Galvanometer, Ammeter and Voltmeter: Meters Compared
A galvanometer is a sensitive instrument that detects small currents and shows their direction. An ammeter and a voltmeter are actually modified galvanometers: an ammeter measures larger currents by adding a low resistance in parallel, and a voltmeter measures potential difference by adding a high resistance in series.
Understanding all three together is important because every electrical measurement in a lab depends on choosing the right meter. Connecting the wrong one, or one with unsuitable resistance, can disturb the circuit and give a perfectly neat but wrong reading.
Key Concepts
Galvanometer G: coil in a magnetic field; deflection θ is proportional to current I. Resistance G is typically tens of ohms and full-scale current Ig is small, in milliampere. Ammeter: measures current in ampere (A), connected in series, must have very low resistance so it does not reduce the current it measures. Voltmeter: measures potential difference in volt (V), connected in parallel, must have very high resistance so it draws negligible current. Shunt S: low resistance in parallel with G to make an ammeter. Multiplier: high resistance in series with G to make a voltmeter.
Formulas
Ammeter shunt: S = Ig G/(I − Ig), where I is desired full-scale current in A, Ig in A and G, S in Ω. Voltmeter series resistance: R = V/Ig − G, where V is desired full-scale voltage in V. Assumption: galvanometer current at full scale Ig is fixed by its coil and spring. Limit: a real ammeter still has small resistance and a real voltmeter finite resistance, so both slightly perturb the circuit.
Worked Example
A galvanometer has G = 100 Ω and Ig = 1 mA = 0.001 A. Convert it into (a) a 1 A ammeter and (b) a 10 V voltmeter.
- Ammeter: S = 0.001 × 100/(1 − 0.001) ≈ 0.10 Ω in parallel.
- Voltmeter: R = 10/0.001 − 100 = 10000 − 100 = 9900 Ω in series.
The same movement becomes a 1 A ammeter with a 0.1 Ω shunt and a 10 V voltmeter with a 9.9 kΩ series resistor — the physics is identical, only the added resistor changes.
Common Misconceptions
- An ammeter goes across a component: No, it goes in series; its low resistance across a component would short it.
- A voltmeter should have low resistance to pass current: Opposite — it must have high resistance so the circuit is undisturbed.
- A galvanometer can directly measure amperes: Its coil would burn; large currents need a shunt to bypass most of the current.
Real-World Applications
- Panel meters in power supplies, chargers and vehicles.
- Multimeters, which switch shunts and multipliers internally to cover many ranges.
- Null detectors in bridges and potentiometers, where sensitivity matters more than scale accuracy.
All three meters share one sensitive movement; shunts and series resistors reshape its range, and connecting it correctly — ammeter in series, voltmeter in parallel — keeps both the circuit and the reading honest.