  Mercury thermostat.   
This is a mercury tilt switch thermostat of a type that was very common until quite recently.  A bi-metallic strip coils and uncoils in response to changes in temperature, causing a glass ampule containing mercury to tip left or right, making or breaking a contact. 
Source: Chris Anderson 
Contributor: Chris Anderson 
Acquired: 13 January, 2010 
Text Updated: 13 January, 2010 
Price: Donated 
Size: 4" 
Purity: 99%
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