Ammeter

An ammeter is an electrical measuring instrument used to determine the current flowing through a circuit. Current is measured in amperes (A), from which the instrument derives its name. To obtain a direct reading, the ammeter is connected in series with the component or section of the circuit under investigation. Because any instrument placed in series affects the system, ammeters are designed with very low internal resistance to minimise voltage drop and circuit disturbance. Instruments for smaller currents are known as milliammeters or microammeters, depending on their sensitivity.

Historical Development

The relationship between electric currents and magnetic fields was first demonstrated in 1820 when Hans Christian Ørsted observed that a wire carrying current deflected a nearby compass needle. Early current-measuring devices, such as the tangent galvanometer, depended on the Earth’s magnetic field for operation. These instruments had to be aligned with magnetic north and were thus impractical outside laboratory settings.
By the late nineteenth century, improved designs incorporated permanent magnets, allowing ammeters to be installed in any orientation. Additional turns of wire—known as multipliers—enhanced sensitivity. Experimental instruments such as Wheatstone’s “rheoscope” briefly appeared but did not remain in use. As electrical power systems expanded, panel-mounted ammeters became standard components in industrial and utility installations.

Types of Ammeters

Modern ammeters employ several different operating principles, each suited to particular measurement ranges and electrical conditions.
Moving-Coil (D’Arsonval) AmmetersThese instruments rely on the deflection of a coil placed within the magnetic field of a permanent magnet. Developed into its modern form by Edward Weston, the design incorporates spiral springs that provide a restoring force and ensure a linear relationship between current and needle deflection. Moving-coil meters measure direct current only, since alternating current averages to zero over time. Some models include centre-zero mechanisms to display currents of either polarity.
Moving-Magnet AmmetersFunctioning similarly to moving-coil designs, these instruments instead move a permanent magnet while keeping the coil fixed. They can accommodate higher currents due to the use of heavier wire and may omit delicate hairspring mechanisms.
Electrodynamic AmmetersBy replacing permanent magnets with electromagnets, electrodynamic meters can measure both AC and DC, and can provide true RMS readings for alternating currents. This principle is also used in wattmeters and other power-monitoring instruments.
Moving-Iron AmmetersInvented in 1884 by Friedrich Drexler, these meters operate through the repulsion or attraction of soft iron components placed within a coil. They respond equally to DC and AC and are common in industrial settings. Although their natural response curve is nonlinear, careful shaping of the iron elements produces an approximately linear scale.
Hot-Wire AmmetersIn these instruments, current heats a thin wire, causing thermal expansion that moves the pointer. While relatively slow and less precise, they were once used to measure radio-frequency currents and provide true RMS readings.
Digital AmmetersDigital instruments measure current indirectly by passing current through a shunt resistor and converting the resulting voltage drop via an analogue-to-digital converter. They may provide simple average readings or true RMS values, depending on design. Digital ammeters form the basis for many multifunction devices such as digital multimeters and energy meters.
Integrating AmmetersSometimes called coulomb meters, these devices measure the total electric charge that has passed over time. When combined with voltage measurement, they can estimate energy consumption or monitor the charge state of batteries and capacitors.
PicoammetersHighly specialised instruments designed to measure extremely small currents—down to the picoampere range—require guarded circuitry, triaxial connections, and sophisticated shielding to minimise leakage and electrical noise. They often use logarithmic measurement techniques to avoid range switching.

Application and Circuit Integration

Because current must flow through an ammeter for measurement, it is always placed in series with the circuit element of interest. The low internal resistance ensures that the measurement produces only minimal disturbance.
Key considerations include:

• For small currents, the ammeter itself carries the full circuit current.
• For large DC currents, a shunt resistor is used to divert most of the current, sending only a small, measurable fraction through the instrument.
• For AC circuits, a current transformer provides electrical isolation and scales large currents to safe levels (e.g., 1 A or 5 A).
• For high-voltage systems, the ammeter is often located in the return (low potential) side of the circuit to improve safety.

Ammeters must never be connected across a voltage source because their low resistance would cause dangerously high current to flow, potentially damaging the instrument and the circuit.

Significance

Ammeters are essential tools in electrical engineering, electronics, power distribution, and laboratory science. From simple moving-coil mechanisms to advanced digital and picoampere-sensitive designs, they provide accurate and reliable measurement of electrical current across a vast range of magnitudes and applications. Their evolution reflects the broader history of electrical technology, from early experiments with magnetism to modern electronic instrumentation.