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Magnetic Field for hollow core? Answered

I am working on a project in which I have to control a small setup at the end of a motor shaft using an electromagnet. The electromagnet needs to be around the shaft inbetween the setup and the motor. So I have used a metallic ferromagnetic spool(image attached) and wound 32 gauge wire on it to make it an electromagnet and put the shaft through it.
Since this core made out of spool is hollow, I'm not sure how the magnetic field will behave. I tried powering up the one I made and it' seems to be both attracting and repelling a magnet's pole (edges attract while centre somewhat repels). Anyone got any idea about this?


Have you tried the classic iron filing test?

For this magnet, I would cut a [rectangular] hole that your coil fits in into a sheet of card.

Dampen the card, put the coil in place, switch on and then sprinkle iron filing around it.

When the pattern is revealed, switch off, lift the magnet out, and leave the filings to go rusty for a few hours - they will leave a permanent stain on the card.

That will show you the shape of the field, and give you a good idea of your situation.

The first thing that came to my mind was using iron filings. But don't know where I'll get them. I'll try to make some and will post a photo of the result. Thanks for the suggestion Kiteman.

What Steve wrote. It's a solenoid. You've built a great scale model of exactly the kind of giant magnets used in particle physics experiments (look up "BaBar", "SLD", "ATLAS", or "CMS" to see examples).

The region inside the hollow core is a very uniform axial field (north at one end, south at the other). That's where the particle-physics experiments would put the devices they use to measure the trajectories of charged particles. Outside the core the field is close to an ideal dipole, with field lines looping from the north to the south end.

But... is it?

I know this is your area, but the tube around which the coil is wound is magnetic. Solenoid fields are normally measured with coil alone (on a non-magnetic core) aren't they?

My mental image here is of the ends of the tube being sort of a circular pole. Around the outside, you get the classic bar-magnet pattern, and and almost-uniform axial field down the middle, but with a sort of "dead zone" just outside the mouth of the tube.

Is that at all correct?

It ISN'T correct,, as you say.. On re-reading the OP, he has a ferromagnetic bobbin, so the field will be very largely confined in the volume of minimum reluctance - ie in the metal.

The net field down the middle should be near zero.

Sorry for delayed response. I had an iron filing test and the fields were as expected. As kelseymh said, the field is pretty weak, just enought to pull staple pins weakly. I connected 3 9V batteries instead of 1 but not much luck. Finally I gave up and connected a 9v 500mah wall wart and it worked! Even managed to levitate a stack of magnets in a tube 1-2 cm!
But it quickly got too hot to touch within 5-7 seconds. I currently have about 20 meters of 32 gauge wrapped around the core. Should I use a wire with less gauge(meaning less no. of turns but more current)?
Thank you guys for your enthusiastic help...


But doesn't that mean my magnetic field will weaken considerably?
Quoting wikipedia:

The ferromagnetic core increases the magnetic field to thousands of times the strength of the field of the coil alone, due to the high magnetic permeability μ of the ferromagnetic material. 

Sorry for peskily asking so many questions and thanks for your patience...


It will work MUCH more effectively, though it will be technically weaker. Your ferromagnetic bobbin is short circuiting your magnetic circuit.

"Short circuiting." That's a great description!

In BaBar, most of the detector mass was a set of steel plates (20 each, 8-10 cm thick) arranged outside the superconducting solenoid. That was the (instrumented) flux return, which did indeed "short circuit" the magnetic field, substantially reducing the field outside of the detector.

Great! My main purpose was to create a strong enough field to repel a magnet placed on top of the electromagnet(as shown in the second image I posted) while using minimum current(and minimum heating also). The only twist was that it had to be placed around a spinning motor shaft. That's why I want for a bobbin and thought from it's ferromagnetic property will help strengthen the field.
I'll try making a pure solenoid as you both suggest and hopefully will be able to finish the project and post my first instructable.
Thanks a lot guys for your help. Really appreciate it...


It doesn't seem to me that you're asking too many questions. This is a great learning experience for you (and as you saw from my own reply, even "experts" can learn things!).

I do think Steve's right, if you use a non-ferrous sleeve, you'll get a significant field down the middle (assuming that's useful to you), and a broader (~ 1/r3 mostly dipole) return field outside the windings.

If you want a strong dipole field, as for an electromagnet, then you use a solid core, not a bobbin.

Wups! You're right. I was looking at the picture, which appears to be stainless steel or aluminum.

With a ferromagnetic core, there's going to be very little field down the center, and whatever field is outside will be pretty weak. Blargh.

....Ah; never mind. I was replying to this from my comments list. I see that Steve has already addressed it.

You have made a classic "solenoid", the field is passing through the centre of the coil and looping around outside too.

Yes. But not exactly a solenoid. This does have a ferromagnetic core but it's hollow. So it's something inbetween a solenoid and an electromagnet.
The magnetic field doesn't seem to be like the way you described. Due to the flat circular surface at both ends, it might be getting a bit warped I guess. Will try the iron filing test as kiteman suggests and have a better insight... thanks for your reply.