We recently got a delivery of some electronics (OK, it was a bunch of lasers. You can’t build a death ray without lasers. Oh, wait, did I say death ray? Forget that. I meant clock), and the packaging for each had both a shock sensor and a freeze sensor.

Naturally, I had to take the freeze sensor apart. It’s a bimetal disc that’s either concave or convex, depending on how you look at it. The two metals have different coefficients of thermal expansion, and if you form them in a strip, this will cause them to bend like a springboard as one expands or contracts faster than the other. This is useful for thermostats and indicators.

In this case, the original shape is concave (let’s pick a convention) at room temperature, and that’s the minimum energy configuration. To change it to a concave shape by cooling, the metal on the outside has to contract faster, but it won’t do this continually, as happens with curve changes of flat strips — the other metal is at a low energy, and will have internal forces trying to keep it there. It’s only after the second metal can overcome those forces that the disc changes to convex. By that point, though, there’s a bit of energy stored up in the system, and as the disc pops into its convex shape, it pushes a red indicator button out to show that it has fired. You can calibrate this type of device, and by choosing the right manufacturing conditions, have different indicators fire at different temperatures — many are set to be triggered when the temperature drops below freezing, or thereabouts.

Here’s what it looks like once you’ve taken it apart.

The disc is covered by the white holder with the red plunger, and is covered by the paper mask and plastic container. The plunger pops out when the disc is thermally triggered.

Here’s what it looks like firing in slow motion. I discarded the container and paper mask, and put the disc directly on a cooler freeze-pak, with the holder and plunger resting on it.