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Author Topic: Why does rubbing a coin on steel make vending machines accept it?  (Read 21714 times)

purplerain

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Tahnee Otto  asked the Naked Scientists:
   
Hi Chris,

I have been wondering about these coin machines in shopping centres.

Often the coins fall straight through.  After rubbing them on, normally the steel surface, they are accepted as payment.  What makes that happen?

Kind regards,

Arnoline

What do you think?
« Last Edit: 05/04/2011 10:30:02 by _system »


 

Offline graham.d

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I have to say that I would need a lot of convincing that rubbing coins on steel makes any difference to whether a vending machine accepts it, at least generally. Sometimes machines can reject a coin that is close to its limits then accept it when re-submitted. It may just be that.

Vending machine coin acceptance/rejection systems vary and some are much more elaborate than you would expect. They don't just look at size and weight for example; some machines check the magnetic properties of the metal, the density, whether serrated and whether the serrations have patterning.
 

Offline Jolly- Joliver

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I have to say that I would need a lot of convincing that rubbing coins on steel makes any difference to whether a vending machine accepts it, at least generally.

No they do, some times a coin will be repeatedly rejected, but if you rub it against something else suddenly the  machine expect it. I know that from experience having used a few vending machines, and trust me when you are on a break you do not want to waste 20 mins trying to get a coin to be excepted just so you can have a coffee or a coke or something.

Personally, my oppinion on the issue is, that whatever the device is that checks the coins, rubing the coin on something else metal does in some ways remove some dirt and or add in some static, and for what ever reason the machine will suddenly except it.

Throwing them on the floor also seems to do something, but generally only with the coins made of two different metals, like the Two Euro coin.

So speaking from experience, I have seen that happen- a person spending a five minutes trying to get a coin excepted in a machine, then someone appearing and rubbing it on somthing metal and suddenly it gets excepted first time. And also from doing it myself, and seeing that rejected coins are suddenly excepted if you rub them on something.

As for why I thnk it's down to removing dirt(personel oppinion).
« Last Edit: 05/04/2011 17:20:13 by Wiybit »
 

Offline yor_on

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the only thing I can think of there, well except magic :) is magnetism and static electricity? I think that was one of the things Graham mentioned too as it might influence the magnetic properties of it? Depending on what alloys your coin is made of it may also be magnetic. Which naturally leads us to this :)

Maybe??

"Two coins are borrowed and examined, the Magician then takes them and says the magic words! The Magician touches them together and they stick! Then with a puff of air or the slightest touch the lower coin begins to spin, still stuck to the upper coin! The Magician then drops the coins into a spectator's hand and lets him or her examine them or try to duplicate the trick. They can’t it’s impossible...unless they know the secret!

THE SECRET: This is one of the coin magic tricks that require a special gimmick - a small magnet! If you have one laying around then great, if not the type you are going to need is preferably a Neodymium Magnet."
 

Offline Geezer

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the only thing I can think of there, well except magic :) is magnetism and static electricity?


Nah! Any static charge would be immediately removed when the coin touches the metal slot, and I don't think coins are made from metals with any magnetic properties, although they could be putting scrap iron into Euros for all I know.

Come to think of it, I have a vague recollection that vending machines do test coins to see if they have any magnetic properties, and reject them if they do. Presumably this is to to defeat creative customers who try to substitute steel washers for legal tender.

However, I have observed a similar effect with paper money. A lot of vending machines in the US seem to be a bit reluctant to accept brand new dollar bills. I've no idea why that is, but if you take a new bill that a machine rejects and scrunch it up into a ball then flatten it out again, it will usually be accepted.
 

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