ATP: Hall Effect and Lorentz Force

Care to take a peek under the hood of reality? Hall effect and Lorentz force are big news in the land of the tiny.

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Today we’re going to get small and talk about some of the freaky stuff that happens between current and magnetism. Specifically, we’re going to explore what Hall effect is and, by extension, the Lorentz force. If you’re an engineer, you’ll know that the Lorentz force is a perennial favorite topic. And if you’re not an engineer (yet), this is one of those things you just gotta know.

We’ll look at the actual experimental write-up by Edwin Hall from 1879 (and even try to replicate his results), and talk about CGS units and the right-hand rule. Then we’ll talk about available sensors and applications, along with sensitivity ranges and prices that you may encounter. Last, I’m going to admonish you to go read about this stuff.

Here come some links!

Hall effect - Because that’s our topic.

Lorentz force - This is what’s really going on.

On a New Action of the Magnet on Electric Currents - Direct from the mouth of the guy that did the work.

The Abampere - How would you quantify current in the mid-1800’s?

CGS Units - In the dark days before SI units.

Thomson Galvanometer - Because it’s just too cool.

Comments 14 comments

  • did it in college 45 years ago and worked.

    to make it work i used;

    • a really good common ground, Stick with common point ground where called for
    • 2 941 OP amps in aliasing configuration with very high impedance input
    • used batteries ONLY not a switching supply, noise is far too high
    • electroplated copper on a 2x3 inch plastic sheet to be thin enough, Tin foil was far too thick.
    • several different magnets.
    • IMPORTANT, calibrate your scope probes
    • keep your connections short as possible
    • my Oscope was an old TechTronics made around 1962 and all vacuum tube for very high impedance.

    A bar magnet worked best due to shape of the field. Field shape and alignment will be critical.

    To get around possible induction I made an adjustable wooden holder (glue only) so the magnet would not be moving and carefully taped down all wires out to 6 inches.

    Once done the OP amp produced a steady offset proportional to the current and the distance of the magnet. Proving changing distance worked. The math from these measure followed Lorentz law as expected.

    As a secondary proof of the field to current ratio; I made a saw tooth oscillator for the current source and you could see the ramp on the scope from the OP amp output. Showing the effect of changing current. My prof really liked that idea.

  • I made a go at it. First I tried aluminum foil, but I couldn’t solder to it, so I connected it up with alligator clips. I rounded up my most sensitive ammeter (5µA full scale), hooked it one way, and hooked a bench supply the other (perpendicular) way and ran 1 ampere through it (checking to see if some asymmetry in my setup pushed some current through the meter, but it didn’t) and waved a powerful magnet at it. The powerful magnet yanked all the clips up! I was afraid I’d ram an amp through my sensitive meter and destroy it! Therefore I made another unit of copper foil, soldered wires to it, and set everything up again. I tried waving the magnet, and saw a little deflection! Then I switched off the bench supply and saw the same deflection. It turns out the magnet was affecting the nearby meter. I spaced everything well apart and tried again. No deflection. I realized the light galvanometer Hall used was effectively magnifying the meter deflection, so I tried observing my meter under a microscope, and saw a small deflection! I turned off the bench supply and saw the same deflection: I was observing Henry’s law, I was inducing current in a conductor with a moving magnetic field, not deflecting electron flow with Lorentz force. Cool, but not what I’m looking for. I figure the Hall effect is a pretty low-impedance source (it’s a piece of copper foil), so perhaps I’ll try boosting it with a transimpedance amplifier next. I may also try some different geometries of copper foil (these tests were done with a square piece, with the wires connected to the corners).

    • Pete-O / last week / 1

      Dude that is so AWESOME! I mean, you’re not there yet, but still really cool! I didn’t even get to see induction. But yeah, the field ultimately has to be stationary to prove anything here. And I still think the foil has to be super thin - rather, you’d get better results with progressively thinner pieces of foil.

      I really want to build a Thomson Galvanometer. Can you imagine a 30-foot needle? You could build your own femto-ammeter.

      • As for the Thomson galvanometer, we do have the advantage these days of small, lightweight lasers, so you don’t have to wait for a sunny day and arrange everything just right to get a beam. That would be a wonderful combination of old and new technology!

