What is an Ohm?

Understanding Ohm

Ohm is the named unit for electrical resistance in the International System of Units. Resistance describes how much a material or component opposes the movement of electric charge. The unit's symbol is the Greek letter omega, Ω. A larger resistance value means a circuit restricts current more strongly. This happens when voltage stays the same.

Related glossary terms: Amperage, Voltage Drop, American Wire Gauge.

The unit honors Georg Simon Ohmwhose work linked voltage, current, and resistance. A conductor with one ohm of resistance has a one-volt drop. This happens when one ampere flows through it. Real circuits contain resistance.copper wire, heating elements, motors, lamps, switches, and electronic parts. Even connections and damaged insulation can affect a resistance reading.

How Ohm Works, Is Measured, or Is Used?

Ohm’s law gives the basic relationship between voltage, current, and resistance: V = I × R. You can rearrange the formula to find resistance with R = V ÷ I. For example, a 12-volt circuit carrying 2 amperes has 6 ohms of resistance. The formula describes many ordinary circuits. Some devices don't keep constant resistance as temperature or conditions change.

A digital multimeter measures resistance by sending a small test current through the circuit. It measures the voltage that results. Turn off the circuit before making an ohms test. Disconnect at least one side of the component when other paths could affect results. A continuous tone or near-zero reading may show a low-resistance path. OL or a very high value may show an open circuit. The meter's range affects the reading's meaning. The component's expected value also affects its meaning.

Why Ohm Matters?

Resistance affects current flow, heat, voltage drop, and device performance. A heating element creates resistance by design. It changes electrical energy into heat. A loose terminal can create unwanted resistance at one small point. This may produce heat under load. The rest of the conductor may still appear sound.

Resistance testing helps separate a sound component from a wiring problem. A reading far from the manufacturer's rating can show a failed motor winding. It can also show a broken heater, damaged cable, or poor connection. Resistance alone doesn't prove that a circuit is safe. A meter test can't replace a voltage test. It also can't replace a grounding and insulation check.

When Ohm Matters Most?

Continuity testing matters when checking a circuit, cable, fuse, or switch. The test checks for an unbroken path. A fuse should normally show very low resistance when removed and intact. A failed fuse usually shows an open circuit. A correct reading doesn't prove the fuse has the right current rating. It also doesn't prove the main fault is fixed.

Ohms also matter during motor testing, heating equipment repair, outlet troubleshooting, and ground path checks. Compare readings between similar windings or terminals when instructions provide expected values. Stop and check readings that change sharply when a wire moves. This pattern can show a loose connection or conductor damage.