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Core material for electromagnet that doesn't oxidize

  1. Mar 25, 2012 #1
    Does anyone know of any electromagnetic core materials that either do not oxidize or who's magnetic and physical strength doesn't change much when it does oxidize? I need to make a room temperature electromagnet for hydrodynamic propulsion experiments. My first experiments used an air core but the results were insignificant.


    Electromagnet covered in aluminum (foil) and suspended in the center of an aluminum pipe which is submerged in a saline (salt water) solution. The original magnet had about 1cm diameter and about 10cm length with approximately 300 turns of magnet wire. Two AA batteries were used in series and a potentiometer wired as a voltage divider, the anode and cathode (aluminum) were connected in series with the electromagnet. The resulting lorentz force should be radial causing a blower-like effect where the rotating water creates a propulsion force.

    The actual results were that the rotation of the water within the aluminum pipe was minimal and halted after a few minutes of operation. The observed forces after a few minutes seem to have changed from radial to going directly from the magnet core to the aluminum pipe.

    I suspect that the AIR core is too weak of a magnet to use for this purpose and an IRON core would quickly oxidize (rust) and become useless. So what I need to know is what core materials could be used for an electromagnet which is designed for salt-water operation.
  2. jcsd
  3. Mar 25, 2012 #2

    jim hardy

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    Three coats of polyurethane varnish or good enamel on a hardware store bolt should be good enough for room temperature experiments.

    400 series stainless steel is what we used for electromagnet cores in PWR control rods.

    A connector specialty store like Fastenal should be able to fix you up with 400 bolts. Carry a magnet with you to check them.
  4. Mar 26, 2012 #3


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    tanus, can you sketch your setup, and illustrate the electrical path? I'm thinking the shortcoming might be in your basic design, not the electromagnet per se.
  5. Mar 26, 2012 #4

    Thanks for your response. I was considering some type of paint, plastic, or rubber coating but varnish sounds like it would be perfect since it wouldn't need to be too thick to be water-proof.

    NascentOxygen ,

    I've attached an X3D image of the concept. The actual design places the coil inside the center aluminum "pipe". I am considering the fact that that could be my point of failure in that the coil may be better placed outside the outer pipe.

    I don't see any reason why the design shouldn't function since the current flow should be from the center pipe to the outside pipe, and the magnetic field will be from one end of the pipe to the other. This should create a circular lorentz force around the center pipe. The actual results confirmed this in that there was noticeable movement on the pipe itself (within a few millimeters). The problem is clearly with the magnetic field leaving me with the options of adding a core, moving the coil to the outside or simply adding another coil to the outside.

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    Last edited: Mar 26, 2012
  6. Mar 26, 2012 #5


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    Zip I can do. Gif & jpeg I can do. X3D whatever that is, I can't.
  7. Mar 29, 2012 #6

    You should try downloading blender, X3D is a standardized file format for 3d graphics so most 3d systems will support it. I'm not good with graphics, but I can manage to do basic 3d models which in this case helped me determine where the forces really would be. Either way the design is relatively simple, it is a metal pipe within a pipe where the pipes don't touch and inside the center pipe is strait solenoid electromagnet. Connect one wire of the electromagnet to the center pipe, and apply a dc voltage between the outer pipe and the other electromagnet wire, submerge in salt water.

    Salt water is a good conductor so the current flows between the center to the outside pipes and the magnetic field runs along the pipe. Since starting this design I've come up with a few more design ideas which I plan on trying. One is to eliminate the center electromagnet and surround the outer pipe with a toroid inductor, this wouldn't work like a blower, but is another design which could produce different efficiency. I also plan on trying to strobe the power through the electromagnet with some flash capacitors, anything that can possibly make a decent MHD propulsion device that doesn't require a huge amount of power to run.

    My only real question was what core material could I use other than air that wouldn't oxidize and it seems there is none but that any core material can be used as long as it is water-proofed. With MHD propulsion there are only two things that lead to movement, current and the strength of the magnetic field. The magnet is my primary focus since there are some ways to increase the strength of the magnetic field without increasing the power consumption. There isn't anything I know of that can be done with the current without increasing power since it takes about 2.5 volts to ionize the water.
  8. Apr 1, 2012 #7


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    I was hoping to see a screenshot of your diagram, but apparently I'm not going to.

    I picked up on your use of aluminium electrodes. Aluminium is not suitable for both electrodes where electrolysis is possible because it quickly forms an insulating later with liberated oxygen. You could instead use lead or graphite (or silver, gold or platinum). It may be feasible to power a MHD drive using AC since both current and field would reverse each half-cycle, and with AC the electrodes will be unlikely to polarise.

    I think you are using the current that flows through the water to also power the electromagnet. Is it possible to separate the two currents, and use a heavier independent current for the solenoid?
  9. Apr 3, 2012 #8

    Yes I have considered running the current through the water separately and have considered using AC though many sources warn of toxic compounds which may be created. The resistance of the water is minimal though and my goal is efficiency. If the current through the water is low due to "pollution" in the water than using less power during these times should be more efficient. The experiments I ran were for short times so I didn't have any problem with oxidized aluminum, but I will consider altering the electrodes. My current project is making a water-proof hall effect magnet meter, and since I haven't been able to get my hands on any hall effect sensors I'm going to try to make one myself with a differential amplifier. I did get my hands on some large iron bolts, varnish, resistors and 100watt transistors which I plan on using in the next experimental prototype. The physics behind it is simply the current and the magnetic field. The current is easy to measure, the problem is measuring the magnetic field. once I have that problem solved I should be well on my way to a more successful design. Thanks for all your help.
  10. Apr 3, 2012 #9


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    Can you name such compounds? I find it hard to believe that AC could be more deleterious than DC. The reverse I could believe.
    The limitation is not so much the conductivity of the salt water, but the interface between water and electrode. The electrode becomes coated with an insulating layer of air bubbles when you use DC, even when you are not using aluminium. There may be mechanical methods for removing this buildup of clinging microbubbles, otherwise in short time it will severely hamper current.
    Maybe you did, when you observed "and halted after a few minutes of operation".
    You have obtained a Hall sensor and are using it with a diff amp to boost its sensitivity? The difficulty may be in calibrating the sensor. Good luck!
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