On Washington’s birthday, explore magnetism of currency

How does the candy bar machine know that I put a dollar bill into it? Not that I ever buy candy bars, of course. I’m just asking for a friend. But it’s a good question. How does a machine recognize George Washington’s face on a $1 bill from Thomas Jefferson on a $2 bill? 

The $2 bill had a bad reputation in the 19th century because it was used to place the most common bet at the racetrack. Other businesses didn’t like to accept them because they were sort of considered riff-raff money. If you tried to spend a $2 bill, people assumed you’d been gambling. So I suspect Thomas Jefferson must be embarrassed to be on a $2 bill. Then he gets stuck on a nickel too, like “that is worth a plugged nickel.” Anyway, Jefferson was born in April so he doesn’t have anything to do with Presidents Day, even though he was a president. 

It was Presidents Day this week, you know. Both Abraham Lincoln and George Washington were born in February. Not many people care anymore since now they don’t get the day off. Abraham Lincoln gets his picture on a $5 bill. I think that was to compensate him for getting stuck on the penny, which, with inflation, is more or less useless. By the way, I suggest you never put a $5 bill into a candy machine.

Actually, candy machines, and automatic checkouts at grocery stores, for that matter, don’t recognize the presidents. Paper money is printed with ink that has been impregnated with iron or other magnetic metals, making the denomination recognizable with magnets. You can prove this with one of those really strong neodymium magnets.

First, borrow a dollar bill from someone. Place it in a blender with two or three cups of water and blend, liquefy, puree, and generally make dollar bill soup. (This is why it was important to borrow the dollar.) This will probably take at least a minute. Stop the blender (although this step is optional, you will probably not regret it) and pour the soup into a one-quart Ziplock bag. Zip it! Lay the bag flat on the counter and place the magnet on top of the bag. You might need to agitate the bag a little to draw the metal particles to the magnet. Then if you slowly pull the magnet away, you should see bits of metal-impregnated paper have been drawn to where the magnet was. 

I’m pretty sure this should work, although I haven’t verified it myself. My wife wouldn’t lend me a buck. But it should be a unique way of celebrating George Washington’s birthday, which is today. 

Pennies are not only cheap, they are also ugly. Because they are made of copper and copper oxidizes with air, they darken with time. This seems a further affront to Lincoln. However, science can help with that. If you have oxidized pennies you want to make shiny again, just go to Taco Bell. Order a triple-decker steak and chicken Dorito Taco with extra sour cream. Then use their hot sauce and napkins to clean your pennies. They will shine like crazy again. 

I suppose you could do the same thing at home with just a salt and vinegar solution. That’s the part of taco sauce that un-oxidizes them. But for Pete’s sake, or Abraham’s, in this case, Presidents Day is supposed to be a celebration, you know! How much of an excuse do you need to have a wild night out at Taco Bell? 

I assume that all the candy machines are just magnetic readers and that’s how they know what bill you have put in. I’m pretty sure that all our bills are printed this way. If you would like to know for sure, just send me a couple of Ulysses S. Grants, and I’ll check them out for you. 

Gary McCallister, mccallis@colorado
mesa.edu, is a professor of biology at Colorado Mesa University.


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