By S. Hussain Ather, Updated Mar 24, 2022

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Introduction

The ammeter is the standard instrument for measuring electrical current, the SI unit of which is the ampere. Understanding its operation is essential for accurate circuit analysis.

How an Ammeter Works

Ammeters determine current by passing the flow through a coil with very low resistance and inductive reactance, creating minimal impedance. This design ensures that the meter does not disturb the circuit it is measuring.

In a moving‑coil ammeter, the current generates a magnetic field that opposes the fixed field produced by permanent magnets. The resulting torque moves a calibrated armature over a scale, displaying the current in amperes.

Connecting an Ammeter

To measure current, an ammeter must be connected in series with the load. Its low internal resistance means it draws negligible power, preserving the circuit’s operation. Connecting it in parallel would short‑circuit the circuit, redirecting most current through the meter.

Ammeter vs. Galvanometer

A galvanometer is a sensitive detector that can sense tiny currents and their direction. It relies on a magnetic field to convert current into a mechanical displacement. While highly precise, a galvanometer’s range is limited, and it typically measures only direct current (DC).

Ammeters extend the galvanometer’s principle. DC ammeters employ the moving‑coil method, whereas AC ammeters measure the deflection caused by the time‑varying magnetic field acting on a piece of iron. Consequently, ammeters can handle both AC and DC.

Shunt Resistor – Extending Range

To adapt a galvanometer to larger currents, it is placed in parallel with a small shunt resistor. The shunt diverts most of the current away from the galvanometer, allowing only a fraction to pass through it. The shunt protects the galvanometer from damage and enables accurate readings of high currents.

Let G be the galvanometer resistance and I_g the maximum current for full‑scale deflection. For a total current I, the shunt resistance S is calculated as:

S = (Ig · G) / (I – Ig)

The effective resistance of the combined ammeter is:

Reff = (G · S) / (G + S)

Key Takeaways

• An ammeter’s low impedance minimizes its impact on the measured circuit.
• It must be wired in series to obtain accurate current readings.
• Galvanometers form the basis of ammeters but are limited to small currents.
• Shunt resistors extend a galvanometer’s range, allowing measurement of large currents.