  Silver oxide battery. 
I'm not sure exactly why silver oxide batteries still exist.  Lithium cells provide more energy at lower cost, so maybe it's just that there are still devices out there that want the particular voltage the silver batteries provide?  I think they are used with hearing aids. 
Source: Radio Shack 
Contributor: Theodore Gray 
Acquired: 2 April, 2009 
Text Updated: 7 April, 2009 
Price: $4 
Size: 0.3" 
Purity: <20%
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