Further AC Explorations + Electromagnet

published 2025-12-12

by Christopher Howard

A little while ago I built a coil by wrapping thin transformer wire around a soft iron cylinder that my dad gave me. My coil is — roughly speaking — 600 turns in three layers — with the layers being about 9.5 cm in length, and the cylinder core being 0.5 inches in diameter.

For learning purposes, I tried to calculate the inductance of the coil, by putting it in series with a 470 nF capacitor, and a small value resistor. I hooked up the scope across the resistor, and then used the signal generator and the audio amplifier to find the resonant frequency, looking for a peak of voltage on the resistor. The peak I found was at 3.1 kHz. Using the formula f_r = 1 / (2 pi sqrt(L C)), and solving for L, the result I got was about 16 mH. I was expecting some higher number, since I have 10 mH inductors that are smaller than the end of my pinky. But, on the other hand, the length of my coil is much longer, which reduces the inductance, and I suppose the wires in the tiny coil are smaller and wrapped more tightly. So I suppose 16 mH is a plausible value.

Something fun is hooking up the coil to my variable power supply and using it as an electromagnet. The power supply is digital and has a current limiting feature, so that I can just set the max voltage very high, specify the current I want, and then it will lower the voltage appropriately. I found that I start to see some noticeable pulling force at around 100 mA, with it holding onto small metal washers.

Bringing the current up slowly, I found that the electromagnet starts to get noticeably warm at 1 A, so I'm using that as my practical upper limit. I tied a bunch of heavy metal washer together, and I found that, at 1 A current, the electromagnet is able to solidly hold up to 0.578 pounds of metal.

There is a formula in my old electronics handbook, which is T = A x B, where T is the electromagnets pull in pounds, A is the cross-sectional area of the pole, and B is the flux density in lines/in². I don't know if it is quite appropriate to use this formula, when we are dealing with donut shaped metal washers. But if I plunge forward anyway, and plug in the cross sectional area of the core (0.195 in²) and a value of 0.578 lb for T, I can solve for B, the flux density. The result I got was 2.96 lines/in². The seems like a small number, so I am in doubt. The example of flux density given in the book was much larger — over 10,000 lines/in² — but the example electromagnet was also much, much larger, so maybe 3 lines/in² is plausible for my small electromagnet.

I'd like to build an electromagnetic gun. The basic idea is you have several metal balls on one side of the electromagnet, touching it. And then on the other side you place another ball slightly away from the electromagnet. Then when you turn on the electromagnet, the solitary ball comes flying at the electromagnet and hits it, and then the moment is transferred to the ball on the other side which is furtherest away from the magnet, causing it to go flying away from the electromagnet. The metal balls would be easy to find, but I need some kind of rail for them to roll on, which might be a trickier matter — that is, finding one of the right size.

Something else that would be really neat to build is a plasma rail gun. But I think the parts and power supply would be a bit more expensive.

Copyright

This work © 2025 by Christopher Howard is licensed under Attribution-ShareAlike 4.0 International.