What it does
A moving-coil galvanometer is a very sensitive meter: a coil in a magnetic field turns when a tiny current flows, and the needle shows how much. A typical one gives full-scale deflection at about 1 mA and has a coil resistance of tens of ohms. On its own it cannot measure big currents or voltages, but adding one resistor fixes that:
- Ammeter: a small shunt S in parallel lets most of the current bypass the coil. S = Ig G / (I₀ − Ig).
- Voltmeter: a large resistance R in series limits the current, so the full range V₀ just drives Ig. R = V₀ / Ig − G.
This page works out S or R with your numbers, shows the meter’s own resistance (an ammeter should be tiny, a voltmeter huge), and redraws the meter face with the new scale outside and the coil current inside. Move the Test it slider to put a current or voltage through it and watch the needle and the current split.
It follows the Class 12 Physics chapter on moving charges and magnetism (the moving-coil galvanometer and its conversion into an ammeter and a voltmeter) and the matching practical. Meters are used in the ohm's law lab; a galvanometer finds the balance in the wheatstone bridge, and biot–Savart lab shows where the coil’s turning force comes from.
How to use it
- Choose Make an ammeter or Make a voltmeter.
- Set the galvanometer’s coil resistance G and its full-scale current Ig (in milliamperes).
- Pick the new range.
- Read the shunt or series resistance, the formula with your numbers and the meter’s resistance.
- Slide Test it from 0 to 120% of the range. Past 100% the needle hits the stop.
Worked example
A galvanometer has G = 50 Ω and Ig = 1 mA. To read up to 2 A, the shunt is S = 0.001 × 50 / (2 − 0.001) = 25.0 mΩ. The ammeter’s resistance is G S / (G + S) = 25.0 mΩ too, almost all from the shunt. When 1 A flows, 0.5 mA goes through the coil (half of full scale) and the rest through the shunt.
To read up to 10 V instead, R = 10 / 0.001 − 50 = 9950 Ω, so the voltmeter’s resistance is 10 kΩ: 1000 ohms per volt, whatever the range. Across 5 V it draws 0.5 mA, half of full scale.
Tips
- An ammeter goes in series, so it must have a very low resistance; a voltmeter goes in parallel, so it must have a very high one.
- A more sensitive galvanometer (smaller Ig) makes a better voltmeter: more ohms per volt, so it disturbs the circuit less.
- Shunts for big ranges are tiny, often a short thick strip of metal.
FAQ
Why can the range not be smaller than Ig (or Ig G)? The galvanometer already reaches full scale there; a shunt or series resistor can only extend the range.
Do real multimeters work like this? Old analogue ones do, with a switch choosing different shunts and series resistors. Digital meters measure a voltage electronically instead.
Is this an official NCERT resource? No. fibub is independent; the page follows the topics in the syllabus in its own words.
Where the numbers come from
- Shunt S = Ig G / (I₀ − Ig), series resistance R = V₀ / Ig − G, meter resistances G S / (G + S) and G + R, and the current divider I S / (G + S): standard circuit results.
- Checked by hand (G = 50 Ω, Ig = 1 mA: a 2 A ammeter needs S = 25.0 mΩ; a 10 V voltmeter needs R = 9950 Ω) and by checking that the full range always drives exactly Ig through the coil.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.