What Is the Difference Between Resistance and Inductance, and Why Does It Matter to a Technician?
Resistance and inductance are both properties that affect how current behaves in a circuit, but they work in very different ways. Resistance opposes the flow of current itself, caused by the material the wire or component is made of. Inductance opposes a change in current flow, caused by the presence of an electromagnetic field. That distinction matters more than it sounds, one affects how much current flows, the other affects how quickly it can change.
Resistance
Resistance is measured in ohms (Ω) and stays constant for a given material at a given temperature. It determines how much current flows through a circuit at a given voltage, per Ohm’s Law. Unusually high resistance, often from a loose connection, corrosion, or a damaged wire, concentrates heat at that point in the circuit, which is why a bad connection can get hot even when the rest of the wiring is fine.
Inductance
Inductance is measured in Henries (H), named after Joseph Henry, one of the two scientists credited with discovering it (see “The Discovery of Inductance” below). Inductors resist changes in current, which makes them useful for smoothing out voltage or current fluctuations, but it also introduces a delay in how a circuit responds, which can affect timing in circuits that depend on it.
Why This Matters on the Job
These two properties call for different diagnostic approaches. If a circuit isn’t functioning properly, a technician checks resistance first, with an ohmmeter, to rule out a bad connection, damaged wire, or failed component. A winding with resistance far outside spec is a strong sign of a failing motor.
Inductance shows up differently. If a motor is slow to start, hums without turning, or trips a breaker on startup, the issue may be in the inductive side of the circuit, a bad start or run capacitor, for example, which exists specifically to help the motor overcome its own inductive lag at startup. Testing resistance alone won’t catch that, since a capacitor or winding issue tied to inductance can look electrically fine on an ohmmeter while still causing a real performance problem.
The Discovery of Inductance
Inductance was uncovered independently by two scientists working an ocean apart in the early 1830s. Joseph Henry, an American physicist, observed the effect in 1830 but didn’t publish his findings right away. Michael Faraday, working in England, demonstrated electromagnetic induction and published his results in 1831, earning him credit as the first to publish. Henry continued his own research and published his findings on self-induction in 1832, work significant enough that the standard unit of inductance is named after him. Both are considered foundational figures in electromagnetism, and their work laid the groundwork for the motors, transformers, and inductive loads technicians work with every day.
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About the Author, James R. Leichter
James is a successful entrepreneur and master mechanic widely referred to as Mr. HVAC. His accomplishments include being the CEO and founder of Aptora, the President of RA Tax and Accounting, the founder of MrHVAC.com, and a partner with Pro-American Investments. He is also the creator of Flat Rate Plus®, which is hosted on this flatratesoftware.com website.
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