Exact-Temp^™ bottle thermometer 

Thermometer is mounted in a square shaped shatter-proof plastic bottle filled with a liquid to insure accurate readings despite the opening and closing of instrument doors. The thermometers are spirit-filled, thoroughly annealed, safety-coated with PTFE, and have been tested against a standard calibrated at the National Institute of Standards and Technology (N.I.S.T.). The bottle has a double magnet attached on one side so it can be affixed to the walls or doors of instruments to reduce space and breakage problems.

Exact-Temp is a trademark of Ever Ready Thermometer Company.