An ammeter is an instrument designed to measure the flow of electric current through a conductor. Current is the rate of charge flow and is commonly expressed in the unit ampere. To measure current directly, an ammeter must be inserted into the circuit so that the measured current passes through the instrument; the physical quantity being measured is often referred to as electric current.

Basic principles and characteristics

Good ammeters are built to have very low internal resistance so they do not significantly change the current they measure. Important performance characteristics include range, accuracy, resolution and burden (the voltage drop introduced by the meter). Many ammeters include a shunt resistor to extend the range, allowing large currents to be measured while the meter movement measures only a small portion of the current.

Common types

  • Analog moving-coil (D'Arsonval) meters: sensitive and used for DC measurements; they show current deflection on a scale.
  • Moving-iron meters: can measure AC and DC with robust construction.
  • Digital ammeters: use electronic converters to display numerical values and often offer higher accuracy and additional functions.
  • Clamp meters: measure current without breaking the circuit by sensing the magnetic field around a conductor, useful for high-current or live-wire measurements.

Most panel ammeters have two terminals and a selector switch for range selection. In circuits with high currents a low-resistance shunt is placed in parallel with the meter movement; the meter reads a scaled value corresponding to the total current.

Uses, safety and distinctions

Ammeter uses include laboratory experiments, electrical troubleshooting, automotive diagnostics and monitoring in industrial equipment. Because an ammeter is connected in series, installing it incorrectly (for example across a voltage source) can create a short circuit and damage the instrument or circuit. Always select the appropriate range and use fused inputs when available. By contrast, a voltmeter measures potential difference and is connected in parallel and is designed to have high internal resistance so it does not draw significant current.

Historically, early ammeters evolved from galvanometers developed in the 19th century; improvements in electronics and sensing techniques have produced the wide variety of instruments used today. When choosing an ammeter, consider expected current magnitude, AC or DC measurement, accuracy requirements and safety standards appropriate to the application.