Eddy Current Squiggly Lines - What the Heck Are They Anyway?

At its heart, eddy current testing is all about sending an electrical signal into a probe, creating magnetic fields, and watching how those fields interact with the material you’re testing.

When a probe scans over a part:

• The alternating magnetic field induces eddy currents in the material.

• Those currents change when they hit things like cracks, pits, corrosion, or weld defects.

• The instrument detects changes in amplitude (signal strength) and phase (timing shift) in the electrical signal.

• These changes become visual patterns on your display screen — that’s the famous impedance plane display or “Lissajous figure.”

Where Did the Display Come From?

The idea of using “squiggly patterns” to represent electrical signals goes back centuries:

• Nathaniel Bowditch (1800s): Discovered the patterns that became the foundation for oscilloscopes.

• Jules Lissajous (1850s): Used vibrating mirrors to create the first Lissajous figures — those graceful loops you see when plotting two sine waves against each other.

• Charles Steinmetz (1890s): Brought in the idea of the phasor — a spinning arrow representing amplitude and phase.

Fast forward to today, and modern ECT instruments combine these concepts into the impedance plane display, letting you visualize material conditions in real time.

How Does the Instrument Turn Physics Into Pictures?

Here’s the basic process inside your eddy current tester:

1. Excitation: The instrument sends a carrier signal (sine wave) to the probe.

2. Interaction: The probe’s magnetic field interacts with the test material, and any flaw modulates the signal.

3. Modulation: Flaws change the signal’s amplitude and phase, “bending” the wave.

4. Noise Filtering: The instrument amplifies the useful signal and filters out junk like:

   • Tube support signals

   • Deposits

   • Cable bumps

   • Electrical noise (even from nearby cranes!)

5. Demodulation: The device mathematically extracts the meaningful data from the carrier wave.

6. Display: The clean signal is plotted as a dynamic pattern on your screen.

   
   

What Do the Patterns Mean?

On the screen, you’re seeing a moving dot (or “vector tip”) tracing a path in real time:

• For a differential bobbin coil passing over a through-wall hole, you often see a figure-8 pattern — one loop for each coil, 180° out of phase.

• Dents, cracks, erosion, and deposits each create distinctive shapes, shifts, or rotations in the display.

• A skilled analyst reads those patterns like a fingerprint — interpreting the phase angle, amplitude, and motion to characterize flaws.

Why Are the Eddy Current Signals So Math-Heavy?

Eddy current signals are fundamentally sinusoidal (they follow wave equations), so the math behind them comes from:

• Trigonometry (sine, cosine)

• Complex numbers (phasors)

• Circular motion (unit circle)

While the math explains why the signals look the way they do, you don’t need to solve equations to read the display — the instrument already does the heavy lifting. Your job is to understand the patterns and their meaning.

Takeaways for the Field

• Don’t be intimidated by the math behind the screen.

• Focus on pattern recognition and understanding phase/amplitude shifts.

• Remember that what you’re seeing is the end result of:

  1. Electromagnetic interaction

  2. Signal modulation

  3. Noise filtering

  4. Mathematical processing

• Well-balanced probes, good electronic setup, and controlled noise are key to getting clean, interpretable signals.

Conclusion

Understanding the eddy current signal doesn’t have to be a mystery. By breaking it into its basic parts — physics, math, electronics, and pattern recognition — you can confidently interpret what’s happening on your display. Next time you see that figure-8 dance across the screen, you’ll know you’re looking at the language of the material itself, decoded in real time.

Explore more on this topic at eddycurrent.com, your one-stop shop for everything eddy current testing.