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Speaker 1: Welcome to Brainstuff from house stuff works dot com where

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Speaker 1: smart happens. Hi, I'm Marshall Brain with today's question, how

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Speaker 1: does a dollar bill changer work? Creating a good bill

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Speaker 1: changer is a tough problem. The device has to work

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Speaker 1: with all sorts of bills, from crisp new ones to

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Speaker 1: ragged old ones, and it has to be reasonably good

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Speaker 1: at telling real bills from fakes. The changer also has

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Speaker 1: to be able to sense the denomination of the bill.

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Speaker 1: In order to accomplish all this, dollar bill changers use

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Speaker 1: a variety of technologies. Here are several of the techniques

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Speaker 1: that have been tried. Many of the early bill changers

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Speaker 1: took advantage of the fact that US bills are printed

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Speaker 1: with magnetic inc They could therefore use magnetic heads like

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Speaker 1: the ones you'd find at a cassette tape recorder to

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Speaker 1: pick up signals from the bills. Areas of the bill,

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Speaker 1: like the background behind the President's base, would generate signals

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Speaker 1: at a specific frequency, and the detection of this frequency

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Speaker 1: would validate the bill. Different bills generate different frequencies, so

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Speaker 1: the frequency can also help to determine what the denomination is.

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Speaker 1: Genuine dollar bills have a certain conductivity, and checking for

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Speaker 1: the resistance of the bill can authenticate it. Inks and

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Speaker 1: papers have different fluorescent properties, and these properties can help

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Speaker 1: tell real from counterfeit bills. The optical properties of a

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Speaker 1: bill or a strip of a bill can be sensed

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Speaker 1: with a photo cell or a camera and compared to

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Speaker 1: valid patterns stored in memory. Different patterns from different bills

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Speaker 1: can determine the domination. A good bill changer will use

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Speaker 1: several of these techniques. Because people seem to have a

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Speaker 1: very strong desire to outwit bill changers. Bill changers have

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Speaker 1: to handle such things as people inserting just half of

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Speaker 1: a bill, people inserting bills with strings attached so they

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Speaker 1: can jerk them back out, people who try unplugging the

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Speaker 1: machine with the bill half inserted, and so on. For

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Speaker 1: more on this and thousands of other topics, is it

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Speaker 1: How Stuff Works dot com