Curiosity strikes. I have no immediate need for this, but I could see it coming in handy on a gizmo of some sort.
Can you get rubber magnets (like a fridge magnet) made with custom magnetization patterns? I know that the typical refrigerator magnet is made with stripes so the field close to the thing goes N-S-N-S -- I'm wondering if there's anyone out there that'll make the magnetic regions to my print.
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My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
No idea, But the frig magnet 'thingie' is cool. I thnik they are really Halbach arrays.
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Which not only has them going N/S but sideways in the middle. (You've got to draw in your own field lines to get an idea of hwo it works.) At the FEL where I worked for a while the wiggling magnets used a similar trick.
Tim - yes they can be purchased but I don't know a supplier. Over a decade ago I worked for an automotive R&D company we specified & purchased lots of different configurations of magnets for small motocycle engine magnetos.
They varied from full annular magnets to individual segments. Some were soft plastic, some were hard "ceramic" like material. Different pole configurations & pole strengths were specified depending on what they were trying to do.
Hmm that's a pretty good wiki page. We've pondered using a Halbach cylinder (further down on the page) to make a uniform field. With two nested cylinders you can get a variable field strength (as you rotate one cylinder inside the other), but the cost of magnetic materials these days makes this kinda expensive at the moment.
It'd be a nice science fair project to measure the near field of a frig magnet. Could you do this with a GMR(giant magneto resistance) head from a hard drive? Do those things tell you anything about field direction or is it only the magnitude? (that's a question to the group and not to you in particular.)
One would think so, but those usually have hard-to-detach preamplifiers. Magnetoresistance is also a direction-independent measure (before GMR, it was 'bismuth resistor' for those probes). I'd like to use a rotating-coil gauss meter.
More usual, these days, is a Hall probe (harder to explain the workings to students, so the rotating coil would be better).
Most accurate, would be a fluxgate magnetometer. Also, VERY hard to explain to a firstyear class.
Oh, I'd never heard of a rotating coil gauss meter...
From wiki article on magnetometers....
"The magnetic field induces a sine wave in a rotating coil. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the sine of the angle between the rotation axis of the coil and the field lines. This type of magnetometer is obsolete."
Obsolete, sounds like a challenge.
But I'd like something realy small that you could stick right on the frig magnet. (You can take a little permenant magnet and run it along the frig magnet and feel the ridges.)
We sell an Earth's field NMR, which is easy to set up outdoors. But inside users have problems because a magnet field gradient causes the signal to de-phase rapidly.
I'd love some cheap way to measure field gradients. (cheap is perhaps ~$100 so we could sell for ~$300.)
The big problem with rotating coils is the slip rings. You might be able to do something fun with the lariat approach, which gives you continuous rotation without winding up the cord.
It works by moving the rotating object to the opposite side of the pivot on alternate turns, so that the helicity inverts. If you hold a soda can in your hand, you can rotate it continuously in one direction by using a figure-8 motion, under your armpit and then next to your ear.
No, mechanically. Say you need to measure a static magnetic field with a SQUID, which doesn't give you an absolute measurement, only a difference. Measure the change between the pickup coil one way and flipped to the other.
Would this (these? how many tensor components do you need to measure?) GMR sensors be sensitive enough?
OK, I think I 'see' the lariat. I'm picturing a coil rotating in the plane of the coil, with the wires (rope) attached to one side. (In my mind the wires are attached to a stick and I'm spinning the coil over my head like some circus clown.) But I was not able to follow the can/ figure eight part. Is this rotation in the plane of the coil or does it rotate out to the plane as well?
But never mind, your lasso gave me an idea. Most gradient things I've seen use two detectors and then take a difference signal. This has all sorts of problems. (Zeroing and drift of the two detectors to mention two) It'd be much better to measure the gradient directly. So I'm seeing a coil vibrating back and forth, (for starters it's moving in the plane of the coil.) Then as long as the coil is not rotating out of the plane as it vibrates it has a signal proportional to the magnetic field gradient. Since it?s only the ends of the coil that are doing any work. (I?m assuming it is moving a distance less than the diameter) I can collapse the coil in the middle and just have two smaller coils connected in series.
So I could shake the coil a distance ~d, but measure gradients over a distance L. To get the gradient for the field in the direction of motion I could have a second set of coils rotated 90 degrees to the first such that the motion is along the axis of the coils.
Now the thousand dollar question is how to keep the plane of the coil(s) perfectly flat as I vibrate it back and forth? I?ve got a few ideas, but I?ll take R.V. Jones ?Instruments and Experiences? on my walk this afternoon. He was a genius when it came to mechanical things. (It?s a beautiful Friday afternoon here, time to cut out a bit early and head to the woods with a beer and good book. :^)
Interesting, thanks. (Why the Oersted units... An Oersted is a Gauss, right?)
Anyway I'd need to sense the Earth's Field ~0.5 Gauss. (Which looks to be below the minimum field of that sensor) We've got a sample volume that's about 2" diameter by 4" long. (~10cm) The NMR frequency is about 2.5 kHz, In 'good' fields we get free induction decays of several seconds, but maybe 100ms is enough to see a signal... so frequncy across the sample can change by ~ 1 part in 500? So field gradients on the order of 1mG/ 10 cm.... 0.1mG/cm
You can do it with an open can of soda without spilling. Palm the bottom of the can, at waist level, and turn your hand around under your armpit. Lift it across your body and in front of your face, then swing your hand outwards again, lower it to waist level, and repeat as many times as you like. (Typically the charm wears off after about three turns.) The can's motion is roughly like a figure 8.
You can also do it holding the top, of the can but it's a much more awkward motion. Holding the side will spill it for sure, unless you shift your grip as you go round.
Great book. I have a copy on my lab bench. If you use a lock-in with decent harmonic rejection, you don't need the motion to be perfectly planar--the out-of-plane motion will be mostly second harmonic.
Have a good brewski, and don't try the can-turning trick till you're nearly done with one. (It would be more entertaining after nearly three, of course.) Enjoy.
Ahh, got it. I think I've seen that used as an analogy to 1/2 spin statistics... you've got to go around twice to get back to the beginning.
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Re: RV Jones. I finally ordered my own copy today..(It came up on my wish list at amazon for 'only' $150.) I've got one on semi permenant loan from a friend, but I can scribble in a copy of my own (in pencil of course) and return the other.
I was reading his section/paper on 'Some uses of elasticity in instrument design', Gotta love the title. (Is anyone writing papers like this anymore?)
Re: 2nd harmonic stuff, Woah! I hadn't thought of that. OK now I only need to wiggle the coil fast... (no magnetic parts allowed)
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