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from: Stash (Direct from the Physics classroom)
cache-ntc-aa01.proxy.aol.com
05-02-28
11:09:58
"In the 20th century for the first time there developed a 4th group, ELECTRONIC". there are still only 3 because for electronic to be heard requires a vibrating membrane, either within a speaker cabinet or headphones it's the same an oscillating magnet vibrates a membrane.
