Making a Thomson Coil?

This page has the Permeability of several materials.

Is there any reason to think about thickness of your material. Why or why not. Why many rods instead of one nice 3.8cm 2/3 meter long ferrous rod. Answer could be technical, economic, or ease of construction.

E.

sing Ohm's law V=IR, we'll try to calculate the inductance needed for typ ical 120V sockets and at a maximum current of 15A. So, we solve for inducta nce and get L = V / (2(pi)*I*f), which when we insert the numbers is L = 120 / (2*(pi)*15*60) = .02H

l a 3.81 cm diameter cylinder that is 1/3 of a meter high to obtain an indu ctance of .02H to impede the N turns of wire around the core. I don't know how many turns of wire are on the 500ft wire spool of 12AWG wire.

e inductor core. I need to know if there is a better alternative that I can realistically acquire.

Answer is current induced in the core. Iron core transformers are made with thin laminations to limit the area of the loop around which a changing mag netic field can induce a current. Higher frequency iron core transformers u se thinner laminations because a faster changing magnetic field induces big ger currents.

Making the core out of wire or rods is another way of limiting the size of the loop, provided - of course - that rods or wires are covered with insula ting enamel (in the same way as laminations) to prevent current flow betwee n adjacent conductors

--
Bill Sloman, Sydney

I built one a long time ago. It was a lot of fun to play with. One day I absent-mindedly held an old telephone exchange relay coil too close to it, and inadvertently built a step-up transformer and burnt my thumb with the arc - I didn't do that again.

I don't think very high inductance is necessarily what you want. If you want to fire rings well, you want more voltage per turn, which means not too many turns, but if there are too few turns, then the inductance may be too low, (especially if the core saturates but maybe even if it doesn't), and you may trip a breaker. From memory I think the coil I made was with 1.7mm diameter wire, wound onto an old spool about 100mm long, 100mm diameter. The core was about 35mm diameter and 400mm long, made of a bundle of welding rods stuffed tightly into a PVC pipe.

For the coil, make sure you use enamelled winding wire, not ordinary insulated wire, as the insulation needs to be thin in order to allow the copper to be thicker (for a given overall wire diameter) and the resistance to be as low as possible for a given number of turns. This will minimise heating, though it will still only stay cool enough for a few seconds at high power. If you want to run it for longer (which is definitely interesting), use a variac or wire it in series with a heater to reduce the current to a value that won't cause overheating.

You might find that for the core, dull steel wire for oxy welding is better than shiny copper plated steel wire intended for TIG welding, because the dirt and oxide on the surface of the oxy wire will help to reduce eddy currents. The thick plastic coating of florists' wire may use up too much space and reduce the iron that you would fit into the core. The voltage between iron wires isn't much so you don't need very good insulation, just a bit of dirt will do. If you were really interested in finding the best possible material then there are special materials intended for magnetic cores, such as grain oriented silicon steel, but I don't think using these would add much to the educational experience, and you could spend the effort instead on constructing other models afterwards such as a linear induction motor:

I recall that some of Eric Laithwaite's books contained instructions for building the electrical machines. I would not pay too much attention to his writings about gyroscopes though.

Those were for her to answer, not you! Mikek

GE.

using Ohm's law V=IR, we'll try to calculate the inductance needed for t ypical 120V sockets and at a maximum current of 15A. So, we solve for induc tance and get L = V / (2(pi)*I*f), which when we insert the numbers is L = 120 / (2*(pi)*15*60) = .02H

ill a 3.81 cm diameter cylinder that is 1/3 of a meter high to obtain an in ductance of .02H to impede the N turns of wire around the core. I don't kno w how many turns of wire are on the 500ft wire spool of 12AWG wire.

the inductor core. I need to know if there is a better alternative that I c an realistically acquire.

with thin laminations to limit the area of the loop around which a changing magnetic field can induce a current. Higher frequency iron core transforme rs use thinner laminations because a faster changing magnetic field induces bigger currents.

of the loop, provided - of course - that rods or wires are covered with in sulating enamel (in the same way as laminations) to prevent current flow be tween adjacent conductors

I already knew about that issue, which is why I knew that I needed many sma ller insulated rods, instead of a single large rod. I wouldn't have even me ntioned floral wire (Which used to be and still is sold in iron/steel in so me places) if I didn't know that.

hysics 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, Electroma gnetic Levitation (Of rings), Conductivity, etc).

fun project for introducing lay people to EM in a way that they can interac t with it.

erything EM that I learned in class, although we skipped out on inductance unfortunately (Stupid school semester physics layout, god forbid we learn a ll the school covered-EM topics in the EM physics class right?) At a fundam ental level, I get inductance, and am aware of inductive reactance, althoug h I haven't reviewed that chapters in my Giancoli physics text book on it y et. 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.

rious 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, a nd I've wired up a few outlets/switches, as well as all the wiring I did in my Physics II labs.

rce:

really just consists of a coil, with an insulate d ferromagnetic iron core, such as this DIY Thompson's coil. I could copy t his 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 wel ding, if I'm not mistaken.), but I'm looking to make the best coil that I c an, given the time and budget (Around $130-150) constraints, and I'm not su re if this is the design I should go with, hence my post here.

s? Will it run on 120V AC? What design choices would make it better?

ion that I can realistically get my hands on? I was thinking of using the s teel welding rods, but was advised by a retired particle physicist to use i nsulated floral wire. Are there any other substances that would be better?

sideration I was thinking about was thermal dissipation. It appears that it will overheat at some point due to thermal insulation and no cooling. That 's alright for my application, as long as it doesn't overheat for about 30 seconds of use.

he better, but I could be wrong, my understanding of inductance is rather l imited.

