Neon indicator light. 
This is a small hollow-cathode indicator light probably designed to run off 120VAC with a limiting resistor.  If I find out the right value of resistor to use, I'll probably hook it up. 
Source: eBay seller heruur 
Contributor: Theodore Gray 
Acquired: 24 December, 2003 
Price: $20/3 
Size: 2" 
Purity: >99%
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