  HP-45B hydrogen thyratron. 
A thyratron is a vacuum tube used to switch on high voltages.  They act like a switch that can be turned on by applying a small voltage to a control terminal.  They can only switch on, not off: They turn back off only when the voltage being controlled drops to zero on its own.  The modern equivalent is the SCR (silicon controlled rectifier) and triac. 
This one contains a small (less than atmospheric pressure) amount of hydrogen gas instead of a pure vacuum.  Various different gases are used in thyratrons, and apparently the hydrogen variety is used when particularly fast switching times are required. 
Source: eBay seller tapent 
Contributor: Theodore Gray 
Acquired: 24 February, 2007 
Text Updated: 29 April, 2007 
Price: $5 
Size: 5" 
Purity: <1%
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