Ammeter and Voltmeter — Core Principles

Core Principles

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.

Important Differences

vs Voltmeter

Aspect	This Topic	Voltmeter
Purpose	Measures electric current.	Measures potential difference (voltage).
Connection in Circuit	Always connected in series with the component.	Always connected in parallel across the points.
Internal Resistance (Ideal)	Ideally zero.	Ideally infinite.
Internal Resistance (Practical)	Very low (shunt in parallel with galvanometer).	Very high (resistance in series with galvanometer).
Effect on Circuit (Non-ideal)	Slightly decreases current by adding resistance.	Slightly decreases voltage by drawing current.
Range Extension	By decreasing shunt resistance.	By increasing series resistance.

Ammeters and voltmeters, though both derived from galvanometers, serve distinct purposes and have contrasting design principles. An ammeter measures current, requiring a low internal resistance and series connection to avoid impeding current flow. It uses a small shunt resistance in parallel with the galvanometer. A voltmeter measures potential difference, necessitating a high internal resistance and parallel connection to avoid drawing current from the circuit. It achieves this with a large series resistance. Understanding these fundamental differences is crucial for correct circuit analysis and avoiding measurement errors.