Inductive kick

[magnetoelastic]

Conceding that I have actually never verified this assertion, I tested this idea.  I have a selection of ferrite rods, used for portable AM radio antennas.  The part I selected was 0.5" dia, and 4" long.  I first wound it with 200 continuous turns of 30 AWG, kynar insulated wire.  This covered all but about 1/4 of the core on each end.  It took 27 feet of wire, including 4" free for each end.  I measured the DC resistance to be 2.7 ohms.  I measured the inductance to be 1.70 mH.  Connecting a 1K resistor in series, and using a 9 volt battery as a source, (measured 8.8 volts under load),  I measured a 6.2 Gauss magnetic field directly off the end of the rod.

I then stripped off the windings, cut the wire in half, and re-wound the coil in bifilar fashion.  Then I connected the end of winding #1 to the beginning of winding #2.  The measured resistance was 2.7 ohms, the measured inductance was 1.71 mH, and the magnetic field with the 9 volt battery attached was 6.2 Gauss.

To my satisfaction, the two configurations are essentially equivalent.

[Elvis Oswald]

That is better than a paperclip test, eh?   

I won't be picky about the core material.  Harder metal makes a better magnet... but you would expect the same ratio (1:1) regardless of the material used.

The only explaination must be that my wire was not insulated - neither was the nail.  So - eddy currents?  Stray capacitance between the turns?   Either way it is a poor design and one gain probably equals a loss of something else.

Thanks for taking the time to test it 

[magnetoelastic]

Choice of core materials matters - some.  For a rod shaped electromagnet, if the core is the same size, you get just about the same strength flux output regardless of whether the core is iron, steel, or ferrite.

If you are making a permanent magnet, you want hard steel - carbon or nickel steel, preferably carburized.  The same mechanism that makes steel hard also tends to keep the domains oriented, resisting demagnetization.

If you want to make an inductor, you want soft steel - annealed, for the same reason.  This keeps hysteresis losses low.

More speciallized applications require speciallized core materials, of course.

[magnetoelastic]

Elvis, there is a really great book on inductor design, free for download at

http://www.pmillett.com/Books/Lee%201955%20Electronic%20Transformers%20and%20Circuits.pdf

written by Reuben Lee, one of my old Westinghouse buddies.  It is clearly written, and very helpful in understanding these matters.

[Elvis Oswald]

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