Tyler Yip enquires about the magnetic properties of ink used in U.S. currency. I believe the ink can be magnetized and the bill passed over a magnetic scanner that recognizes a magnetic signature. U.S. Tres. has machines that very quickly pick counterfeits out of a stack of bills using this method. Other things U.S. Tres. does (and you can do) to recognize counterfeits are: 1) Put the bill under UV light. Cheap paper tends to fluoresce. The real thing won't. 2) Look at the treasury seal (the only part of the bill that is multi-colored). Make sure that the denomination printed over the seal is all black and all there. 3) Rub any of the printing against a piece of typing paper hard enough so that the ink from the bill stains the paper. Now look at the bill where you rubbed it. Offset print jobs will smudge -- the real thing won't. Europeans are now going a step further. They are composing the portrait out of concentric circles centered on one of the eyes. If someone attempts to copy the bill using a digital scanner, the resulting aliasing of the circles in the portrait will be very noticeable to even a casual observer. The are also using colors chosen especially to foil color copiers. | (V) | "Tiger gotta hunt. Bird gotta fly. | (^ (`> | Man gotta sit and wonder why, why, why. | ((\\__/ ) | Tiger gotta sleep. Bird gotta land. | (\\< ) der Nethahn | Man gotta tell himself he understand." | \< ) | | ( / | Kurt Vonnegut Jr. | | | | ^ |