Magnetic vs Eddy Current: Principles & Differences

Checkline Europe's Technical Support 10 Jul 2026

Two electromagnetic methods dominate coating thickness measurement on metal, and they are not interchangeable: magnetic induction reads coatings over a ferromagnetic base such as steel, while eddy current reads them over conductive non-ferrous metals such as aluminium, copper or brass. Which method is valid is decided by the substrate, not by the gauge — so understanding what each one actually senses is the difference between a stable reading and one that drifts. This page sets out the two principles, where each applies and how to choose between them.

1. What Magnetic Induction Measures

Magnetic induction measurement depends on the interaction between the probe field and a ferromagnetic substrate such as steel. A non-magnetic coating acts as the spacing layer between the probe and the steel base. As that spacing increases, the magnetic coupling changes in a predictable way, and the instrument converts that change into coating thickness.

The key dependency is magnetic permeability. That is why the method is intended for coatings on ferromagnetic substrates and why substrate variation, alloy differences and magnetic condition can affect the result.

2. What Eddy Current Measures

Eddy current measurement works on electrically conductive, non-ferromagnetic substrates such as aluminium, copper or brass. The probe creates an alternating electromagnetic field that induces circulating currents in the metal beneath the coating. The strength and phase of that induced response change with the probe-to-substrate distance, which corresponds to coating thickness.

The key dependency here is electrical conductivity rather than magnetic permeability. That is why eddy current is suited to non-ferrous conductive bases and why it should not be treated as a generic fallback for every metal surface.

3. Why the Difference Matters in Practice

At the user level, the most important practical rule is simple: magnetic induction for ferrous substrates, eddy current for conductive non-ferrous substrates. Instruments that auto-switch between methods can be useful, but they do not remove the need to understand what is being sensed. Mixed materials, duplex systems and thin metallic coatings can still complicate interpretation.

Method choice also affects calibration strategy, sensitivity to substrate variation and the kinds of error that are likely to appear. Magnetic induction is often more affected by changing permeability or steel condition. Eddy current is more sensitive to conductivity, alloy variation and some surface-related effects.

4. Where Method Selection Goes Wrong

Assuming all metallic substrates can be measured the same way: they cannot. Substrate family determines the valid electromagnetic principle.
Ignoring alloy variation: the nominal substrate type may be correct while its actual magnetic or conductive behaviour still shifts the result.
Misreading duplex systems: layered metallic systems can respond in ways that are not obvious unless the operator understands which layer the method is really sensing.
Treating auto-sensing as infallible: automatic mode switching helps, but it does not replace correct setup and method awareness.

5. How to Choose Between the Two

The first question is always the substrate: steel or other ferromagnetic base suggests magnetic induction; conductive non-ferrous base suggests eddy current. The second question is whether the coating and part geometry fit the method well enough for stable readings. Roughness, curvature, very thin metallic layers and complex assemblies may all require extra caution.

If the job includes multiple substrate families, users should treat that as a calibration and verification challenge, not as an afterthought. Correct principle selection is part of the measurement process, not something separate from it.

7. Next Step

Once the method difference is understood, the next step is usually narrowing the gauge choice either by measurement principle or by the substrate family involved.

Magnetic vs Eddy Current Coating Thickness Gauge if the main buying decision is which electromagnetic method best fits the inspection task.
Select a Coating Thickness Gauge by Substrate if the substrate and coating structure are the clearest way to narrow the choice.

8. Frequently Asked Questions

1. Can magnetic induction be used on stainless steel?

Only if the specific stainless steel behaves sufficiently as a ferromagnetic substrate. Many stainless grades do not, so the method may be unsuitable or unstable.

2. If a dual FN gauge auto-detects the substrate, do I still need to know which method it used?

Yes. Auto-sensing handles routine work well, but on duplex systems, very thin metallic coatings or unusual alloys it can pick the wrong principle. Knowing which method should apply lets you challenge a reading that looks wrong rather than trust it blindly.

3. Are readings taken on aluminium directly comparable with readings on steel?

No. They come from different physical principles with different sensitivities, so absolute values are not comparable across substrate families. Judge each reading against its own substrate-matched reference and acceptance limit.

4. Do I need to recalibrate when moving between ferrous and non-ferrous parts?

Verify on each substrate family. A reference set that suits steel does not represent aluminium, so even a dual FN gauge should be checked on each base material before its readings are trusted.

5. When should another method be considered entirely?

When the substrate is not suitable for either principle, the layer system is too complex, or the application requires layer-resolved measurement beyond what electromagnetic methods can provide.

9. Glossary

Magnetic Induction	Coating-thickness method that senses distance to a ferromagnetic substrate by monitoring magnetic coupling.
Eddy Current	Coating-thickness method that senses distance to a conductive non-ferrous substrate through induced circulating currents.
Ferromagnetic Substrate	Base material such as steel or iron that responds strongly to magnetic fields.
Conductive Non-Ferrous Substrate	Metallic base such as aluminium, copper or brass that conducts electricity but is not ferromagnetic in the same way as steel.
Magnetic Permeability	Property describing how readily a material supports magnetic field formation.
Electrical Conductivity	Property describing how readily a material carries electrical current.
Auto-Sensing	Instrument function that attempts to identify the substrate type and select the appropriate method automatically.
Duplex System	Layered structure in which multiple metallic or coating layers can complicate which interface the measurement is responding to.