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All Pop-Up Thermometers are not constructed exactly the same way but the operation principal is similar.
The Pop-Up described here, is from the last turkey I purchased. It was removed prior to cooking for these tests.
Image #1 on the left, is an x-ray photo of the Pop-Up as purchased (in the un-tripped state)
Image #2 is an x-ray photo of the Pop-Up (in the tripped state).
Image #3 is the Pop-Up indicator stem. I've included this separately because it wasn't clearly visible in the x-ray photos. Note the metallic appearance at the tip of the stem. This is the dark area visible inside the spring in Image #2 and at the bottom of image #1.
How it works
The indicator stem is anchored at the bottom of the housing, in this case with a eutectic alloy (like solder).
This is the metallic area in image #3 and the dark areas in image 1&2. The spring is being held in compression because the stem is anchored at the bottom of the housing with this alloy.
When the melting point of the alloy is reached the bond between the stem tip and bottom of the housing is broken and the spring pushes the indicator stem up.
A sample of the alloy was removed and tested using a differential scanning calorimeter (DSC). The melt point was approximately 176 degrees F (80 degrees C).
The USDA recommends "the temperature must reach 180 degrees F in the thigh of a whole turkey".
Looks like this Pop-Up thermometer would have done a pretty good job of meeting that temperature recommendation.
DSC scan pictured below:
Food Safety of Turkey ... from Farm to Table - USDA
Better Homes - Turkey Tips
Thanksgiving hot lines
Ten Steps To A Picture Perfect Turkey - Butterball
Holiday Turkey Guide - Reynolds Kitchens
All About Turkey - Honeysuckle White