Magnetic Field for hollow core?

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?

Picture of Magnetic Field for hollow core?
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Kiteman5 years ago
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.
Antzy Carmasaic (author)  Kiteman5 years ago
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.
kelseymh5 years ago

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.
Antzy Carmasaic (author)  steveastrouk5 years ago
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...
Use a NON ferro magnetic bobbin, or a sleeve.
Antzy Carmasaic (author)  steveastrouk5 years ago
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.
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