My aluminum conductors arrived last week in coil form, almost perfectly filling two USPS flat rate boxes.

One is bigger and heavier than the other, and in much better shape (literally). Maybe that one can be used as a 400 turn electromagnet coil

*without* being rewound. A 3" diameter steel pole piece, not yet procured, might be persuaded to slide through that core with no layers of cardboard peeled off.

I immediately cut off a sample from the larger coil, on which to measure the material's thickness, weight, and electrical resistance.

The aluminum has a clear anodized coating on both sides, which I scraped away in a few places for electrical connections.

**Questions up front. Can anyone cite experience to complement my Internet research on:**

How to strip anodized coatings without rapidly etching the aluminum?

What fluxes and filler metals can solder aluminum at temperatures below 600F or 300C?
Here the sample is conducting 5 amperes from a benchtop power supply. Voltage drop is measured with a fancy DMM that can resolve microvolts. Keeping one meter probe at a fixed location, I used the other (with sharp pointy tip) to penetrate the anodized coating and map the potential.

These contours are 1/2 millivolt apart, so the resistance between lines is 0.1 milliohms. The sample is barely long enough to demonstrate a rule of thumb for current spreading: Current density is practically uniform at places more than about 1 strip width away from a point source.

**The sheet resistance worked out to be 0.24 milliohms per square, an unexpectedly high value.** (Another rule of thumb: 1 ounce copper foil is 1/2 milliohm per square.) If my Al were 7 mils thick then its resistivity would be 427e-8 ohm-cm, a plausible value for 3004 alloy. But this is supposed to be 1100, practically pure Al, at around 300e-8 ohm-cm.

- unimet_comp.jpeg (30.51 KiB) Viewed 6261 times

*By the way, it's easy to remember the value for 100% IACS, a popular and practical reference value for stating the conductivity of metals. The International Annealed Copper Standard, which is 100 years old in 2013, adopted a standard resistance at 20 degrees C of a copper wire 1 meter long and 1 mm^2 in area: 1/58 ohm. Today we'd say 58 megasiemens per meter. It works out to 172.4e-8 ohm-cm. Modern copper wire routinely exceeds 101% IACS.*
The discrepancy was resolved by careful thickness measurements, and some investigation of the maker's label inside the cores. Overall thickness is about 9.5 mils (0.24 mm), including the 2.0 mil (0.05 mm) clear plastic film. Originally I, like the used metal vendor, had peeled back the film and measured 7.0 mils. But that includes an adhesive layer that takes up 2.5 mils (0.06 mm). When that's cleaned off, the metal thickness is only about 5.0 mils (0.13 mm), consistent with resistivity of 305e-8 ohm-cm.

The labels inside the core say Adhesive Research, which is still in business. There's a special part number, but I think what I got is closely related to ARclad 5795 "EMI shielding foil".

http://www.adhesivesresearch.com/Docume ... 0Sheet.pdf
So I got only about 28 lbs of Al in a 38 lb coil. The magnet power estimate must be revised upward, unless I want to strip that adhesive and use a thinner insulating film. On the other hand, original power estimate based on 7 mils used a very conservative value for resistivity.

Let's close with the one indirect measurement that's probably accurate to within 1%.

As wound, the thickness per layer is 9.73 mils (0.247 mm). I measured the metal ID and OD, and counted the 401 layers like rings on a tree.

The same measurements tell us that the strip is 803 feet long (1676 squares, so about 0.40 ohms). The AR label says the coil originally held 1200 lineal feet.

Thank y'all for reading this far. Both of you!

All models are wrong; some models are useful. -- George Box