Making a Thompson's Coil

[Sarah Szabo]

I'm interested in making a Thompson's Coil after seeing my University's physics II (EM) professor demonstrate Faraday's Law to us using it. It seems like a fun device to demonstrate various principles of Electromagnetism to lay people (Induced Voltage by Faraday's Law, Resistive Heating, Electromagnetic Levitation (Of rings), Conductivity, etc).

I should emphasize that this isn't for any university project, just as a fun project for introducing lay people to EM in a way that they can interact with it.

How should I go about building this? I have a functioning knowledge of everything EM that I learned in class, although we skipped out on inductance unfortunately (Stupid school semester physics layout, god forbid we learn all the school covered-EM topics in the EM physics class right?) At a fundamental level, I get inductance, and am aware of inductive reactance, although I haven't reviewed that chapters in my Giancoli physics text book on it yet. I'm on break for the next month, having just finished a semester at the university. So I have plenty of time to plan and execute the project.

I'm also well set up for woodworking/steelworking equipment for making various components, although I have more of a specialization in woodwork for now. I also have been trying to get into electronics ever since taking the physics II class. I have multi-meters (Including a clamp on one), but don't really have a power supply yet, I also have a 30W Weller soldering iron, and I've wired up a few outlets/switches, as well as all the wiring I did in my Physics II labs.

Functionally, from what I've seen, a Thompson's Coil (Pictured Below)(Source: http://kossover.squarespace.com/journal/2012/5/1/improved-elihu-thomson-coil-jumping-ring.html), really just consists of a coil, with an insulated ferromagnetic iron core, such as this DIY Thompson's coil. I could copy this method outlined in the blog post of the person who made this Thompson's coil (He used the spool, with iron welding rods, which are used in TIG welding, if I'm not mistaken.), but I'm looking to make the best coil that I can, given the time and budget (Around $130-150) constraints, and I'm not sure if this is the design I should go with, hence my post here.

Is this an acceptable design? Will it give good results for demonstrations? Will it run on 120V AC? What design choices would make it better?

 

[Sarah Szabo]

One thing that I noticed is that in this design, the ferromagnetic core is not removable (Or else short circuit), whereas other designs had a removable central stalk. I suppose that the wires were wrapped around their iron cores and the central stalk was detachable (Although, the fields would fight you for it, if the remove attempt was done while it was on.)

 

[hevans1944]

The soft iron core is necessary to raise the inductive reactance high enough to prevent overheating of the wire. It also concentrates the induced magnetic field to allow a greater repulsion effect on a single shorted turn (a conductive copper or aluminum ring) placed over the PVC core. You should measure the inductance of a 500-foot spool of THHN insulated 14 AWG before and after inserting a soft iron core. Then calculate how much current will flow with and without the soft iron core inserted. Also do a rough order-of-magnitude calculation of the temperature rise, and the rate of temperature rise, to be expected in the THHN-insulated wire. I doubt you will be able to operate continuously with 120 VAC applied, but THHN (basically nylon) can tolerate pretty high temperatures. Problem is, there is no easy way to remove I²R heating losses originating in the center of the spool of wire. Really BIG electromagnets use hollow conductors and circulate coolant through the coil.

Welding rod or even modestly ductile ceiling hanger wire is a poor choice of core material. Better would be green-painted soft iron florist wire, which is made from annealed iron. Search eBay for vendors selling appropriate lengths, either as cut straight pieces or on spools that you measure to length and cut yourself. Another source might be your local florist or their distributor of floral accessories. Worse comes to worst, you can gather up a few hundred old-fashioned metal coat hangers and straighten them out. Coat hangers can also be further annealed in a kitchen oven, but not as well as pure iron florist wire. Whatever source you use for the core, make a final insulating coat with an acrylic spray paint before closely packing the wires into the PVC tube.

An interesting extension to the original Elihu Thomson (NOT Thompson) coil is a position feedback circuit that allows you to "float" a copper ring at a specific height using negative position-feedback to modulate the AC applied to the coil. This would be an advanced project for just about anyone because of the highly non-linear nature of such a system. But imagine the thrill of dropping a ring over the PVC tube and watching the ring fall to a stable position half-way up the PVC tube, perhaps oscillating up and down a few times in a rapidly damped cycle, instead of being rapidly repelled up and off the end. It's a sweet trick balancing the force of gravity against the repulsive force created by induction currents in the ring, but if your position sensor is up to it you can position the ring anywhere along the length of the PVC tube simply by adjusting a position-setting potentiometer. That ought to be good for a little extra credit next semester.

Good luck with your project and welcome to Electronics Point.

 

[Sarah Szabo]

Thanks for your reply. It was really insightful. I'm somewhat new to electronics, but I've done some fun things with it, and it's already been rather useful to me as a skill. (I was able to repair an old turntable, and wire up new lights for my mom's car among other things, and I plan on making chemistry lab equipment (My brother & I are both science majors at Stony Brook University), and we have a home lab for replicating experiments or exploring chemistry in our free time. We plan on making a heating mantle for boiling flasks using insulated resistive wire and a PID Temperature Controller.

I was checking the inductance of various objects. It appears that the higher the iron content the better (Obviously), unless you're using some exotic substance, or alloys with things like cobalt, which I don't have access to. One thing that I know has a decent amount of iron is low carbon steels, both in the form of rods, and flat-bar steel as well as cast-iron balls of various sizes, which I have lying around all over the place.

According to the carbon steel wiki, low carbon steels are less than 3% carbon with minor impurities. Would these make for acceptable inductor core material? The cast iron tends to be of a high carbon content, so I'm guessing I shouldn't use that. But what about the flat-bar or rod steel stock

I was also thinking of ways to raise the inductance (I've been busy since the post, but with spring break coming up, it's a great time to finally try my hand at the mutual induction demo). Perhaps using an iron or steel pipe instead of PVC would raise the inductance higher?

I also has a question about geometry. Assuming that I did go for using the cast-iron steel balls, the balls are rather small, like marbles. I'm guessing that not having a solid stalk of iron (We can imagine a single large rod of iron) (The spheres would be touching, but not chemical bonded together. I'm aware of induced magnetism, but would this effect diminish over length compared to a solid region of iron?)

I also had a question about the florists wire that you mentioned. I was searching for what you meant, and I found this (https://www.homedepot.com/p/The-Hil...uge-Green-Floral-Wire-Twister-57223/203753737), it says that it's made out of steel, which in my mind seems similar to the steel rod for welding (https://www.homedepot.com/p/Everbilt-1-2-in-x-36-in-Plain-Steel-Round-Rod-802457/204273960)

I'm also guessing that the core should be spray-painted to prevent eddy currents due to the AC current, and that the reason why this will work is the skin effect?

 

[Sarah Szabo]

I was thinking of replacing the PVC core with something more inductive like EMT (Steel) conduit like this: https://www.homedepot.com/p/1-in-EMT-Conduit-101568/100400409