Hi Sarah, I've seen these 'jumping rings', but I've never bothered to figure out how they work. If you search for "jumping ring physics" you get tons of links.

lots more,

I say ignore the nay-sayers and give it a try. AC is dangerous so use caution, (and a lab partner) the incandescent bulb in series sounds like a good idea. (at least to start.) (lots of us got our start by building stuff according to a recipe.)

Since you are at a university, one thing you could try is to go and talk to the 'lab/demo guy/ gal' in the physics department, (or wherever you saw the demo.) You'll have to figure out who this person is. Then go talk with them, they might be interested in helping you... again I'm thinking it will work better if you can find a partner to help, and stay safer.

There's a ton of literature, you should read a bunch of that. (report back, tell me/ us what's any good. :^)

George H.

Yeah, or use DC. DC by contrast is totally harmless as we all know. ;->

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Right, by AC I meant the wall outlet. Here that's 120V and maybe a 20A fuse. GH

GE.

using Ohm's law V=IR, we'll try to calculate the inductance needed for t ypical 120V sockets and at a maximum current of 15A. So, we solve for induc tance and get L = V / (2(pi)*I*f), which when we insert the numbers is L = 120 / (2*(pi)*15*60) = .02H

ill a 3.81 cm diameter cylinder that is 1/3 of a meter high to obtain an in ductance of .02H to impede the N turns of wire around the core. I don't kno w how many turns of wire are on the 500ft wire spool of 12AWG wire.

the inductor core. I need to know if there is a better alternative that I c an realistically acquire.

with thin laminations to limit the area of the loop around which a changing magnetic field can induce a current. Higher frequency iron core transforme rs use thinner laminations because a faster changing magnetic field induces bigger currents.

of the loop, provided - of course - that rods or wires are covered with in sulating enamel (in the same way as laminations) to prevent current flow be tween adjacent conductors

Don't be silly. This isn't a forum for school teachers.

--
Bill Sloman, Sydney

Ok, so looking over the wiki, I guess you have no interest in buying

99.5% iron wire annealed in Hydrogen, so I would go with Iron wire, I don't know if the earlier suggested annealing process will oxidize the wire enough to create an insulating layer. Or you might find a trashed welder and disassemble the transformer and use the laminations, the trade off, your filling a round section with a square group of laminated wire whereas the wire could fill the whole area. Also cutting and fitting the transformer laminations would be a job. (I doubt you will be able to lift the welder transformer) Go to an electric motor shop to get epoxy insulated wire, you don't want the plastic insulation of house wiring. I would have my form all ready with a proper exit for the internal lead and ask if the shop will wind it for you. They need to wind it on something, might as well be your form.

You probably should do some work to decide the dimensions of your coil. Then you can find how many turns you can get with what ever size wire you choose, using this.

You earlier suggested 0.02 Henries, Don't forget the resistance of your wire, Then you need to calculate the inductance.

Opps, you're going to install a core of magnetic material. I would get my form built and install the magnetic material, then wind

50 to 100 turns of wire on it and measure the inductance and calculate the A sub L (others may use a different term) from that you can find the number of turns for the inductance you want. I expect you will have several iterations, wire size vs number of turns.

Wire characteristics

nuff for now Mikek

** FYI: recently I disassembled a 6 inch loudspeaker on the bench. I remov ed the cone and voice coil from the magnet assembly. The VC was 28mm in dia , had four layers and measured 6.7 ohms. Next I measured it's inductance an d found it to be a tad over 1mH.

So I popped it back in the magnet gap and found the inductance had increase d by no more than 50%, depending on the test frequency.

What can you conclude from this?

.... Phil

removed the cone and voice coil from the magnet assembly. The VC was

28mm in dia, had four layers and measured 6.7 ohms. Next I measured it's inductance and found it to be a tad over 1mH.

My physics (like much else) is very rusty. But could it be that the permanent magnet is like a saturated core?

BTW is the measured inductance different if the coil is jammed so unable to move?

Mike.

** The above Q was directed to amdx, but Mike got his reply in first.

** You are very close. The magnetic gap region of a loudspeaker is made from mild steel - the permanent magnet is either a ferrite or Alnico ring.

However, steel parts are magnetised to saturation so almost eliminating permeability.

As the aim of a Thomson Gun is to saturate the iron core ASAP, one can only rely on the air cored L value to limit AC current flow.

** Not at test frequencies above 2kHz.

.... Phil

e

oved the cone and voice coil from the magnet assembly. The VC was 28mm in d ia, had four layers and measured 6.7 ohms. Next I measured it's inductance and found it to be a tad over 1mH.

sed by no more than 50%, depending on the test frequency.

It's tightly wound so that all the loops are pretty much coupled already.

George H.

One could design it systematically using math e.g.:

But oh no look at all them scary squiggles and vectors

Sarah, Are you still looking at this project? Mikek