Moving-coil galvanometer – conversion to ammeter, voltmeter and ohmmeter
PhysicsElectricity & CircuitsAges 16–17Virtual lab
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A moving-coil galvanometer in a radial magnetic field: the torque NBIA on the coil is balanced by the spring torque kθ, so the deflection θ = (NBA/k)·I is proportional to the current. Find the resistance of a lab galvanometer by the half-deflection method and its figure of merit, then convert it into an ammeter with a shunt, a voltmeter with a series resistance and an ohmmeter with a non-linear scale, checking each one against a standard meter in a results table.
Lesson: Moving-coil galvanometer; current sensitivity and figure of merit; half-deflection method; conversion into an ammeter, a voltmeter and an ohmmeter
What it shows
In a moving-coil galvanometer a coil of N turns and area A carrying a current I sits in a radial magnetic field B, so the torque NBIA is the same at every angle. A spiral spring gives a restoring torque kθ, and the coil comes to rest where NBIA = kθ: the deflection is proportional to the current and the scale is linear. The figure of merit is the current per division. A small shunt in parallel turns the galvanometer into an ammeter, a large resistance in series turns it into a voltmeter, and a cell with a series resistor turns it into an ohmmeter with a non-linear scale.
How to use
Start in Moving coil and change Number of turns N, Magnetic field B and Current I to see the torque and the linear scale; press Record to plot θ against I. In Half deflection, close K₁, choose R for a large deflection, close K₂ and adjust S until the deflection halves, then record and enter G and K. In Ammeter and Voltmeter, calculate the shunt or the series resistance, type it in and compare your readings with the standard meter.
Parameters you can change
- Screen Moving coil, Half deflection, Ammeter, Voltmeter, Ohmmeter
- Lab galvanometer Galvanometer 1, Galvanometer 2, Galvanometer 3
- Number of turns N 20–300
- Magnetic field B 0.05–0.5 T
- Coil area A 0.5–6 cm²
- Spring constant k 0.5–10 ×10⁻⁶ N·m/rad
- Current I (moving coil) -1000–1000 μA
- Cell emf E (half deflection) 1–4 V
- Ammeter range I₀ 5–100 mA
- Voltmeter range V₀ 1–20 V
- Show the given G and K
Questions to explore
- Why does a radial magnetic field make the galvanometer scale linear?
- Why must the shunt of an ammeter be much smaller than the galvanometer resistance?
- Why are the divisions of an ohmmeter scale crowded together at the high-resistance end?
