What Is Eddy Current Metal Detection? (Coil Physics)
Eddy-current metal detection uses an alternating-current coil to create a changing magnetic field. That field induces circulating currents in nearby conductive metal. The currents create an opposing field, which changes the coil’s impedance, or opposition to current. A detector measures that change; the result depends on the target, coil, frequency, and spacing.
For a practical test, setup is often simple: connect the instrument, position the coil, establish a baseline, then scan a known area. Reliable results take more care. A loose hand, nearby steel, or a change in coil spacing can alter the reading. I treat the detector as a measuring tool, not a metal-identification oracle.
How eddy-current metal detection works
Eddy-current detection is a noncontact method for sensing conductive materials. An alternating current in a coil makes a changing magnetic field. Near a conductor, that field induces circulating currents. The currents make their own magnetic field, which alters the electrical behavior of the coil.
Faraday’s law describes how a changing magnetic field induces voltage. Lenz’s law says the induced current acts in a way that opposes the change that caused it. In a detector, this interaction changes the coil’s effective resistance and inductance. The instrument measures the resulting impedance change:
ΔZ = ΔR + jωΔL
Here, ΔR is the change in resistance, ΔL is the change in inductance, ω is angular frequency, and j represents a 90-degree phase shift in the mathematical description. Instruments may display these changes as amplitude and phase, or as a point on an impedance plane.
This method works with conductive metals such as copper, aluminum, and steel. It does not require the target to be magnetic. However, magnetic permeability also affects the signal, so a ferromagnetic metal such as steel can produce a response that differs from copper or aluminum.
ISO 15549 describes general principles for eddy-current testing. ASTM E1004 covers electromagnetic measurement of electrical conductivity. These standards provide relevant testing frameworks; they do not make every instrument or setup interchangeable.
Planning the eddy-current measurement
Planning means defining what the test must detect and what could affect the reading. Before choosing a coil or frequency, note the target material, approximate size, expected position, and nearby metal. These details determine whether a signal can be separated from background effects.
Start with the job, not the detector’s highest sensitivity setting. Decide whether you need to find a conductive object, compare conductivity, or inspect a defined area for a change. Those are different tasks and may call for different probes and reference samples.
Record the expected target and test conditions:
- Material, if known, and whether it may be ferromagnetic
- Target dimensions, shape, and orientation
- Expected distance from the coil
- Nearby wiring, metal fasteners, fixtures, or other conductors
- Whether the coil can be held at a consistent angle and spacing
Frequency affects how current is distributed in the target. Coil geometry affects the field and the area being sampled. A small coil can focus on a smaller region, while a larger coil samples a broader area; the exact response depends on the instrument and probe design.
For a homogeneous conductor, the skin-depth estimate is:
δ = √[2/(ωμσ)] = 1/√(πfμσ)
Skin depth, δ, is the depth at which current density falls to about 1/e, or 37%, of its value at the surface. Here, f is frequency, μ is magnetic permeability, and σ is electrical conductivity. It describes current distribution, not a guaranteed detection depth.
| Reference material | Assumptions | At 1 kHz | At 10 kHz |
|---|---|---|---|
| Copper | 5.8 × 10⁷ S/m; relative permeability about 1; near 20°C | About 2.1 mm | About 0.66 mm |
| Aluminum | 3.5 × 10⁷ S/m; relative permeability about 1; near 20°C | About 2.7 mm | About 0.85 mm |
These are calculated reference values, not universal instrument specifications. Target size, shape, spacing, and coil design also influence detection. For steel, permeability can vary with alloy, heat treatment, stress, and magnetic state. Do not apply the nonmagnetic copper or aluminum estimates to steel as if they described its response.
Coil position, lift-off, and clearance
Lift-off is the distance between the coil and the target. A small change in lift-off can change the detector response, even when the target stays still. Repeatable coil spacing and orientation help separate a real target signal from a change caused by how the probe is held.
Before scanning, place the coil in its intended operating position and note the unloaded response. Check nearby metal, wiring, and fixtures for background coupling. If the baseline changes as you move the coil over the area, first identify whether the cause is a target, a nearby object, or inconsistent spacing.
Keep the coil’s face and angle consistent. Avoid tilting the probe at the edges of a surface, where the coil may see a different amount of material. If a fixed spacer or probe guide is compatible with the instrument, it can help keep lift-off repeatable. Do not assume that a signal change means metal is present until you have checked for movement and nearby objects.
Calibration and controlled scanning
Calibration compares the instrument’s response with a known reference under controlled conditions. Use a target whose material and size are relevant to the task. A baseline without a reference can show that the reading changed, but it cannot establish how a known target should respond in that setup.
Use a staged process:
- Set the test conditions. Choose a frequency and coil arrangement suited to the material and target size. Keep the instrument settings fixed during comparison.
- Record the baseline. Position the coil as it will be used, away from the reference target. Note the reading and check for nearby metal or other sources of background coupling.
- Check a representative target. Place a known reference in the expected orientation and at a controlled spacing. Record the instrument response.
- Scan consistently. Move the coil at a controlled speed and keep its orientation and lift-off steady. Mark or record where indications occur.
