(c)1989 William J. Beaty

Also see: electricity projects

There are a number of science museum exhibits which require many tens of amperes of electric current in a thick cable to generate strong magnetism. One example is a raft of compasses with a 200-amp cable running through the center of the raft. Or, three 100-amp cables with three-phase AC powering them, where the resulting field rotates and can spin a conductive object by induction.

Rather than trying to build a 2-volt, 200 ampere supply, there is an easier way. Think:

In stranded cable, the DC electric current divides equally among all the strands.

If a 200-amp cable has 200 strands, then each strand actually has only 1.0 ampere. So instead of using a length of thick cable, why not wind a hoop-coil of very large diameter? (e.g. a hoop that's 3ft in diameter.) Wrap the coil with black electrical tape so that it resembles a circle of heavy black cable. Send 1.0 amperes into the coil's connections, and you have a circular "cable" which has one ampere within each "strand", and 200 amperes within the cable as a whole. There is no difference between a segment of this "coil" and a segment of a thick electrical cable with an enormous current inside.


Depending on the number of turns and the thickness of the wire you use, you can then power your coil with a 20-volt, 10-amp supply, (or a 200-volt, 1-amp supply if you used thin wire.) Even alkaline 9V battery might give interesting effects. If the resistance of your coil is high enough, it may be possible to plug your coil directly into 120VAC with no gigantic power-supply box needed, just use a diode! (Make sure the total wattage isn't too high. Your coil can be warm, but you don't want the insulation in the center to melt! And better use a fullwave bridge rectifier to get smoother DC.)


When I saw that the above could be done, I felt really stupid for not realizing it earlier. But then I began noticing that no other science museum was already using this idea. Can it really be that novel? Those neat Exploratorium magnetism exhibits with thousand-amp conductors are within reach of anyone, simply by using one hundred strands of 10-amp conductors! Huge power supplies can be replaced with a tiny FW-bridge rectifier, and the whole thing plugged directly into 120vac!


But isn't this a form of "cheating"? An electromagnet coil is not a single cable after all. WRONG! If we connect a thick cable to a power supply and send 200 amperes through it, then we have created a 1-turn electromagnet coil. Whenever there is direct current in a cable, there is ALWAYS a 1-turn coil present. And whether you send 100 amps through one wire, or 1.0 amps through each of 100 strands of wire, the magnetism is the same. Power supplies are charge pumps after all, not charge sources. The path for current is continuous through the power supply as well as through the cable, so even if you use a single thick wire, result is a long, flexible, hoop-shaped electromagnet. But if we instead wind a 100-turn coil and send 1.0 amperes through it, the end result is the same: the "cable" still has 100 amperes within, and has a very strong, circular magnetic field outside. A hoop-coil is the same as a cable, both are electromagnet coils, the only difference is in the number of turns used.


Speaking of cables, here is an additional trick. Obtain many feet of some thick multi-strand telephone cable; the sort used in older office telephone systems. A "100-pair" telephone cable contains 200 conductors. Place the ends of the long cable near each other to form a loop, and then solder the strands in a very special manner to electrically convert the cable into a coil. (e.g. solder the red to the green, the green to the blue, the blue to the orange, etc., until only two ends are left.) The path for current in each strand should repeatedly come out of one end of the cable and dive back into the other end, so the loop of cable now forms a coil. The result is a loop of thick plastic cable (of any length you wish!), with two little wires sticking out. Send a current through them, and it will seemingly multiply the applied current by 200 times.

If you only want a short "100-amp cable", it's proably easier to wind yourself a hoop-coil using thin insulated wire. The above "phone cable" idea requires a lot of soldering, and it's easy to make mistakes. The "phone cable" is best when you need to make a very long "magnetic cable." Eg., when you want to wrap a coil around your house, apply a few hundred watts of 5KHz, then run your toys with little 5KHz resonator coils (no batteries needed.) Courtesy of Nikola Tesla, no?


  • - Compasses surrounding a 'single' thick conductor to display the circular shape of the magnetic field

  • - Pairs of flexible cables held near each other will magnetically attract or repel other depending on the direction of current.

  • - A powerful AC magnetic field around a 'single' 100-ampere cable can light a coil/lamp assembly held nearby.

  • - A spiral of thick cable wound through holes in a compasses-table makes a dipole field pattern.

