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Electromagnetic Devices

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How Do Vending Machines Know What Type of Coin You're Putting in?
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The connection between electromagnetism and vending machines

Credit: Todd Ehlers
Source: http://www.flickr.com/photos/47325272@N00/19354775/in/photolist-2Hcv2-2fwnSJ-76LcLF-4moghY-5FFRaV-aUNecB-uYX2G-dNmQwp-85m3dn-85pen5-6yemCu-9ZYEo-2coaQ-85pejo-85pefw-85pe5E-85mgit-85m5zP-85pe8C-85mbMv-85pe3f-85pqZS-85pkmj-85m5oR-cvAUs7-8anAdx-vtRTz-5AwN-4y4LAV-4y4LNi-4y91xd-acMd72-4y91DG-aL4BZk-8SCTjh-iRfq3-4Q88kZ-5LWsTP-4bmZce-puodk-yCibS-7T1G8M-4Z4LQ5-6nto2E-2KPGQ6-2KPGPX-3q1jM-8qoK8T-3QAVdj-97PzGZ-a7UXLj
License: CC BY-NC 3.0

[Figure1]

Can you tell the difference between a penny and a dime?  Sure, that’s easy you say!  But what about for a machine?  How does a vending machine know what coins are you putting in?Although vending machines vary on the method they use to determine what coin is put in, most vending machines use the same electromagnetic phenomenon as a metal detector. After the coin is dropped in, it will pass through a small electromagnetic field that creates ripples that can be detected and analyzed. Each coin has a distinct pattern based on the size and metal of the coin. If a coin does not match a known set of signatures in the vending machine, it is rejected. This makes it pretty hard to fool a vending machine now.

Creative Applications

  1. An electromagnetic device is any device that uses electromagnets, which is a large portion of all the electronics we use today. A common example is a vending machine. How is a vending machine an electromagnetic device?
  2. What type of metals are different coins made up of? How do you think specific metals affect the signature? 
  3. How would changing the composition of a coin affect vending machines?
  4. It’ll be hard to build a coin detector at home, but you can make a simple electromagnetic motor. Watch this video and give it a try!

Resources:

http://boards.straightdope.com/sdmb/archive/index.php/t-245619.html

http://wiki.answers.com/Q/How_does_a_vending_machine_know_what_kind_of_a_coin_you_put_into_it

http://blogs.howstuffworks.com/2010/03/31/32069/

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