Rubbing a coin and vending machines

[McCrunchy]

Hi all,

 

I was wondering why is it that some coins need to be rubbed in order to be accepted by vending machines. Is it making the edge more conductive by removing some grease and thus allowing for a resistivity measurement inside the machine ? Any bright ideas, references ?

 

Thanks in advance,

 

McCrunchy

[swansont]

I'm pretty sure coin validators make use of magnetic properties, rather than depending on surface conductivity/resistivity.

 

I recall there was an issue in making the Sacajawea dollar coins have the same properties as the SBA coins.

 

edit:

found it

http://findarticles.com/p/articles/mi_m1200/is_14_157/ai_61617778/

 

I was wrong:

What's more, different companies use different frequencies of alternating current in their coin acceptors, compounding the problem. "The higher the frequency, the shallower the penetration into the coin," Brauer explains. "At higher frequencies, you measure only the surface conductivity. Lower frequencies penetrate into the core."

 

So surface conductivity, or that of surface contaminants, is an issue. So you may be right.

Edited January 6, 2010 by swansont