Eddy currents arise from the interaction of conductive metals and magnetism. A brief history & explanation of the phenomenon is presented below along with some simple experiments for demonstrating the effect.
What Are Eddy Currents?
In 1820, Hans Christian Ørsted discovered that an electrical current moves a compass needle. That began a series of experiments that changed the world. Only ten years later, Europe had telegraphs and Michael Faraday had invented the first transformers and generators. Faraday’s Law 1 describes how a moving magnetic field causes an electrical current in a nearby conductor. The usual demonstration involves a magnet moving in and out of a coil of insulated wire, and observing the induced current in a circuit connected to the coil.
Magnaprobes are simple but sensitive analog instruments that respond to magnetic fields. The white version, the Mark I, has a jewel-mounted magnet that moves to fields as weak as 0.3 gauss. It’s even able to measure the declination of the Earth’s magnetic field. The black one, the Mark II, is a handy teaching tool for indicating the poles of magnets or outlining magnetic fields.
Lenz’s Law
The “Left Hand Rule” indicates the direction of the current in the coil. If one’s thumb points in the direction of motion of the magnet, the induced electric current flows in the direction of the fingers on the left hand. Of course, that current also produces an associated magnetic field. If the direction of that field caused the current in the coil to increase, it would cause a runaway condition that would violate the Principle of Conservation of Energy. In 1834, Emil Lenz 2 addressed the issue by showing that the magnetic field produced by that induced current must oppose the original moving magnetic field.
Magnetic Induction
Fortunately, the induction effect occurs in any conductor exposed to a moving magnetic field, and the ends of a cylindrical magnet are where the magnetic force occurs at right angles to the flat faces. So the face of such a magnet moving through a metal pipe induces a current in the ring-shaped area close to that face edge. THAT current radiates a magnetic field that opposes the motion of the magnet. The magnet’s longer fall & exit than expected surprises most students. It is counter-intuitive, usually quite fascinating and a great way to generate questions about “what must be happening?”.
A Mark I Magnaprobe, detected eddy currents as far away as 39cm.
Inverse Square Law
Magnetic force obeys the Inverse Square Law 3. The closer the magnet is to the side of the tube, the more dramatic the effect. Indigo Instruments Metric Neodymium Disc/Rod Magnets come in sizes best suited for use with standard imperial measure pipes.
Our metric N42 neodymium rare earth rod magnets are great for producing eddy currents. The 10 & 12.5mm wide versions are a good fit for 1/2″ diameter copper or aluminum tubes.
Eddy Current Experiment
The eddy current effect is put to good use in amusement & adventure parks. Eddy current brakes can be found on roller coaster rides & drop towers & ziplines.
Eddy currents induced by a neodymium rod magnet falling inside a copper tube, in turn, impede the fall of the magnet. The video shows a 15x25mm N42 grade neodymium rare earth rod magnet dropped down a 0.5m (20″) long x 19mm (3/4″) diameter copper tube. The experiment was repeated with a 0.5m (20″) long x 12.7mm (1/2″) diameter copper tube and a variety of 10 & 12.5mm diameter N42 grade neodymium rare earth rod magnets.
Experimental Results with 7 Sizes of Nd Rod Magnets
The first table shows the average drop time for the different magnets. Note how much longer the wider 12.5mm rods take to fall. This indicates greater attraction is directly proportional to tube wall proximity. Another way to interpret the results would be to calculate how far the magnet would have traveled in a free fall. Perhaps a more dramatic illustration of the braking effect.
| Magnet Size (mm) | Trial 1 (seconds) | Trial 2 (seconds) | Trial 3 (seconds) | Trial 4 (seconds) | Trial 5 (seconds) | Average (seconds) |
|---|---|---|---|---|---|---|
| 10x10mm | 4.71 | 4.56 | 4.86 | 5.01 | 4.91 | 4.91 |
| 10x12.5mm | 3.86 | 3.97 | 3.78 | 4.01 | 3.92 | 3.91 |
| 10x15mm | 4.06 | 4.36 | 4.21 | 4.33 | 4.28 | 4.25 |
| 10x25mm | 3.03 | 2.81 | 2.84 | 2.86 | 2.91 | 2.89 |
| 12.5x10mm | 6.45 | 6.41 | 6.36 | 6.48 | 6.39 | 6.42 |
| 12.5x12.5mm | 6.88 | 6.76 | 6.91 | 6.69 | 6.81 | 6.81 |
| 12.5x25mm | 4.18 | 4.16 | 4.09 | 4.11 | 4.08 | 4.12 |
The second table shows how sensitive the Mark I Magnaprobe is to the eddy current-induced magnetic field. This device is actually more sensitive than the best digital magnetic field detecting devices. Scientists at CERN once used them to find a magnetic leak so small no other device could find it. This was critical since the leak was still large enough to deflect the particle beam. Other demanding leak detection scenarios have involved MRI/NMR devices, magnetoencephalography & more.
| Distance from Magnaprobe (mm) | 1mm | 10mm | 50mm | 100mm | 200mm | 390mm | 391mm |
|---|---|---|---|---|---|---|---|
| Does the Magnaprobe Detect the Magnet in the Pipe (Yes or No) | Yes | Yes | Yes | Yes | Yes | Yes | No |
References
- Faraday’s Law from the University of Colorado
- Lenz’s Law from Florida State University
- Inverse Square Law from Wikipedia
- Indigo Instruments Nd magnets are certified N42. One customer’s review: “I purchased N42 Neodymium magnets, they are the real deal. I had previously bought magnets on Amazon that were advertised as N52; they were just another cheap Chinese fake. These folks sell authentic, high-quality stuff and get it to you fast….”
