By David Robinson
Updated Mar 24, 2022
Measuring the flow of electrons—known as electric current—is fundamental for diagnosing electrical systems. Current is expressed in amperes (amps), with one ampere defined as the flow of one coulomb of charge per second. For precision, that equates to approximately 6.24 × 10¹⁸ electrons every second.
In practice, the current must overcome the resistance of the conductor, so its value varies with the circuit. A reliable measurement provides a quick check of circuit integrity and performance. The procedure outlined below requires only a standard digital multimeter (DMM) or ammeter, and no prior electrical training.
Ensure the circuit is completely de‑energized. Set the DMM to voltage mode and confirm zero volts across the points of interest. Only proceed once you are certain that no power is present.
Open the circuit at the point where you wish to measure current. This can be done by disconnecting a wire or removing a terminal from a junction box. The ammeter must be inserted in series to accurately capture the flow.
Switch the DMM to the appropriate current mode: AC for mains‑powered systems or DC for battery‑driven circuits. If your meter offers an auto‑range feature, select it; otherwise, set the highest available current range to avoid overloading.
Attach the test leads to the two open ends of the broken wire. Polarity is irrelevant for current measurement, but secure, low‑resistance contacts are essential to prevent spurious readings.
Double‑check all settings, then restore power to the circuit. The current appears on the DMM display. Record the reading, power down the circuit, disconnect the meter, and re‑close the circuit.
Insert an ammeter in series—never in parallel—to obtain a true current reading.
Exceeding the meter’s current rating can generate excessive heat, potentially leading to fire or electric shock—even at low voltages. Loose probe contacts increase resistance, causing a drop in measured current and creating misleading results.
Follow these steps carefully to protect both yourself and your equipment.
