Magnetic Paper: Write Your Ode to Lode
This "Magnetic Field Paper" is simple enough, reacting to the mysterious power of the lodestone. I am unsure if baby's blood is necessary to activate its fell power, but I figure it always pays to play it safe.
From the product page:
Developed back in the 1970s, with ideas that it might become a replacement for paper, this paper makes magnetic fields visible. Any other magnet can be used, and the paper will allow to to make the magnetic field visible.That would imply to me that it is reusable, sort of like a cross between an Etch-A-Sketch and those iron filling-filled mustache man magnet kits. It's $11 for a three-by-three-inch square.It consists of a thin plastic sheet containing tiny cells containing liquid. Each little cell contains colloidal iron. Hold a magnet near, and the iron in each of the cells will align, and form a pattern on the paper.
Catalog Page [Grand-Illusions.com via Coolest Gadgets]
Previously: Lancome "Le Magnetique": Magnetic Nail Polish [BBG]

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Yes, it is "reusable".
If $11 seems steep, Edmund Scientific sells "Magnetic viewing paper" in 20mmx50mm pieces for two bucks each:
http://scientificsonline.com/product.asp_Q_pn_E_3082237
The larger and more expensive Grand Illusions "Magnetic Field Paper" in the article apparently constrains the iron within cells. I'm not sure how much difference this makes?
I seem to recall a similar item with which you could clearly see the magnetic domains making up the tracks and data on a credit card's stripe, but I don't know whether ES or GI's products is sensitive enough to work for this?