What is an Ohm?

Understanding Ohm

Ohm is the unit used to express electrical resistance. It describes how much a material or device opposes electric charge. The unit uses the symbol Ω, the Greek letter omega. Higher resistance limits current more when voltage stays the same.

Related glossary terms: Ohm's Law, Current, Voltage.

The name honors German physicist Georg Simon Ohm. Resistance depends on the material, length, cross-sectional area, and temperature. Copper wire has low resistance. Rubber and glass have very high resistance under normal conditions. A resistor adds a controlled amount of resistance to a circuit.

How Ohm Works, Is Measured, or Is Used?

Ohm's Law connects resistance, voltage, and current. The formula is R = V ÷ I. R represents resistance in ohms. V represents voltage in volts. I represents current in amperes. For example, a 120-volt load drawing 10 amperes has 12 ohms of resistance. This applies under those operating conditions.

An electrician measures resistance with an ohmmeter. They can also use a digital multimeter set to the Ω function. Power must be disconnected before a resistance test. Outside voltage can damage the meter or cause a false reading. The meter sends a small test current through the isolated path. It then calculates resistance from the measured voltage and current.

Test results need context. A continuous wire should show very low resistance. An open wire may display OL or an out-of-range indication. A component's expected resistance comes from its specifications. Temperature can change the result. Measuring across an energized circuit is a voltage test, not a resistance test.

Why Ohm Matters?

Resistance affects current flow, heat, voltage drop, and equipment performance. Excess resistance at a loose connection can create heat. It can also reduce voltage at a load. A heating element, motor winding, transformer winding, or control component may fail. This can happen when measured resistance differs greatly from its expected value.

Resistance readings help separate several fault types. Near-zero resistance between isolated points may indicate a short circuit. Infinite resistance across a conducting path may indicate an open circuit. A single reading doesn't prove the cause. Technicians compare test points, equipment data, and visible wiring conditions.

When Ohm Matters Most?

Ohm readings help during electrical troubleshooting, equipment installation, and inspections. They can identify damaged conductors and failed heating elements. They can also identify open motor windings, poor connections, and unwanted paths to ground. Testing can verify circuit continuity before power is restored.

Use the correct meter setting. Confirm that the circuit is de-energized before testing resistance. Disconnect sensitive electronic controls when equipment instructions require it. A meter's test current can affect some devices. Compare the result with the manufacturer's specification. Don't treat a general low or high number as a universal pass or fail.