The Foerster–Hochschild Connection: Germany’s Gift to American Eddy Current Testing

When most people in the NDT world think of foundational figures, names like Maxwell, Faraday, and Tesla come to mind. But if you're in the know—especially in the field of eddy current testing—you also recognize Dr. Friedrich Förster as one of the most pivotal forces behind the science we use today. What many don’t realize is how Förster’s work didn’t just influence Europe—it was transplanted, nurtured, and further developed in the United States through the efforts of a lesser-known but equally significant figure: Richard Hochschild.

This is the story of how a German physicist and an American engineer bridged continents and ushered in the era of modern eddy current testing. And more importantly, why it still matters today.

Before There Was F90, There Was Fg

Today, many ET professionals rely on simplified “F90” formulas to choose their test frequency—a handy method that provides around 90° phase separation between near- and far-side flaws in tubing. But in the 1940s and 50s, this convenience didn’t exist. There was no “look-up chart.” Instead, Dr. Förster had to derive the math from scratch, and his solution came in the form of a formula for limit frequency (fg).

His formula—originally expressed in metric units with conductivity in mΩ·mm²—wasn’t just a math exercise. It allowed you to calculate a scaling factor (fg) specific to the geometry and conductivity of your test piece. Once you had fg, you could use an impedance graph indexed to f/fg ratios to select the best operating frequency for your test. And those graphs weren’t generic—they were tailored to product form: thick-wall tubes, thin-wall tubing, solid bars, etc.

In short, fg wasn’t a frequency you would actually test at—it was a reference point that allowed you to place your actual test frequency on a normalized scale.

The American Connection: Enter Hochschild

Now, here’s where the story gets even more interesting.

In the early 1950s, American industry was hungry for more precise NDT methods. The war had highlighted the importance of reliable component testing, and scientists were eager to learn what their European counterparts were developing. That’s when Richard Hochschild, working under the sponsorship of the U.S. Atomic Energy Commission, was sent on a six-month visit to Institut Dr. Förster in Reutlingen, West Germany.

While there, Hochschild essentially conducted a knowledge transfer mission—absorbing everything he could from Förster and his lab. This wasn’t just a “factory tour.” He immersed himself in impedance theory, scaling laws, instrumentation, and test applications. Upon returning to the U.S., Hochschild wrote a detailed technical report—essentially a brain-dump of what he had learned.

That report became one of the key catalysts for eddy current innovation in the U.S.

From Theory to Practice: The Rise of Simplified Formulas

Thanks to Förster and Hochschild, American NDT professionals now had access to the Similarity Law, impedance plane analysis, and quantitative signal interpretation. But not everyone wanted to—or could—use impedance plane nomograms for every inspection. Over time, American engineers and scientists simplified things.

One of the byproducts of this was the introduction of F90 as a practical anchor frequency for flaw detection. It gave a nice middle ground—not too much phase separation (which would confuse analysts), not too little (which would obscure flaws). It was intuitive, adaptable, and—thanks to computer modeling—easy to bake into automated systems.

Today, we rarely see the original fg formula or those f/fg impedance charts in day-to-day practice. But they weren’t discarded because they were wrong—they were the foundation. And that foundation still holds up.

Did You Know?

🔍 Fg and Fc are not the same thing. When original formulas like Förster’s are altered or expressed using modern units (like %IACS instead of mΩ·mm²), the result is often labeled Fc to differentiate it. This subtle change is almost never taught in certification courses—but it’s one of the reasons confusion persists around which “scaling frequency” you’re really using.

So Why Should You Care Today?

If you’re working with eddy current testing in any serious capacity—whether that’s training analysts, selecting test frequencies, or developing inspection procedures—knowing where our tools came from makes you better at using them.

You’ll also avoid some of the common pitfalls that come from relying on simplified tools without context. For example:

• Assuming F90 is “optimized” for every material (it isn’t).

• Using modern equations while misapplying older reference materials.

• Confusing fg with actual test frequency or misinterpreting impedance graphs.

Understanding Förster’s Similarity Law and Hochschild’s role in propagating it lets you connect the dots—not just between past and present, but between physics and practice.

Conclusion: Standing on the Shoulders of Giants

If we’ve made one thing clear, it’s this: Foerster didn’t have a manual. He wrote it.

And thanks to Hochschild, that manual crossed the Atlantic and became embedded in the DNA of modern American NDT.

The next time someone tosses out an F90 frequency like it’s just a number, remind them—it’s the tip of the iceberg. And it took two brilliant minds on two continents to build the part we now take for granted.

✅ Want to learn more about the deep history and practical science behind eddy current testing?Head over to eddycurrent.com—your one-stop shop for everything ET, from used instruments and training resources to historical archives and expert blogs.