Why must an ammeter have very low resistance, ideally zero?

An ammeter is connected in series with the circuit component whose current is to be measured. If the ammeter had significant resistance, it would add to the total resistance of the circuit. According to Ohm's Law ($I = V/R$), an increase in total resistance would lead to a decrease in the total current flowing through the circuit. This means the ammeter would measure a current that is lower than the actual current flowing before the ammeter was introduced, thus giving an inaccurate reading. To minimize this error and ensure it measures the true current, an ammeter must offer negligible resistance, ideally zero, to the current flow.

Why must a voltmeter have very high resistance, ideally infinite?

A voltmeter is connected in parallel across the two points where the potential difference is to be measured. If the voltmeter had low resistance, it would provide an alternative, low-resistance path for the current. This would cause a significant portion of the current to divert through the voltmeter, reducing the current flowing through the original component and consequently altering the potential difference across it. To ensure it draws negligible current from the circuit and measures the true potential difference without disturbing the circuit, a voltmeter must offer very high resistance, ideally infinite.

What happens if an ammeter is connected in parallel in a circuit?

If an ammeter, which has very low internal resistance, is connected in parallel across a component, it effectively creates a short circuit across that component. Due to its extremely low resistance, a very large current will flow through the ammeter, bypassing the component. This excessive current can damage the ammeter itself (by burning out its coil) and potentially the power source or other circuit components due to overheating. It will also give an incorrect, usually very high, current reading that doesn't reflect the component's actual current.

What happens if a voltmeter is connected in series in a circuit?

If a voltmeter, which has very high internal resistance, is connected in series in a circuit, it will introduce a substantial resistance into the circuit path. This large added resistance will drastically reduce the total current flowing through the circuit, potentially to near zero, effectively making the circuit an open circuit. Consequently, the voltmeter will read a voltage that is not representative of any meaningful potential difference across a functional part of the circuit, and the circuit itself will cease to operate as intended.

How does increasing the shunt resistance affect an ammeter's range?

Increasing the shunt resistance ($R_s$) in an ammeter means that a smaller fraction of the total current will bypass the galvanometer and a larger fraction will pass through the galvanometer itself. Since the galvanometer has a maximum current ($I_g$) it can safely handle for full-scale deflection, increasing $R_s$ would mean that the total current ($I$) required to achieve $I_g$ through the galvanometer would be smaller. Therefore, increasing the shunt resistance *decreases* the range of the ammeter. To increase the range, the shunt resistance must be decreased.

Ammeter and Voltmeter

Physics

NEET UG

Version 1Updated 22 Mar 2026

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An ammeter is an electrical instrument used to measure the current flowing through a circuit or a specific branch of it. It is always connected in series with the component through which the current is to be measured. An ideal ammeter possesses zero internal resistance to ensure it does not alter the current it is measuring. Conversely, a voltmeter is an electrical instrument designed to measure t…

Quick Summary

Ammeters and voltmeters are essential tools for circuit analysis, both derived from a galvanometer. A galvanometer is a sensitive device that detects and measures small currents. To convert it into an ammeter, a very low resistance, called a shunt resistance ( $R_s$ ), is connected in parallel with the galvanometer.

This allows the ammeter to measure larger currents by diverting most of the current through the shunt, while only a safe fraction passes through the galvanometer. Ammeters must be connected in series and ideally have zero internal resistance to avoid altering the circuit current.

The formula for shunt resistance is $R_s = \frac{I_g R_g}{I - I_g}$ .

To convert a galvanometer into a voltmeter, a very high resistance, called a series resistance ( $R_{ser}$ ), is connected in series with the galvanometer. This allows the voltmeter to measure larger voltages by dropping most of the potential difference across the series resistor, limiting the current through the galvanometer.

Voltmeters must be connected in parallel and ideally have infinite internal resistance to avoid drawing current from the circuit. The formula for series resistance is $R_{ser} = \frac{V}{I_g} - R_g$ . Understanding these configurations and their ideal characteristics is crucial for correct circuit measurement and problem-solving in NEET.

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Key Concepts

Ammeter Range Extension

To extend the range of an ammeter, meaning to enable it to measure larger currents, a smaller shunt…

Voltmeter Range Extension

To extend the range of a voltmeter, meaning to enable it to measure larger voltages, a larger series…

Loading Effect of Practical Meters

Practical ammeters and voltmeters have non-ideal internal resistances, which can affect circuit measurements.…

Galvanometer: —

R_g

(resistance),

I_g

(full-scale current).

Ammeter Conversion: — Shunt resistance

R_s

in parallel with

R_g

- Formula: $R_s = \frac{I_g R_g}{I - I_g}$ - Ideal Ammeter: $R_{eff} = 0$ , connected in series.

Voltmeter Conversion: — Series resistance

R_{ser}

in series with

R_g

- Formula: $R_{ser} = \frac{V}{I_g} - R_g$ - Ideal Voltmeter: $R_{eff} = \infty$ , connected in parallel.

Key Principle: — Ammeters in series, Voltmeters in parallel.

Ammeter: All in Series, Shunt in Parallel, Small resistance. (Ammeter, All in Series connection, Shunt is in Parallel, Shunt is Small resistance)

Voltmeter: Very Parallel, Series resistor, Large resistance. (Voltmeter, Very Parallel connection, Series resistor, Series resistor is Large resistance)