  • - Ring-core halves with polished faces, held around the conductor, become impossible to separate once touched together. When the core halves form a complete ring with no air gaps, the magnetic field becomes incredibly strong. This also demonstrates that magnetism is NOT based on individual "poles" (where are the magnet poles in a closed ringlike magnet?) It also demonstrates that extremely high flux can seemingly arise from nowhere (after all, before the ring was closed, the magnetic field around the cable was relatively weak.)

  • - Drive three hi-amp cables with audio power amplifiers driven by tiny oscillators with 60Hz square waves in 3-phase relationship. Hold the three cables near each other, and it creates Nikola Tesla's spinning magnetic field. If a conductive object is suspended by a thread and held near the cable, it will rotate, which demonstrates how induction motors work.

    What would you do with your own thousand-amp magnetism cable?

    From Mon May 18 12:00:56 1998
    Date: Mon, 18 May 1998 12:00:18 -0700 (PDT)
    From: William Beaty <>
    Subject: Re: magnetism cable

    I was playing with a 700-amp DC welder and finding that its cable would cause the iron filings on the machine-shop floor beneath the cable to align. I tried it with the above 200-conductor phone-cable coil powered by several 9v batteries in series, and I obtained nearly similar results. Pretty impressive, considering that the power supply on the welder was the size of a small desk. The phone cable heated up quickly though. Conductive cooling doesn't work well through all that PVC insulation.

    I smirkingly though that an educational supply company could sell this as a "magical current multiplier" device. Supply a black box which has a loop of heavy cable permanently attached, and which has a compartment for a couple of 9V batteries. When switched on, hundreds of amperes flow in the thick cable. Just what sort of incredibly-efficient solid-state switching circuitry must be inside?! Heh heh. The box is empty. The thick cable is actually a coil.

    One cool demo: I had some iron half-rings (two c-shaped iron cores) which I found in an electronics surplus store (Radar Electric in Seattle.) They had been part of a clamp-on meter which measures alternating current. The two rings would fit face to face perfectly, forming a solid ring. The faces had been ground very flat. When either of these iron objects was held near the "hundred-amp cable", the attraction force was too small to notice. But if the two c-cores were placed together to form a ring around the cable, something impressive happened. Just before the faces of the cores were about to make contact, there was a "snap" sound as they came together. Once touched together, the attraction between them was incredible. I could not use my bare hands to get them apart. I couldn't even slide them, the friction on the smooth iron faces under the enormous force was too high!

    When current was switched off, the core halves remained stuck (there was still considerable remanent magnetization, and they formed a ring-shaped permanent magnet.) But this was soft iron transformer core material, an alloy which is designed to be nearly unmagnetizable. When pulled apart and then stuck together again (still with no current) the two c-cores did NOT stick together anymore. That significant remanent magnetization had vanished. Briefly separating them caused them to demagnetize! (I think this happens because opening the ring is moving the operating point on the hysterisis curve, and once the magnetization is lost, it doesn't return.)

    I also found that if I included a piece of Saran wrap (tm) plastic film between the faces of the c-cores, the attractive force was greatly diminished. Clamp the two c-cores around the hi-ampere cable with .0001" plastic film in the way, and they can easily be separated by hand. Tiny gaps in the iron flux-path have enormous effects.

    From what I understand of the reluctance of flux paths and the motion along the BH curve of soft iron, all of the above phenomena are expected and explainable. Weird though, yes?

    Besides demonstrating some magnetic phenomena, I found that this demo upset some of my incorrect magnetism concepts and rubbed my nose in the fact that magnetism is not based on "poles." It is actually Special Relativity mixed with moving charge. I COULD NOT use my "north pole" and "south pole" concepts to explain these phenomena to myself, and this made me realize that both conventional bar magnets and electromagnet coils have the illusory "north/south pole" concept built into them. Playing with circular fields and c-cores gave me a gut-level feel for the TRUE nature of magnetism: there are no poles, the lines of flux are circles, and net flux can vary enormously. This is quite different than the concepts accidentally communicated by playing with bar magnets, where misleading "poles" seemingly appear, and where the net flux extending from these "poles" is fairly constant. Bar magnets cause magnetism to seem akin to electrostatics, while the above demonstrations with the c-cores shows the spots where it is quite different.

    P.S. For those who haven't stumbled across it, see my Phys Demo page:
    (includes a large section on electrostatics)

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