# How is it possible that one face of my electromagnet produces stronger magnetic field than the other?

I work in an experimental research lab. I made an electromagnet by having the machine shop cut through a toroid core and hand-winding wire around it. The objective is to have a region of uniform magnetic field above the gap for testing my devices. (The cuts on top of the magnet allow for placing a sample holder on top.) The faces of the magnet in the cut are parallel to each other--at least, by the naked eye. I used a Gaussmeter (425 Lakeshore) to measure the strength of magnetic field as a function of position, slightly above the gap, by recording the field strength at every 1mm interval going from one side of the gap to the other.

EDIT: Added arrow in photo to indicate approximate path of where I took measurements. Imagine that this is slightly above the gap.

This is the result I get:

As you see, there's an overall slope, and the part where the field strength starts to rapidly drop off on the right is lower in magnitude than on the left. I need the area in the middle to be uniform across, not sloping.

What is the explanation for why this is happening? It is a single block of the same material, so I would expect the material to be magnetized uniformly everywhere. So each face of the electromagnet should have the same density of magnetic poles, and therefore the field strength should be the same on each side.

Is it that the winding of the wire is uneven on one side compared to the other? That's the only explanation I can think of so far. Maybe if the density of wire windings is greater on the left than on the right, the magnetization would be stronger on the left?

What if I wound it such that only the bottom half of the toroid has wire around it? That would probably reduce the field strength but would it make the magnetization on each face of the cut more equal?

• Your graph doesn't surprise me. I would have expected that the magnetic field strength wouldn't be perfectly constant between the pole surfaces as a function of height and that there would be a small slope because the geometry of the magnet isn't symmetric as a function of height about the mid-height point of the poles. Look up "Helmholtz coil" for a better magnet design for field uniformity.
– user93237
Apr 21, 2019 at 20:50
• Is the gap really 25 mm wide? And does the horizontal axis of your graph correspond to a horizontal position in your photo? If the field nonuniformity is due to external fields as suggested by @janlalinsky and SamuelWeir, turning thr magnet upside down or at 90 degrees should change the measured field distribution. Apr 22, 2019 at 3:58
• Ok, for extra background (probably should have mentioned in my OP), I have made a previous electromagnet with the exact same design, but both the core and the gap are smaller, and that one DID come out symmetric. Or at least much closer to perfect symmetry than this. It was also tested in the exact same setup and location, so what I want to achieve is doable in principle. I wanted to make a bigger one with a slightly larger gap to have a slightly larger area of uniformity. Larger gap means the core has to be bigger so that the faces of the cut have larger area. Apr 23, 2019 at 16:38
• @S.McGrew The gap is actually supposed to be 0.95 inches wide. So it's a bit smaller than 25mm. I didn't start measuring at exactly the edge of the gap because it doesn't matter as long as I capture enough points to show where the uniformity starts to rapidly fall off. Yes, the horizontal axis in the graph corresponds to horizontal position (in mm) going left/right in the photo. That's a good idea of a way to prove whether it's due to external (non-uniform) fields or the magnet--to rotate it and redo the test. I'm pretty sure it's the magnet, though. Apr 23, 2019 at 16:55
• Oh and also, of course I know about Helmhotz coils. There are certain reasons why it doesn't really work here. The #1 issue is that it's much harder to produce a strong field due to the lack of a magnetizable core. Apr 23, 2019 at 17:05