (small); voltmeter $= G + R$ (large).

Scenarios to explore

Galvanometer Conversion — Shunt → ammeter, multiplier → voltmeter.

Real-world applications

Every analog multimeter is one galvanometer + switched shunts/multipliers.
Current sensing shunts in battery management systems.
Voltmeter loading errors in high-impedance circuits.

JEE exam tips

n-fold range extension: S = G/(n−1) where n = I/I_g.
Voltmeter sensitivity spec: 'Ω per volt' = 1/I_g.
An ammeter in parallel or voltmeter in series = classic circuit-blunder questions.

Common mistakes

Swapping the topologies (series shunt / parallel multiplier).
Forgetting to subtract G in R = V/I_g − G.
Using I instead of I − I_g in the shunt equation.

Exam traps to avoid

The shunt carries almost ALL the current — it must be thick wire.
Ideal voltmeter reads EMF only when no current flows (infinite resistance).