# Ammeter and Voltmeter Conversion > Ammeter and voltmeter conversion for NEET Physics: shunt design for galvanometers, multiplier resistance, ideal meters and circuit loading. - Canonical URL: https://prepelephant.com/topics/neet-ug/physics/ammeter-voltmeter-conversion - Exam / course: NEET-UG · Subject: Physics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Ammeter and Voltmeter Conversion", PrepElephant, https://prepelephant.com/topics/neet-ug/physics/ammeter-voltmeter-conversion ## Direct answer A galvanometer becomes an ammeter when a small shunt resistance is wired in parallel, diverting the bulk of the current around the delicate coil: S = I_g G/(I - I_g) for a meter of resistance G and full-scale deflection current I_g. The same movement becomes a voltmeter when a large multiplier resistance is wired in series: R = V/I_g - G. The design logic follows from placement — an ammeter is a low-resistance series element (ideally zero), a voltmeter a high-resistance parallel element (ideally infinite). Conversion numericals are plug-ins; the deeper NEET questions concern what non-ideal meters do to the circuit they measure. ## What you must remember - **Shunt formula:** S = I_g G/(I - I_g); for a galvanometer of G = 100 Ω and I_g = 1 mA converted to a 1 A ammeter, S ≈ 0.1 Ω — always far smaller than G. - **Multiplier formula:** R = V/I_g - G; the same galvanometer as a 10 V voltmeter needs a series resistance of 9900 Ω, total 10 kΩ. - **Resulting meter resistance:** the converted ammeter's resistance is GS/(G + S), essentially the shunt; the converted voltmeter's is essentially the multiplier — the design difference in one line. - **Ideal instruments:** ammeter resistance zero, voltmeter resistance infinite; every real reading carries the loading error of falling short of these ideals. - **Placement discipline:** ammeter in series with the element, voltmeter across it; an ammeter connected across a battery is effectively a short circuit and can burn out. - **Shunt protection:** most of the heat dissipates in the shunt, which also protects the coil from overcurrent — a parallel resistance, never a series one. ## Converting one galvanometer two ways Start with a galvanometer of G = 100 Ω that deflects fully at I_g = 1 mA. As an ammeter of range 1 A: the shunt must carry 999 mA when the coil carries 1 mA at the same 0.1 V (the coil's full-scale drop), so S = 0.1/0.999 ≈ 0.1 Ω, matching the formula S = I_g G/(I - I_g). The converted meter's resistance is now about 0.1 Ω — inserting it in series with a 10 Ω resistor changes the circuit current by barely 1 per cent, which is the whole point of the low-resistance design. As a voltmeter of range 10 V: at full scale the total resistance must pass 1 mA at 10 V, so the total is 10,000 Ω and the series multiplier is 9900 Ω. Placing this across a 10 kΩ resistor in a circuit effectively halves that branch's resistance — a severe loading error that a 100 kΩ meter would reduce to under 10 per cent. One galvanometer, two personalities: the conversion is Ohm's law applied to what the meter is allowed to do. ## Loading and the traps it sets The exam's sharpest questions are about the non-ideal meter's effect. A voltmeter of resistance R_v connected across R reads the voltage on the parallel pair R and R_v — if R_v is comparable to R the reading is systematically low, and questions give the two resistances expecting you to compute the pair before anything else. The ammeter's error runs the other way: its small series resistance raises the total circuit resistance slightly, lowering the current it is trying to measure. The second trap is the miswired meter: an ammeter in parallel with a supply is a near-short (0.1 Ω across 12 V would pass 120 A if the source could give it), while a voltmeter in series simply adds its huge resistance and kills the current. Third, the conversion formulas are direction-sensitive: increasing the ammeter's range means decreasing the shunt (more diversion), while increasing the voltmeter's range means increasing the multiplier — "to double the range" sends the two instruments in opposite directions. ## Frequently asked questions ### Why is a shunt connected in parallel with a galvanometer? To divert most of the line current around the coil while both share the same voltage, letting the milliammeter-scale movement indicate currents of amperes. ### How is a galvanometer converted into a voltmeter? By connecting a high resistance in series, R = V/I_g - G, so that the full voltage range corresponds to the coil's full-scale current. ### What are the ideal resistances of an ammeter and a voltmeter? Zero for the ammeter (so it disturbs no current) and infinite for the voltmeter (so it draws none) — real meters approach but never reach these ideals. ### Why should an ammeter never be connected across a battery? Its resistance is deliberately tiny, so it would pass a destructively large current — effectively short-circuiting the source through the meter. ### What happens if a voltmeter of resistance comparable to the circuit's is used? It draws significant current and lowers the very voltage it is meant to measure — the loading error; a voltmeter's resistance should always be much larger than the element it tests.