      • I’m still playing with it. I built a transimpedance amplifier out of an LF356 and hooked it to a meter to yield a much more sensitive meter. I’m powering it from a 9V battery for both low noise and so I can float it at whatever voltage is required. Pushing a microamp into it gives good meter deflection. I’m playing with a mylar balloon to get a thin enough foil. I’ve verified that it’s conductive, but I’m having trouble making good contact with it, as this particular balloon is decorated and the ink isn’t conductive. I’ll see if I can remove it with solvents, but I may have to go buy a plain silver one to play with. I’m also thinking I could buy some fake gold leaf from eBay and try that. I figure I can make contacts with either conductive paint, or try lightly clamping copper foil for leads using something like plastic paper clips – I learned my lesson about using anything ferrous anywhere near the big magnet!

        • Mylar… that’s an interesting choice. Primarily for the thickness? How big is the piece you’re using? What kind of resistance is it offering and how much current are you pushing through it?

          • Oops, I missed a word: aluminized mylar: a very thin layer of aluminum on a mylar substrate. I tried cleaning off the decorations with flux remover, which worked fairly well, then I cut out the sensor shape, which was an amazingly fiddly process, with the very thin material that wanted to curl up. Then I removed the label from a Sparkfun box with a heat gun, which left a sticky surface to which I adhered the aluminized mylar. Then I used 4-40 nylon screws to attach strips of copper foil to the corners and hooked it all up. No dice. While I had measured some pieces of it and got very low resistance (fractions of an ohm), the piece I had made appears to have discontinuities in it, probably scraped the aluminum while scrubbing/cutting/attaching it. It really is thin aluminum, I can see light through it. I think I’ll go shopping for a non-printed aluminized mylar balloon or one of those “space blankets”, which might not be as ready to curl up.

            • Oh… OK. Yeah, that sounds like a really good candidate for a hall plate. I’d be very curious to hear the results.

  • Now that I’ve managed to get through the internet delays and watched the whole video (I’m by no means a “YouTube guru”), I’ve got three things to offer:

    First, it’s a pretty fair description of the Hall effect. It’s always frustrating when a demo goes awry – I can sure sympathize with that!

    Second, a nit to pick about “vectors” – in two or three dimensions one representation of a vector is a direction and magnitude – another is simply an X-Y-Z value (for 2D, X-Y) of “distance to the tip” from the origin. This latter has a quite useful extension in mathematics when we get into higher dimensions, indeed, we often talk about “test vectors” for testing the functionality of ICs that can have as many as 1024 “dimentions”. In mathematics, any collection of values that can vary independantly can be thought of as a “vector”, and can be subjected to things like cross products and dot products. (I would have refrained from this if you’d just said “in 3 dimentions, a vector is just a magnitude and a direction”.) (Sorry, my “math nerd” is showing…)

    My third minor comment is that if you really wanted to use gold foil, you could probably find it at the local arts & crafts store (such as Michael’s or JoAnne’s). Even with gold prices at a bit over $1200/troy ounce, it shouldn’t be too exhorbitant. (I haven’t noticed it in several years, but I recall it being around $30 for several “leafs”. It’s incredibly thin, so just like the gold in our ICs, a troy ounce goes a long way.) I should also mention my “standard advice”: before you buy at the arts & crafts store, check the newspaper and/or junk mail for coupons! They often have a coupon for the likes of 40% off any one item… but carefully read the fine print.

    • Pete-O / last week / 1

      LOL You went easier on me than I thought you would!

      1) Thanks! The demo… meh. I mostly wanted to see… anything. But it clearly takes more effort than tin foil a scope and a magnet. But how cool would it have been if it worked?

      2) I was really trying to steer clear of anything that looked like vector math and keep it as conceptual as possible. But you just had to bring us there… ;) As far as being a math nerd, ain’t a thing. I’ve been eyeing my calc books for a little of the ol' self-inflicted pain. I really do believe that math can be fun (certainly algebra - that’s just a game!).

      3) I dunno… that almost would have been too much effort. Ironically, I would have totally gone the distance to hammer the tin foil thinner, given a little more time. So maybe too much money.

      • 3) I dunno… that almost would have been too much effort. Ironically, I would have totally gone the distance to hammer the tin foil thinner, given a little more time. So maybe too much money.

        Not much effort at all. I know there are artsy people at SparkFun. Tell one of them to pick up that pack of gold leaf that they want, you’ll pay them back for the cost on one sheet of the gold leaf. Very little effort on your part. ;-)

  • I’m not sure this is the problem, but it looks like your foil is wider than your magnet, so the electrons get pushed a little to the side, then leave the field and go back to the direction the electric field is pulling them. Try a narrower foil (or a wider magnet).

    • Pete-O / last week / 2

      I don’t know that there was any one problem (I think maybe it was entirely built of problems). But the dimensions of the foil vs the size of the magnet is a good place to start.

      • “Entirely built of problems”, now that is a beautiful turn of phrase!

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