- Repeat the check. Return to the reference target after scanning. A changed reference reading may point to changed setup, settings, or instrument conditions.
If the instrument displays phase or impedance as well as amplitude, record those values where possible. They can help compare responses, but they do not by themselves prove a material’s identity. Keep notes on the probe, frequency, spacing, orientation, and reference used so another test can be compared fairly.
Signal verification and interpretation
A signal is a change in the coil’s measured response; interpretation is deciding what caused it. Signal strength alone cannot reliably identify a metal. Repeat an indication and compare it with a suitable reference before drawing a conclusion, especially when the target may be steel.
Useful checks include:
- Scan the same location again at the same speed and spacing.
- Rotate the coil or change its direction of travel, then compare the response.
- If the instrument allows it, repeat at another frequency while keeping other conditions consistent.
- Compare the indication with known reference targets and the recorded baseline.
- Check whether a nearby edge, fastener, wire, or fixture could explain the change.
An indication that moves or changes greatly when the coil angle changes may reflect geometry or lift-off rather than a different material. Edges can also affect the field and produce a response that differs from a broad, flat target. A second frequency can provide more information, but it does not remove the need for calibration.
For ferromagnetic targets, use a representative steel reference when possible. Steel’s permeability can vary with its composition and condition, and its response may be dominated by magnetic effects rather than conductivity alone. I would not label an unknown object from one amplitude reading.
Illustrative measurement scenarios
These scenarios show how to apply the method; they are not reports of field trials or guarantees of performance. In each case, the key is to hold test conditions steady and compare the indication with a relevant reference.
Scenario 1: Comparing a copper target at two frequencies. A tester uses the same coil and target position at 1 kHz and 10 kHz. The calculated copper skin-depth estimates are about 2.1 mm and 0.66 mm, respectively. The difference helps explain why the current distribution changes with frequency. It does not predict whether a particular instrument will detect the target at a given depth.
Scenario 2: Checking for a conductive object near a fixture. A reading changes near a metal bracket. The tester first records the response with the coil in its intended position, then checks the area with and without the bracket affecting the field, where that can be done safely and practically. If the indication follows the bracket, it may be background coupling, not a separate object.
Scenario 3: Evaluating an unknown steel piece. A response differs from a copper reference. That difference alone does not identify the sample. The tester repeats the scan, uses a representative ferromagnetic reference, and records coil angle, frequency, and spacing. Changes across these checks help show whether geometry or magnetic properties may be influencing the signal.
What eddy-current readings can and cannot tell you
An eddy-current reading can show that the coil’s electrical response changed under a given setup. With suitable equipment, reference targets, and controlled conditions, the method can support tasks such as conductivity measurement or comparative inspection. The result is tied to the material, target geometry, coil, frequency, and spacing used.
It cannot promise a fixed detection depth based only on skin depth. Nor can a single amplitude value identify a metal with certainty. Nearby conductors, target edges, lift-off, orientation, and magnetic properties can all affect the response.
For conductivity measurements, follow the instrument’s procedure and the relevant method, such as ASTM E1004. For broader eddy-current testing principles, consult ISO 15549. If the result affects a safety-critical decision, use a qualified testing procedure and competent personnel rather than relying on an informal scan.
Conclusion
Eddy-current metal detection works by measuring how induced currents in a conductive target alter a coil’s impedance. The physics is well described by Faraday’s and Lenz’s laws, but reliable interpretation depends on careful setup. Define the target, establish a baseline, use a representative reference, control lift-off, and verify indications before drawing conclusions.
Frequently asked questions
What does an eddy-current detector measure?
It measures a change in the coil’s impedance caused by its interaction with induced currents in a conductive target. Instruments may report amplitude, phase, or related values.
Does eddy-current detection work on nonmagnetic metals?
Yes. It can detect conductive nonmagnetic metals such as copper and aluminum. A target does not need to be magnetic for eddy currents to form.
Can it detect steel?
Yes, but steel’s magnetic permeability can strongly affect the response. Calibrate with a representative ferromagnetic reference, and do not identify steel from signal amplitude alone.
What is skin depth?
Skin depth is the characteristic depth at which induced current density falls to about 37% of its surface value. It is not a guaranteed detection depth.
Does higher frequency always improve detection?
No. Frequency changes current distribution and the measured response, but it does not guarantee greater detection range or better results. The right frequency depends on the target and test setup.
What is lift-off in eddy-current testing?
Lift-off is the distance between the coil and the target. Changes in that distance can alter the signal, so keep spacing consistent during calibration and scanning.
Can one reading identify an unknown metal?
No. Signal amplitude depends on several factors, including material, size, shape, orientation, frequency, and spacing. Compare repeat readings with suitable known references.
Why establish a baseline before scanning?
A baseline records the coil response before the target indication is assessed. It helps reveal background coupling from nearby metal, wiring, or fixtures.
Which standards are relevant?
ISO 15549 covers general principles of eddy-current testing. ASTM E1004 covers electromagnetic measurement of electrical conductivity. Use the applicable standard and instrument procedure for the specific task.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
