# Winding an electromagnet ring coil to get radial magnetic field

How can I wrap an electromagnet ring coil radially where the north pole is in the center and the south pole is at the outer ring? Is this possible to wrap?

Example:

I know I can wrap a simple coil to get north at one end and south at another but can one be wound radially to produce a magnetic field like in the picture above?

• By 'wrapping', I assume you mean 'winding'? Commented May 4, 2021 at 12:50
• Yes thank you I'll make the change Commented May 4, 2021 at 12:51
• if the "radial" case is at a specific point, then just "wrap" two points- 90 degrees away from that point or put your coil inside or outside the ring. If it is radial but all around, think of what the complete field would look like (draw it from all north to nearest south) the end result is the same as the usual winding.
– Abel
Commented May 4, 2021 at 12:56
• If you already know how you'd wrap it to get your first diagram (hint, a coil sits above each demarkation line between red and green), then you should be able to figure out where to place the coils for the second diagram (hint, exactly the same hint wording) Commented May 4, 2021 at 13:05
• You can make a transformer with 2 coils but the H field goes thru the ring centre axis. falstad.com/vector3dm/… Commented May 4, 2021 at 17:36

1. You can wrap several bar magnets. (At least 2, but the more the better.) Pay attention to wind them equally.

2. Align the bar magnets in parallel, so all north poles point inward. Like this

3. Fix them in place. If you don't, they will mutually push eachother apart when you energize them.

4. It is suggested to power them serially. That way each carries exactly the same current. If you wire them serially sequentially, though, you will also have a small axial magnetic field as the current traverses the circle once. You can get rid of that by wiring their series configuration in a way that moves e.g. from left to right.

Here is a Falstad link with 4 magnets facing in on each other. As expected the central field is zero. and the field lines exit both sides in axial direction creating large axial fields beyond the plane of the circle. The axial direction can be stuffed with more magnets facing in on the center. Falstad calls this configuration fittingly "monopole attempt". As magnetic monopoles cannot be created in this way. The field will always exit through the gaps.

• can you make 4 Tesla magnetic field in a palm size paddle with 150 to 300 Hz BW and up go 30 pps pulse rate using 1kW max? With fan cooling.... This has great therapeutic value with magnetic stimulation of muscles without pain. Commented May 4, 2021 at 17:39
• @TonyStewartEE75 my guess would be no. The axial field is strong in this configuration, but 4 Tesla is way beyond saturation of iron, so at this range the funneling effect of the cores is extremely weak. To reach 4 Tesla with 1 kW would require tremendous sized coils. At low temperatures it is easy, e.g. by using Holmium pole pieces. Commented May 5, 2021 at 7:15
• I was wondering how the Tesla Stym does it by a company in Slovakia. They would need exotic transformer steel for sure. A few years ago they were rated at 4T then 3T , I see now it's 2.5T iskramedical.eu/images/brochures/tesla-stym/… Commented May 5, 2021 at 13:19
• @TonyStewartEE75 Afaik there is no material with higher saturation magnetization than CoFe at room temperature, which is around 2.4 T. You can go a little beyond this with clever arrangement of the poles. But going far beyond either requires massive power, massive coils or higher magnetization pole pieces (such as Holmium, but only at ~10K). Factor of 1.5 above saturation would be a very clever pole geometry. I cannot test their claims. Commented May 5, 2021 at 13:35
• Yes it is a puzzle to me even to get the MOSFET drivers to be efficient with thus reactive power. I should ask a former colleague who designed a 7T MRI for NRC but that’s at RF with massive cooling. It imploded all the CRT raster scans in the NRC building even with a Faraday cage. All built with a hydraulic plastic gantry. Commented May 5, 2021 at 17:13

It's possible, look for axial flux machine. But the machine has alternating poles, so the flux has a path trough metal stator and rotor, in your case you could wind all in one direction and then place a steel disk on top of that, but I have no idea why would someone use this.

Another simpler way is to wrap the wire around a steel core, like solenoid coil.