Galvanometer to Ammeter Conversion Explained

A moving coil galvanometer (MCG) is a sensitive instrument primarily used to detect and indicate small electric currents. Its core component is a coil suspended in a magnetic field, which deflects when current flows through it. However, galvanometers are typically designed for low currents and have a relatively high internal resistance (often denoted as $R_g$).

Purpose of an Ammeter

An ammeter, on the other hand, is designed to measure the electric current flowing through a circuit. Ammeters are connected in series with the component through which the current is to be measured. To accurately measure the current without significantly altering the circuit's behavior, an ideal ammeter should have very low resistance.

Modifying a Galvanometer for Ammeter Functionality

To convert a sensitive moving coil galvanometer into a practical DC ammeter capable of measuring larger currents, a modification is necessary. The galvanometer coil itself cannot withstand large currents without damage. Therefore, a mechanism is needed to divert most of the current around the galvanometer coil, allowing only a small, known fraction to pass through it.

This is achieved by connecting a resistor of very low resistance, known as a shunt resistor ($R_s$), in parallel with the galvanometer coil. The galvanometer and the shunt resistor together form the ammeter.

Analyzing the Options

• Low resistance across the meter: This refers to connecting a low-resistance component (the shunt resistor, $R_s$) in parallel with the galvanometer (across its terminals). This configuration diverts the majority of the current through the low-resistance path (shunt), leaving only a small current to pass through the galvanometer coil. This is the correct method for converting a galvanometer into an ammeter. The total current $I$ is divided, with the galvanometer current $I_g$ given by $I_g = I \times \frac{R_s}{R_s + R_g}$. To keep $I_g$ small, $R_s$ must be much smaller than $R_g$.
• High resistance in series with the meter: Connecting a high resistance in series with the galvanometer increases the total resistance of the circuit element. This setup is used to convert a galvanometer into a voltmeter, which measures voltage drops across components. Voltmeters are connected in parallel, and a high series resistance limits the current while allowing the voltage drop across the high resistance (proportional to the circuit voltage) to be measured.
• Pure inductance across the meter: While inductance relates to magnetic fields, it's typically relevant in AC circuits. For DC measurements, adding inductance in parallel doesn't serve the purpose of creating an ammeter. Furthermore, inductance itself has resistance, but it's not the standard or correct component for this conversion.
• Capacitor in series with the meter: A capacitor blocks the flow of direct current (DC). Connecting a capacitor in series with the galvanometer would prevent any DC current from passing through the coil, making it impossible to measure DC current.

Conclusion Summary

The correct way to convert a moving coil galvanometer into a DC ammeter is by connecting a low resistance (shunt resistor) in parallel across the galvanometer terminals. This allows the galvanometer to measure currents larger than its original sensitivity range.