On July 10, 1856, in a small village in Smilijan (now modern-day Croatia), a child was born during a midnight lightning storm. The nurse reportedly called it a bad omen. His mother is said to have replied that he would be "a child of light." It’s a small but memorable part of the story, but in this case, the real truth is even more interesting than the legend. The boy was Nikola Tesla, and more than a century later, his name still shows up wherever magnetic fields are measured, from the force inside an MRI scanner to the quiet magnetic pulse of the Earth.
For those of us who spend our working lives measuring magnetism in billionths of a tesla, his birthday is a day worth pausing on. To understand why, you must start with the idea that made him famous.
Most people remember Tesla for alternating current (AC) and his rivalry with Thomas Edison, but at the heart of his work was something deeper: magnetism.
In 1882, Tesla discovered the principle of the rotating magnetic field, the idea that alternating currents could create a magnetic field that rotates without any moving parts driving it. According to the story, the breakthrough came to him while he was walking through a park in Budapest. In that moment, he sketched the basic design in the dirt with a stick.
It may sound abstract, but its impact is all around us.
A rotating magnetic field can pull metal into motion, which turns electricity into rotation. That idea became the foundation of the AC induction motor, which Tesla patented in 1888, and the same basic principle still powers everything from washing machines and refrigerator compressors to power tools and electric vehicles.
Combined with Tesla’s polyphase AC system, the rotating magnetic field became one of the most important uses of magnetism in the modern world.
Tesla didn’t discover magnetism, but he showed the world how to put it to work on an industrial scale. So, when the scientific community needed a name for one of magnetism’s most important measurements, his name was the obvious choice.
Here’s the part most people have never heard of.
For a long time, magnetic fields were commonly measured in gauss, an older unit named after the German mathematician Carl Friedrich Gauss. The gauss worked, but as science and engineering moved toward stronger magnetic fields and a cleaner worldwide system of measurement, it started to feel outdated.
That changed in 1960. At an international meeting in Paris, the modern metric system — known as the SI system was being put into place. During that meeting, the official unit for magnetic flux density was updated to the tesla, with the symbol T.
The idea to honor Tesla came from France Avčin, a Slovenian electrical engineer. It was a fitting choice. Tesla had built his career around finding practical ways to use magnetic fields, and now his name would be attached to one of magnetism’s most important measurements.
The unit itself was defined simply: one tesla equals one weber per square meter. It also equals exactly 10,000 gausses, which made the connection to the older system easy to understand.
So the next time you see “1.5 T” on the side of an MRI machine, that number is carrying Tesla’s name forward. It means the magnetic field is incredibly strong, roughly tens of thousands of times stronger than Earth’s magnetic field, and its strength is measured in honor of the man who helped make magnetism move.
Here’s the catch—one tesla is a lot of magnetism.
A hospital MRI runs around 1.5 to 3 teslas. The magnets in the Large Hadron Collider reach beyond 8. A refrigerator magnet is only a few thousandths of a tesla. And Earth’s magnetic field, the one that moves every compass needle and helps shield the planet from the sun, is only about 25,000 to 65,000 nanoteslas, depending on where you are.
That is why scientists often use the nanotesla, written nT, one billionth of a tesla. It is the scale used by satellites that map Earth’s magnetic field and monitor the magnetic storms that can threaten power grids and communication systems.
But the nanotesla is not just the language of satellites looking down from space. It is also the language of instruments listening to the ground beneath our feet, and that is where this story comes home for us.
In 1937, decades after Tesla came up with his rotating magnetic field, and years before his name became the official unit for measuring magnetism, a young German physicist named Dr. Friedrich Förster noticed something most engineers were trying to get rid of, and quite frankly didn’t truly understand.
He was studying the magnetic properties of metals when he realized the Earth’s own magnetic field was affecting the coils in his test equipment. To most people, that was a nuisance signal, or interference. To Förster, it was the thing worth paying attention to.
Trained at Göttingen under Nobel Prize-winning physicists James Franck and Max Born, Förster had the kind of eye that could see value where others saw noise. If the Earth’s magnetic field could disturb a measurement, then perhaps it could become the measurement itself. That idea led him to build sensors sensitive enough to read those faint changes on purpose.
The result was the fluxgate magnetometer, later known around the world as the Förster probe. In 1948, he founded the company that still proudly carries his name: Institut Dr. Foerster, known today simply as FOERSTER. From the beginning, the company was built on the determination of one physicist’s to pay attention to a signal everyone else was trying to filter out.
And that same idea is still at the heart of what we do here at FOERSTER.
Our FEREX fluxgate magnetometers listen for tiny disturbances in the Earth’s magnetic field caused by buried iron objects. Those disturbances are measured in nanoteslas, billionths of a tesla. Because the instrument can detect changes that small, an archaeologist can trace the outline of a lost settlement. A survey crew can clear a field of unexploded WWII ordnance. An engineer can locate a buried steel drum or find a flaw hidden inside a finished component. When the field shifts by only a few billionths of a tesla, the instrument feels it, and something hidden becomes something you can see.
That has been FOERSTER’s work for more than seventy-five years, making the hidden visible.
And this is where Tesla and Förster stop feeling like two names connected by coincidence. Tesla gave the world a way to move magnetic fields. Förster found a way to hear them. Their connection runs deeper than the “T” on the readout. It lives inside the probe itself.
Inside every fluxgate sensor is a small coil wrapped around a magnetic core. Alternating current keeps that core moving between magnetic states, back and forth, repeatedly. That constant reversal is what makes the sensor work, and it is built on the same kind of restless magnetic field Tesla helped bring into the modern world.
Förster’s breakthrough used one of Tesla’s central ideas: don’t let the magnetic field sit still. Keep it moving, keep it alternating, and it will reveal what is hidden around it.
Every time a FOERSTER instrument reads in nanoteslas, two ideas collide. Förster’s probe listening, and Tesla’s alternating field giving it a voice. Billionth by billionth, the measurement belongs to both men, even if the measurement unit only bears one name.
Nikola Tesla died alone in a New York hotel room on January 7, 1943. He was nearly broke, his greatest dreams still unfinished, and the fortune he once had long gone.
There is something heartbreaking about that. A man who helped light the modern world spent his final days with very little of it shining back on him.
He never knew that seventeen years later, a room full of scientists in Paris would place his name permanently into the language of physics. He never knew that across the ocean, a young physicist named Friedrich Förster would take the same restless alternating field Tesla had spent his life advancing, and build it into the heart of a new kind of instrument.
That simple idea, “never leave the field still, keep it moving,” would live on inside every Förster probe and every FOERSTER instrument that followed. Long after Tesla’s own pulse had stopped, and he had taken his final breath, his idea would still be listening beneath fields, factory floors, and finished steel.
But that is exactly what happened.
And so today, every time a geologist sweeps a field for hidden ore, every time a satellite measures the trembling of the magnetosphere, every time a doctor slides a patient into the glowing tunnel of an MRI, the reading comes back in his name.
For a man who died with so little, Nikola Tesla left behind something almost impossible to measure, a name that still moves through the invisible forces of the world.
Not bad for a child born in a lightning storm.